Eduardo and Maria were recounting their experience in Sticky World and Slippery World to their Uncle Hubert, who was also a middle school science teacher. They weren't sure he believed that those two worlds existed, but he went along with them anyway and asked many questions.
Finally Eduardo was fed up with so many questions and decided to ask one of his own. "So, what makes something have more or less friction?" he asked, figuring that Uncle Hubert wouldn't be able to explain it.
"That's a hard question!" answered Uncle Hubert. "To answer that, we need to see what the molecules are doing. Molecules are too small to see, so I have a computer model that will help us picture what's going on."
Pre-test
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6) Box A had a mass of 100 kg and Box B had a mass of 200 kg, but otherwise they were the same. For each box, three measurements were taken of the force needed to pull it along the floor.
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Driving question
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1
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Explore what happens when two surfaces rub together and create friction.
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Exploring the question
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2
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3
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Push your hands together and rub them hard.
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4
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What does friction do?
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1
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| Read the story of "Martin and Emily's Wonderful Summer: A Frictional Adventure" by using the forward and back buttons below. |
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Chapter 1
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2
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|
"I'm bored!" said Martin.
"Me, too!" said Emily. |
"Now children, school has only been out for a week," said their mother. "Surely you can't be bored so soon!"
"There's nothing to do," said Emily.
"All our friends are at camp," said Martin.
"Well," said their mother, "you're going to do something much better than camp! Next week you're going to visit your friend Albert. The week after that you'll stay with Jennifer."
"What's so great about Albert and Jennifer?" asked Martin. "We haven't seen them since they were little kids."
"You were little kids too back then," reminded their mother. "You all are almost the same age. Besides it's not just Albert and Jennifer who are exciting – it's where they live! Albert lives in Sticky World and Jennifer lives in Slippery World. Just wait till you see what that's like!"
Do the names
Sticky World
and
Slippery World
give you any clues what those places will be like?
I predict that
Slippery World
will have slippery things, like ice. And
Sticky World
will probably be covered in sticky things, like tape.
What do you think
Sticky World
and
Slippery World
will be like?
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3
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|
"Sticky World sounds icky!" said Emily, "Who would want to live in a
place where everything sticks together?"
"Yeah," agreed Martin, "and I bet Slippery World is slimy, too!" |
"You're both wrong," said their mother. "Sticky World and Slippery World are just like here except the friction is different."
"What's friction?" asked Martin. He thought he knew, but he wasn't sure.
"Friction is the force that keeps things from slipping and sliding on each other," explained Emily. "Like sandpaper."
"What do you mean, like sandpaper?"
"Remember when we sanded that old rocking chair?" asked Emily. "Remember how hard we had to push to slide the sandpaper up and down?"
"And it tore," said Albert, "and I got a splinter!"
"Right," said Emily. "Well, that's friction."
"Sticky World has a lot more friction than we do," said their mother, "and Slippery World has a lot less. That makes things very interesting, you'll see!"
"OK," said Martin, "but I hope I don't get another splinter!"
This is a good place to make a connection. Have you used sandpaper?
Sandpaper can be hard to move back and forth on wood.
Sandpaper increases the friction between surfaces.
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Chapter 2
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4
|
|
A week later Martin and Emily were on the bus to Sticky World.
"Notice how slow the bus is going?" asked Martin. He was looking out the window at the trees inching past. |
"Yeah," agreed Emily, "It's been doing that ever since we entered Sticky World. The motor sounds like it's working really hard, too!"
"I hope we make it!" said Martin.
Just then the bus driver called out, "Ladies and gentlemen, we're about to make our first stop in Sticky World. Mind your step as you exit the bus – things are a little different here from what you're used to!"
The bus came to a sudden, lurching stop. Emily and Martin looked out and saw Albert and his mom and dad standing on the sidewalk. They waved to them. Then they got their luggage out of the rack and ran to the front of the bus to be the first ones off.
"Careful!" said the bus driver, but Martin and Emily weren't paying attention. They jumped off the bus and tried to run to their friend, but as soon as their feet touched the pavement they stuck there. Martin and Emily fell over and lay on the sidewalk, looking up at Albert who was coming to them as fast as he could.
There was something strange about the way Albert was moving. For one thing, he wasn't moving very fast. Also, he was picking his feet up awfully high with every step he took.
"Hi!" said Albert, when he finally got close enough to talk without yelling. "I'm glad you made it! Did you have a good trip? Here, let me help you up." Albert held out a hand to Emily and yanked her to her feet. Then he did the same thing for Martin.
I notice that the bus is going slow and the motor sounds like it's working hard. Albert also moves funny.
What do you notice already that's different about Sticky World?
Notice the way the bus moves and sounds. Also, pay attention to the way Albert moves.
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5
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"Don't be embarrassed about falling down like that," Albert
said. "Everybody does it the first time. Sticky World takes a
little getting used to."
|
"Yeah, Mom warned us about that," said Martin. "She said I might get a splinter!"
"A splinter?" Albert looked confused.
"Don't mind him," said Emily, "He got a splinter once sanding a chair with sandpaper and now he's convinced that..."
"Oh, we don't use sandpaper here," Albert interrupted. "We just use an old rag or something and it works fine. Everything sticks to everything else in Sticky World."
"I get it!" said Emily. "That's why our feet stuck to the pavement when we got off the bus."
"And we tripped!" said Martin. He was inspecting a scrape on his elbow that he'd gotten when he fell.
"I saw," said Albert. "I hope you didn't hurt yourselves. Around here you have to learn to walk like this." And he showed them how to pick their feet straight up so they wouldn't get stuck.
"This stinks!" said Martin. He said it softly, but Albert heard him anyway.
"Oh, no, it doesn't!" he said. "Wait till you see all the neat things you can do in Sticky World!"
What is the force of friction like in Sticky World?
What does the story say about the force of friction in Sticky World?
The story tells me that the force of friction is greater in Sticky World.
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Chapter 3
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6
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| As soon as Martin and Emily had met Albert's parents they all piled into their car and took off for home. On the way, Martin and Emily noticed that the car was going awfully slowly even though the engine was roaring and the road was perfectly flat. They were getting sadder and sadder – what fun could you have in a world where everything stuck together like this? |
Albert's father tried to cheer them up. "How do you like our new car?" he asked. "We're getting great mileage – three miles to the gallon on the highway!"
"That's nothing," said Martin. "My dad's hybrid goes 45 miles on a gallon of gas."
"Oh, but that's in Normal World," said Albert's father. "Around here most cars don't do much better than one mile to the gallon."
"What a ripoff!" said Emily. "I bet you pay the same for gasoline as we do, but you don't get as much out of it. That's not fair!"
"You're right about that," Albert chimed in. "A gallon of gasoline costs about the same here as in your world ‘cause there's just as much energy in it. But here we seem to lose a lot of the energy somehow."
"Oh, we don't lose it," said his dad. "We just don't use it very efficiently here."
"A lot of the energy in the gasoline goes into heat," Albert's mother explained. "That's what happens when there's friction. Things slow down and their energy turns into heat."
"I don't get it," said Emily. Sticky World was getting stranger by the minute. She was beginning to feel homesick already.
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7
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"I think I understand," Martin said. "It's like when it's
really cold and you've lost your gloves..."
"You're always losing your gloves!" said his sister. |
"No, it was just that one time. Anyway, when your hands get really cold like they did that time, you can rub them together to warm them up."
"Very good, Martin!" said Albert's mom. "That's a good example of what happens when friction turns motion into heat!"
"And in Sticky World it really works well!" said Albert. "Try it!"
So Martin and Emily tried rubbing their hands together. They were surprised at how hard it was to move them against each other. After a very short while, though, they had to stop.
"Wow!" said Martin. "It feels as though my hands are burning up!"
"I bet you could start a fire real easily here by just rubbing two sticks together," said Emily.
"Oh, yes," said Albert, "we do that all the time. We never use matches in Sticky World."
You can do this, too. Try rubbing your hands together.
When you rub your hands together, what do you notice? What happened to Eduardo and Maria when they rubbed their hands together?
When I rub my hands together, they get warm. Eduardo and Maria's hands also get warm, but they get too warm. Why?
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Chapter 4
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8
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| After what seemed like an hour they finally got to Albert's house. As they pulled into the driveway, Martin and Emily could see that it was a very pretty house, made of brick and two stories high with an extra room way at the top where the chimney came out. There were boxes under every window full of red and yellow flowers, and a tall oak tree in the front yard. |
"That's my room," said Albert, pointing to the little room at the top. "Watch how I get to it."
Albert walked over to the house and then a surprising thing happened! Albert walked right up the wall, using his hands and feet like Spiderman! Martin and Emily were amazed!
In no time at all, Albert had reached the roof of the house. He walked along the wall sideways until he came to his room. Then he opened the window with one hand, holding on with the other. He crawled in, turned around, and reappeared at the window, smiling down at them and waving.
"How did you do that?" yelled Emily. "What are you, some kind of acrobat?"
"Anyone can do it," Albert called down. "Come on, you try!"
So they did. It was a little scary at first, but Emily and Martin found that it was really pretty easy to walk up and down walls in Sticky World. It wasn't quite as good as being Spiderman, but almost. Their hands and feet did slip a little on the wooden walls of the house, but if they found rough spots they could hold on to them and pull themselves up. They spent the rest of the afternoon practicing, and pretty soon they were almost as good as Spiderman himself.
Maybe Sticky World wasn't going to be too bad, after all!
Wow, the kids can all move like Spiderman!
Are there clues in the story about why the children can climb the walls like Spiderman?
To climb walls like Spiderman, there must be a greater friction force to keep the children from sliding down the walls.
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Chapter 5
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9
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|
The week had gone by incredibly quickly.
"Goodbye," said Emily. "I had a wonderful time!" |
"Me, too!" said Martin. "It's going to feel strange going back to not sticking to things."
"I can't imagine what that's like," said Albert, who had lived his whole life in Sticky World. "I'd probably fall down all the time."
"That may happen to us, too," said Emily. "Remember, we're not going home yet. We're going directly from here to our friend Jennifer's house. She lives in Slippery World where there's practically no friction at all. I wonder what that will be like."
"Here comes your bus," said Albert's mother. "Do you have all your stuff with you? Do you have your tickets?"
Martin and Emily got on the bus. "Have a good time," said Albert's mother. "I'm sure you'll find interesting things to do in Slippery World!"
How might Slippery World be different than Sticky World?
Re-read the story for clues about what is different in Slippery World.
The story says that there is almost no friction in Slippery World.
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Chapter 6
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10
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| "I'm going to be really careful getting off the bus this time," said Emily. They had just stopped at Slippery World. Emily and Martin could see Jennifer waving at them madly. She was trying to say something, but they couldn't hear her through the bus window. |
"Watch your step!" said the bus driver, as Martin stepped out of the bus. "It's kind of slippery out there!"
Oops! Martin was careful, but not careful enough. His feet slipped out from under him and he sat down rather suddenly. From the bus, the ground where he was sitting had seemed perfectly level, but now that he was sitting on it he found that it sloped slightly. He started to slide downhill, going faster and faster, until he banged up against a wall and stopped. The experience reminded him of the very first time he had been on skis. He fell at the top of the bunny slope and slid all the way down to the bottom.
"Look out, here I come!" Emily yelled. She had fallen down, too, and now she was sliding right at Martin. She slammed into the wall beside him. "Oof!" she said.
"Boy, this place is really dangerous," said Martin. "I wonder how Jennifer can stand it!"
"Well, you can ask her," replied Emily, "'cause here she comes."
Jennifer was gliding over to them. She was wearing ordinary shoes, but she seemed to be ice skating. It looked very graceful and easy. It also looked like a lot of fun!
Eduardo and Maria get hurt! Why do you think Slippery World is dangerous?
Look in the story for clues about what makes Slippery World dangerous.
Slippery World is dangerous because without friction, Maria and Eduardo moved easily on a slightly downhill surface. They slid into a wall.
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11
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| "Welcome to Slippery World!" said Jennifer. "I'm sorry you fell. There's a bit of a hill, I'm afraid. They really should move the bus stop. The tourists are always sliding up against this wall. Are you OK?" |
Emily's arm was scraped where she had hit the wall, but she said, "No, I'm fine."
Martin said, "I guess we're going to have to learn to walk all over again."
"It's not hard," said Jennifer. "You just dig your heel in like this and push off." And she demonstrated, gliding away just as if she were on skates. "The thing is," she added, over her shoulder, "there isn't much friction here, so you have to pull or push on something if you want to stop or change direction." She demonstrated by digging in her heel again, making little ruts in the ground and coming to a stop.
"It's as though you're wearing heelys!" said Emily.
"What are heelys?" asked Jennifer.
"You don't know what heelys are?" Martin was amazed. "They're shoes with wheels on them so you can roll places."
"Why would you need wheels," asked Jennifer, "when you can just slide?"
"Where we come from," Emily tried to explain, "friction keeps you from sliding. The wheels on the heelys roll and that makes it much easier."
"Oh, now I remember," said Jennifer. "We learned about friction in school, but I didn't think it was real! It must be very strange to slow down all the time, even when you're not going uphill."
"You get used to it," said Martin.
Do you think it would be difficult to move around in Slippery World?
What does the story say about how to move in Slippery World?
The story says that you have to pull or push on something if you want to stop or change direction in Slippery World.
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12
|
|
Martin and Emily practiced "skating" without skates in Slippery
World. "You better learn to do it," said Jennifer, "'cause this is
how we're getting home."
"You mean you didn't drive here?" asked Emily. |
"No," said Jennifer. "My house and this bus stop are at about the same level and there's a valley in between, so we can coast all the way."
"I don't understand," said Martin. "What does a valley have to do with it?"
"You'll see," said Jennifer with a smile.
Have you ever used any kind of skates?
Think about skating and what you need to do stop. How do the children in the story stop?
Eduardo and Maria must carefully dig their heels into the ground to stop.
|
Chapter 7
|
13
|
|
Martin and Emily practiced and practiced until they were
pretty good at sliding around in Slippery World. To get moving
they learned how to dig the side of their foot in and push off
onto the other foot. To turn, all they had to do was lean one
way or the other. Stopping was the hardest. When they tried to
dig their heels in the way Jennifer had done, they would lose
their balance and sit down very suddenly. And then they would
continue sliding until they hit something.
|
It took quite a while for them to get it right, but after an hour or so they were able to slow down and stop without falling – most of the time, anyway!
"OK," announced Jennifer, "I think it's time for us to go home now. Follow me." Jennifer pushed off and slid slowly down the sidewalk, looking around to make sure that Emily and Martin were following her. When she got to the corner she dug in her heels expertly, stopped, and waited for them to catch up.
"This street leads to my house," she told them. "It dips down a bit, so you'll get going faster and faster, but don't worry – after a while it goes up and that will slow you down. Just keep both feet on the ground, don't try anything fancy, and you'll coast to my house with no effort at all."
"How far is it?" Martin wanted to know.
"About three miles."
"And we can coast the whole way?"
"Sure! Sometimes we'll be going pretty fast, like down in the valley, and other times we'll go more slowly, but we'll get there. As long as you don't hit anything!"
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14
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| And with that Jennifer pushed off just a bit and headed slowly down the street. Martin and Emily followed her cautiously. Although they hadn't noticed it before the street sloped downward just a bit, and without even trying they found that they were going faster and faster. |
"This must be what Jennifer meant when she talked about going into the 'valley,'" said Emily.
"It's not much of a valley!" said Martin.
"It's not much, but it's enough to get us going pretty fast, anyway!" said Emily. Indeed, she and Martin were going so fast that she could feel the wind in her face. She started to worry that she was going to hit a tree or something.
Emily was just about to try digging in her heels to slow down, when the street started to slope upward. As before, the slope was almost unnoticeable, but the little hill was enough to slow them down.
"Whew!" said Emily. "That was scary!"
"I hope we make it over the top of this hill," said Martin. "I don't think I know how to skate uphill around here."
Jennifer was pretty far in front of them by now, and wouldn't be any help if they stopped and started to slide back down the hill. But that didn't happen. By the time they reached the top of the hill they had slowed almost to a walk, but then they were over it, and as they headed down the other side they started to speed up again.
And that's the way it went all the way to Jennifer's house. When they were going downhill they went faster and faster, when they went uphill they slowed down, and when they were on level ground they just slid along at a constant speed. It was a lot of fun and they were almost sorry when, turning a corner, they came upon Jennifer standing to one side of the street in front of a low single story house. She clapped her hands as they skated up, looking at them with a big smile on her face.
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Chapter 8
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15
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| They were moving pretty slowly as they approached Jennifer's house, but even so Emily and Martin nearly went sailing past it. "Dig in your heels!" yelled Jennifer. "Otherwise you'll coast right into the next town!" |
"That was neat!" said Martin as he awkwardly came to a stop several feet beyond the spot where Jennifer stood.
"We never even had to skate," said Emily, who had grabbed Jennifer's hand as she slid by. "We just coasted over all those hills."
"I know," said Jennifer. "Pretty cool, huh? Still, it's kind of tiring. It's a hot day – why don't we all go inside and my mom will make us an ice cold lemonade."
Jennifer's mom was small and pretty. With her long brown hair and brown eyes, she looked like an older copy of Jennifer herself. She must have seen the kids coming down the street because the lemonade was already on the table. Her voice was soft as she asked Martin and Emily how they were enjoying Slippery World so far. They told her how much fun they had had sliding home from the bus stop.
But Emily, as always, had a question. "I still don't understand," she said, "how come we could coast the whole way here? We could never have done it at home, even on our bikes. Not without peddling."
"That's because the bus stop is higher than any other point along the road," explained Jennifer's mom. "So starting from there you had enough stored up energy to make it all the way here without working at it."
"There you go with that 'energy' again!" said Martin. "That's all Albert could talk about, too. Energy, energy, energy. What is energy, anyway?"
"It's simple," said Jennifer's mom. "Basically, there are two kinds of energy – the energy you have when you are moving and the energy you have when you're at the top of a hill. Oh, and heat energy, too, of course."
Wow, Maria and Eduardo went sliding a long distance!
How were Maria and Eduardo able to keep moving?
The story tells us that Maria and Eduardo had enough stored up energy to slide all the way to Jennifer's house.
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16
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| "What about electricity?" said Jennifer. She had just learned about electricity in school that year and she was proud to show off her new knowledge. |
"Well, yes, there's electricity," said her mom, "and other forms of energy, too, but to answer Martin's question we only need to think about those three: kinetic energy , which is due to motion, potential energy , which you get by going up a hill, and heat ."
"So, let's see…" said Martin. He was scrunching up his face as he thought about energy. "At the bus stop, before we started, we had no kinetic energy 'cause we weren't moving yet. But…"
Emily chimed in, "But we had lots of potential energy, right? Because we were at the top of a hill."
"It isn't a very steep hill," said Martin. "I hardly noticed it at first."
"Remember how you slid when you got off the bus?" Jennifer reminded them. "It doesn't take much of a hill to get you sliding all over the place in Slippery World!"
"OK, OK," said Martin. He was embarrassed about falling out of the bus and he wanted to change the subject. "So when we slid downhill we were turning potential energy into kinetic energy, right?"
"Exactly!" said Jennifer's mom, beaming. "And when you went uphill and slowed down you were changing kinetic energy back into potential energy. You always keep the total amount of energy you started with."
"Kind of like the rechargeable batteries in my iPod!" said Emily. "When we go uphill we're storing energy that we can use later on."
"But what happens if we hit a hill that's too big?" asked Martin. "What if we can only get partway up?"
"That will happen any time you try to get higher than you were originally," said Jennifer's mom. "Luckily none of the hills between the bus stop and here are higher than the bus stop itself, so you had enough potential energy to get over all of them."
There's a lot of vocabulary here. It's important to remember, so let's look at it again.
What does the story say about kinetic energy and potential energy?
The story tells us that kinetic energy is due to motion and that you get potential energy by going up a hill.
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17
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| "OK, I get it," said Martin, "but there's still one little thing I don't understand. Why can't we do this kind of thing in Normal World, where I come from?" |
"Ah, that's where the heat comes in!" said Jennifer's mom. "Remember, heat is energy, too. In Normal World there's a lot more friction than there is here, and friction is the force that turns motion into heat. So in Normal World instead of using all your kinetic energy to go up hills you lose a lot of it by making heat. Even if there are no hills you slow down because your motion is heating things up. That's why you have to keep peddling your bike instead of just coasting everywhere."
"It's even worse in Sticky World," said Emily. "There you really have to work to keep moving at all."
A thought popped into Martin's head. "They make a lot of heat there, too!" he said, excitedly. "Even rubbing your hands together practically makes them burn up!"
"That's because there's so much friction in Sticky World," said Jennifer's mom. "Try rubbing your hands together here."
Martin and Emily rubbed their hands together. Their hands felt slippery and no matter how hard they rubbed them they didn't warm up. It was a weird feeling.
"See?" said Jennifer, "No friction, no heat!"
"So how do you guys make fires?" asked Emily. "Do matches work in Slippery World?"
"Ordinary matches don't," said Jennifer, "but we have specially sensitive ones that light if you rub them on sandpaper. Luckily there is some friction here, otherwise we couldn't use any kind of match."
"And not only that," said Jennifer's mother. "If there were no friction at all nothing would ever stop moving. Imagine that!"
What would a world without friction be like?
Re-read the story for clues about a world without friction.
Without friction, there would be no heat, and everything would be in motion.
|
Chapter 9
|
18
|
|
"It's been a lot of fun," said Martin, "and I'm really going to miss
you, but..."
"I know," said Jennifer, "I'm sure you miss your parents, too." |
"It's not just that," said Emily. "There are a lot of cool things about a world with practically no friction, but ... I don't know..."
"It's just not the same." Martin said. "I mean, it's cool to be able to skate everywhere, and those slides in the park are great, but I'm really looking forward to having my dinner plate stay where I put it."
"You know what?" said Emily, as the bus came into view. "There's just no place like home."
What do the children miss about Normal World? What would you miss?
Think about Sticky World and Slippery World. Why would you want to leave them and return to Normal World?
I would want to get back to Normal World where I don't move too fast or too slow, and where I can rub my hands to get them warm (but not too warm!).
Frictional adventure
|
Discovery question
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1
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In this activity you will measure the temperature of a penny heating up by friction.
|
Engage
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2
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Think about how slippery or sticky your hands are.
Fill in the blanks in the answer above.
Choose the best answer.
Try adding your thoughts to the correct answer above.
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3
|
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Materials
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4
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- surface temperature (fast response) sensor
- 4 pennies
- wood block (optional)
- tape
- piece of cardboard (2 cm x 4 cm)
- waxed paper (or rub wax on a surface with a crayon)
|
Explore
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5
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1. Cut out a 2 x 4 cm rectangle of cardboard.
2. Make a loop of tape, so that it is sticky on both sides. Press it against the cardboard and stick the temperature sensor to it. Then press the tape down on the penny.
The cardboard will keep the penny from being heated by your fingers. Hold the cardboard by the edges when you press down on the penny.
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6
|
for
a technical hint.
4. Put the penny on a flat surface, like a desk. Use a block of wood if you don't have a good surface to rub on.
5. Start the sensor. Click on the magnifying glass
for
a movie showing how to use the data collection graph.
6. Let the sensor run for at least one minute. Wait until the penny is at a steady temperature. It should change less than 0.1 deg C in 5 seconds. If it is not steady, hit Stop and wait for a few minutes.
7. While you are waiting, look at the next two pages. Then come back to this page and hit Stop. Your graph will re-appear on later pages.
Have you ever taken a temperature at home? What did you use to take the temperature?
You need to find the temperature of the penny as you rub it on a block of wood. The temperature sensor allows you to do this.
Your graph should show the results of two parts of this experiment -- rubbing the penny slowly and rubbing it faster.
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7
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Now take a snapshot of your labeled graph.
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8
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9
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9. Expand the temperature scale so that it is only two or three degrees tall. For example, if the penny temperature is 22 deg C, make the temperature scale go from 21 deg C to 24 deg C. Then you will be able to see a small change.
10. Hold the penny down by pressing on the edges of the cardboard. Hold the cardboard by the edges so that your fingers don't affect the temperature of the penny.Push down hard so that the sensor and cardboard stay in place against the penny.
|
10
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Explain
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11
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1. Press the autoscale button to change the temperature scale so that you can easily see the change in temperature of the penny.
2. Label the part where you were rubbing slowly. Label the part where you were rubbing fast. Then take a snapshot of your graph.
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12
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|
Elaborate
|
12
|
Move the sensor and cardboard to another penny and make a new sandwich. Place a sheet of waxed paper on the desktop or wood surface. Start the sensor and note the starting temperature of the penny.
Rub the penny hard for 10 seconds directly on the desktop, then move it to the waxed paper and rub it hard for 10 seconds on the waxed paper, then stop. Look at the graph.
We have waxed paper in my kitchen at home. It feels kind of slippery.
Now you are testing the difference between surfaces. Pay attention to the graph when you move the penny from one surface to the other.
We can see the results of our experiments in the graph. Think about how the graph changed. Are you able to explain the change?
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14
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Which surface causes less friction?
When the penny and the wood are rubbed together there is _____ friction. When the penny and the waxed paper are rubbed together there is ____ friction.
Choose the best answer.
Try adding your thoughts to the correct answer above.
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15
|
The brakes on my bike help me to stop.
You can review the graphs you made to help you answer these questions.
You did an experiment involving rubbing a penny slowly and moving it quickly. Now you need to think about a car's brakes helping it to stop.
How can you get the most warming from friction?
|
Discovery question
|
1
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In this activity, you will explore a molecular model to see how friction creates heat.
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Engage
|
2
|
|
3
|
Using computer models is fun!
Pay attention as you zoom in at each level.
The box and slide look different at the microscopic level. These differences will help us explain friction.
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Even though they look very smooth, even the smoothest surfaces are really very rough!
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Here is a model of a solid. The balls represent molecules. Their color shows how fast they are moving. Red is faster, white is slower.
MW MODEL -- kinetic shading checked
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Predict what you think will happen to the atoms if the red object is dragged to the right.
The atoms will
when the orange object is dragged, because
.
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Why?
| I think it's | because I notice that |
Wow, we can see individual atoms!
You need to pay attention as the model is running to where the two surfaces meet. You can reset the model and run it again.
The model helps us to think about friction at the molecular level by showing us the little bumps and grooves. It takes work to move past those bumps.
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Explain
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Eduardo wasn't satisfied. "Now I can see why rubbing warms things up," he said, "but why does more friction mean harder work to move, like in Sticky World?"
Maria jumped into the conversation. "More friction means more heat, and more heat means more work!"
"That's right!" cried Uncle Hubert, obviously enjoying himself. "If two surfaces have lots of friction, the surface atoms heat up a great deal when they slide across each other. Making heat takes work, because heat is a form of energy and energy has to come from somewhere. Where does it come from?"
"It comes from my muscles pushing my foot along, right?" said Eduardo, who felt more comfortable now. "Yes," replied Uncle Hubert. "And what about a slippery surface?"
"There's no friction and sliding is easy! It doesn't take any energy and it doesn't heat up the surface," Maria replied.
Maria jumped into the conversation. "More friction means more heat, and more heat means more work!"
"That's right!" cried Uncle Hubert, obviously enjoying himself. "If two surfaces have lots of friction, the surface atoms heat up a great deal when they slide across each other. Making heat takes work, because heat is a form of energy and energy has to come from somewhere. Where does it come from?"
"It comes from my muscles pushing my foot along, right?" said Eduardo, who felt more comfortable now. "Yes," replied Uncle Hubert. "And what about a slippery surface?"
"There's no friction and sliding is easy! It doesn't take any energy and it doesn't heat up the surface," Maria replied.
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Elaborate
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Why does a surface heat up when you rub it against another surface?
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Discovery question
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In this activity, you will measure the amount of force needed to overcome the force of friction.
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Force is the amount of push or pull. To make something move, you must pull on it with enough force to overcome the force of friction.
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To start with, don't use the force sensor.
Make a hook on the box by bending a paper clip and poking it through the box. Use a pen to make a hole if you need to.
Put one water bottle in the box.
Pull very gently on the box. Then pull harder and harder until the box starts to move. Then pull the box slowly and steadily across the table.
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Now take a snapshot of your marked-up graph.
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Materials
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- force sensor
- cardboard box large enough to hold two water bottles
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two water bottles (about 1 liter or 1 quart) filled with water
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paper clip
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2. Hook the force sensor through the paper clip.
for
a technical hint. Be sure the force sensor is set to
10 N and not
50 N
.
4. Start recording with the force sensor by clicking on the start button. Click on the magnifying glass
for
a movie showing how to use the data collection graph.
5. Set the force to zero, using the zero force button. The graph should go to force = 0 when you do this.
6. Pull the box very gently with the force sensor. Gradually pull harder and harder until the box starts to slide. Keep pulling so that the box slides slowly at a constant speed.
7. Stop recording. If you want to try again, hit Clear and repeat the experiment. Take a snapshot of each run that you want to save.
We need to find the force required to push the box with one water bottle.
Sometimes I can't push a big object. It takes too much force.
Remember that on this graph, a pushing force is negative. Now, you can better interpret what you see.
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Record the average value of the moving friction force with one bottle in the table below.
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Explain
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Elaborate
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2. Select the dataset "two bottles" and repeat your experiment.
3. Stop recording and look at your graph. Be sure to label the new line you have made on your graph.
4. Record the average value of the moving friction force with two bottles.
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Divide the larger by the smaller. You can use a calculator. What is the exact ratio?
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How strong is the force of friction?
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Discovery question
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In this activity, you will observe the changing kinetic energy and heat energy for a sliding object.
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Engage
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In this picture of the airplane coming to a stop on the runway, smoke is pouring off the front wheel.
Cars, trucks, and railroad trains all use brakes to slow down. In order to stop, they must give up all of their energy of motion - the same energy it took to get them moving!
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The graph shows three values:
- The object's energy of motion ( kinetic energy ) is shown by a black line.
- The heat energy the object makes due to friction is shown by a red line.
- The sum of these two energies is shown by a blue line.
Describe the graph of kinetic energy shown by the black line.
I've pushed furniture across a carpet.
You should be thinking about both friction and heat as you watch the graph.
The graph displays a lot of information. Reread what each colored line represents to help you make sense of the relationship between heat and friction.
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Describe the graph of heat energy shown by the red line.
Time to test another surface!
The graph shows us important information. Interpreting it is your job!
We can find the greatest value for heat energy by looking at the graph. We can also see the time when the heat value was zero.
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Explain
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Elaborate
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Now choose carpet and push it many times, so that you get the box all the way across the screen.
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In this model, the loss of kinetic energy is exactly equal to the gain in heat energy. This is what friction is: the transformation of sliding energy into heat energy in the two surfaces.
When an object slows down, where does the energy go?
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Discovery question
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In this activity you will analyze data from friction experiments.
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Engage
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Suppose you give five students an object to weigh. They each weigh it, using different scales. Here are the results:
| 55 g | 52 g | 56 g | 60 g |
I've weighed myself at home. Sometimes I weigh produce at the supermarket.
Notice that five students get five different results.
The measurements are all lined up. What do you notice about the numbers?
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Here are actual measurements of the height that the ramp can be raised before the shoe starts sliding.
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Explain
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Elaborate
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| 25 kg | 23 kg | 24 kg | 27 kg |
When one measurement is very different from the others, one often discards it in calculating the average. It is called an outlier because it is out of range of the other values. For example, in this list of weight measurements, one of the values is so different that one might assume a mistake was made.
| 53 g | 52 g | 35 g | 56 g | 60 g |
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- Repeat the measurement more times.
- Several different people do the same measurement.
- Use different sets of measuring equipment.
- Make the same measurement at a different place and time.
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Think of a different way to measure the same value.
If a series of measurements vary, how do you decide on the best value?
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Coming soon: summarize what you learned in this unit.
| Teacher password | |
Wrapping up
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Press the Right Arrow below to go to the next page.
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Press the Right Arrow below to go to the next page.
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6) Box A had a mass of 100 kg and box B had a mass of 200 kg, but otherwise they were the same. For each box, three measurements were taken of the force needed to pull it along the floor.
Click the Submit button when you have finished.
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