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Why does a surface heat up when you rub it against another surface?

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Explore a molecular model to see how friction creates heat.


Linker Coach (engage & motivate) Why? Affective

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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."


Here is a box on a slide [picture]. Now zoom in on the surface by pushing the ZOOM button.

insert zoom flash, ending with MW model.

Here is the model of two surfaces that Uncle Hubert showed Maria and Eduardo. Things are so enlarged that you can see individual atoms. The green ones are part of the top object that is moving. The white (black?) ones are part of the surface. Gravity is pushing the green object down on the surface.


Predict what you think will happen to the atoms if the green object is dragged to the right.
The molecules will ___________  when the green object is dragged, because __________


Run the model. Observe and describe what happens to the atoms.  Look very carefully at the ones where the two surfaces meet.
[Make the model stop automatically before the block reaches the end of the frame]

Would you say this is a slippery surface or a sticky surface?

It think it's _______ because I notice that ______________.

Reset the model and check the "KE shading" box. This feature colors the atoms according to how fast they are moving. Red is faster, and white is slower.
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What happens to the color of the atoms in the object as it moves across the surface?

Run it again. This time, watch the color of the atoms and the temperature graph. What does the temperature do?
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a. It goes up.
b. It goes down.
c. It stays the same.


Temperature is a measure of how fast the atoms are moving. Higher temperature means faster atoms. Why do you think the moving box causes the temperature to change?

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.


In your own words, explain why rough surfaces with lots of friction heat up when you rub them. Use the words atoms, temperature, and surface.

In your own words, explain why smooth surfaces with very little friction don't heat up when you rub them. Use the words atoms, temperature, and surface.


Heat energy is wiggling of molecules. The faster they wiggle, the hotter the material. When two surfaces have a lot of friction, the molecules on the surfaces get bumped around a lot when they are rubbed together.
Explain how you think the rubbing of two surfaces with a lot of friction could could create heat energy in the two materials.

When the two surfaces are forced to scrape each other by the rubbing motion, the molecules get bumped around and wiggle faster, which is the same as increasing their heat energy. So they get hotter. If your rub faster or push harder, they get bumped around more and also heat up faster.

If there is very little friction between surfaces, the molecules slide by each other without interacting very much. Explain why they do not heat up when rubbed.

If they don't interact very much, the rubbing doesn't make them wiggle more, so the materials don't heat up.
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