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### Penny heat

#### Discover question

Does friction create heat?

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In this activity you will measure how much a penny heats up when it is rubbed on a surface.

#### Engage

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1. Rub your hands together, first slowly and then quickly. You probably notice that your skin warms up when you rub your hands quickly. Why do you think this happens? ANSWER BOX

 Level 5: essay box only [Cynthia: It makes no sense to have an empty scaffolded field.  At the least, it should say something like "Re-read the question."] Level 4: Think about how slippery or sticky your hands are. Level 3: When two smooth surfaces rub together there is _____ friction. When two rough surfaces rub together there is ____ friction. (ignore italics) Level 2: When two rough surfaces rub together there is a lot of friction that results in ___________. Level 1: When one body in contact with another is moved, heat results due to friction. Explain in your own words why this occurs.

2. Hold a penny against your cheek to feel its temperature. Rub the penny on a hard surface, like a block of wood. Don't use a surface that could be damaged. Rub it in a circle twenty times. Then test the temperature of the penny again by holding it against your cheek.

You probably noticed that the penny heats up. If there were no friction, would the penny heat up? ANSWER BOX

 Level 5: essay box only [Cynthia -- see comments above.] Level 4: Think about what happens when two surfaces that have some resistance are rubbed together. Level 3: When two surfaces are rubbed together, think about why the objects heat up. Level 2: If there were no friction while rubbing a penny on a hard surface, would anything else create heat? Level 1: When there is no friction, the rubbing does not cause heat. Explain in your own words why this occurs.

3. Is it your finger that makes it hot, or the rubbing?
_ finger
_ rubbing
How can you tell? ANSWER BOX

4. Which of these things would make the penny heat up? You could choose more than one. (multiple selection question)
a. Rub it faster.
b. Add wax to make it more slippery.
c. Push down harder on the penny.

#### Materials

• surface temperature (fast response) sensor
• 4 pennies
• wood block
• tape
• rubber cement
• piece of cardboard (2 cm x 4 cm)
• waxed paper (substitute: rub wax on a surface with a crayon.)

#### Explore

1. Now let's test your choices to the above questions using a surface temperature sensor. Cut out a 2 x 4 cm piece of cardboard.

2. Coat one side of the cardboard and one side of 4 pennies with rubber cement.
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3. Make a sandwich with the sensor between the penny and the cardboard.  The sides coated with rubber cement should face each other (inside the sandwich). The rubber cement will provide friction between the penny and the cardboard.
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4. Put the penny on a block of wood. Hold the cardboard by the edges so that your fingers don't affect the temperature of the penny.

5. Start the sensor. Note the temperature of the penny before you start touching, rubbing or pushing the penny.

6. Hold the penny down by pressing on the edges of the cardboard. Have one student rub with the penny and another student count out seconds. Rub the penny slowly for 10 seconds, then stop for 5 seconds, and then faster for 10 seconds on the wood block. Try to push down with the same amount of force! Look at the graph.
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 TEMPERATURE DATA COLLECTOR GRAPH

7. Describe the graph. Which had more heating effect, rubbing slowly or rubbing fast? ANSWER BOX

8. Move the sensor and cardboard to another penny and make a new sandwich. Start the sensor and note the starting temperature of the penny.

 TEMPERATURE DATA COLLECTOR GRAPH

9. Push down lightly with the cardboard and rub the penny for 10 seconds, then stop for 5 seconds, then push down hard and rub it for 10 seconds, then stop. Look at the graph.

10. Which had more heating effect, pushing down lightly or pushing down hard? ANSWER BOX

#### Explain

Heat is a form of energy, so it takes energy to heat up a penny.
Where did the heat come from?

 scaffolding: friction between the surfaces.

Explain why the heating effect of rubbing fast would be different from the heating effect of rubbing slowly.

 scaffolding: rubbing fast takes more energy, so it produces more heat energy through friction between the surfaces.

Explain why the heating effect of pushing hard would be different from the heating effect of pushing lightly.

 scaffolding: pushing hard takes more energy and there is greater friction between the surfaces, so it produces more heat energy in the penny.

#### Elaborate

1. Move the sensor and cardboard to another penny and make a new sandwich. Place a sheet of waxed paper on a part of the wood. Start the sensor and note the starting temperature of the penny.
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2. Rub the penny for 10 seconds on the wood, then move it to the waxed paper and rub it for 10 seconds on the waxed paper, then stop. Look at the graph.

 TEMPERATURE DATA COLLECTOR GRAPH

3. Describe the graph. Which caused more heating, rubbing on the wood or rubbing on the waxed paper?
O wood
O waxed paper
Why do you think this was true?

 Level 5: essay box only [Cynthia: It makes no sense to have an empty scaffolded field.  At the least, it should say something like "Re-read the question."] Level 4: Think which surface is more slippery. Level 3: 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. Level 2: When two rough surfaces rub together there is a lot of friction that results in ___________. Level 1: The wax paper is smoother than the wood surface. Which one will cause the least amount of friction?

#### Evaluate

Automobile brakes heat up a lot when a car comes to a stop. The heat energy comes from the energy of motion of the car. Based on your experiments with a penny, answer these questions.
Stepping harder on the brakes makes the car stop faster. Would this cause more heating in the brakes?
o yes
o no