### Halt!

#### Discovery question

When an object slows down, where does the energy go?

Cannot resolve external resource into attachment.
Observe the changing kinetic energy and heat energy for a sliding object.

#### Engage

In the picture of the airplane coming to a stop on the runway, smoke is pouring off the front wheel. Why? Where is all the heat coming from?

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! How do brakes use up that energy?

#### Explore

Here is a model of a surface with different amounts of friction. The "Push it" button gives the object a shove that starts it with a fixed speed. You can choose different amounts of friction – steel, wood, or carpet.

The graph show three values:

• the object's energy of motion ( kinetic energy ) with a black line
• the heat energy the object makes due to friction with a red line
• the sum of these two energies with a blue line

Run the model with steel.

 PhET MODEL INSERTED HERE, with just 3 choices: steel, wood, carpet
 here's what they will see: Cannot resolve external resource into attachment.

Describe the graph of kinetic energy (black line).
What did it do as the object slowed down?
When was its value greatest?
When was its value zero?

Describe the graph of heat energy (red line). What did it do as the object slowed down?
When was its value greatest?
When was its value zero?

#### Explain

You probably observed that the kinetic energy graph goes down and the heat energy graph goes up as the object slows down. Why do you think this happens?

#### Elaborate

page 1
Now run the model with wood, which has more friction than carpet.

 PhET MODEL INSERTED HERE, with just 3 choices: steel, wood, carpet

How is the graph the same as for steel? How is it different?

page 2
Now run the model with carpet, which has more friction than wood.

 PhET MODEL INSERTED HERE, with just 3 choices: steel, wood, carpet

How is the graph the same as for wood? How is it different?

#### Evaluate

If you added the kinetic energy and the heat energy together, what would be the result?

a. The sum would get smaller.
b. The sum would get larger.
c. The sum would be constant.

Why?

 Document generated by Confluence on Jan 27, 2014 16:49