Get the Gizmo ready: Select Two sleds. Check that Show energy is on. Set the Red sled mass to 150 kg and its height to 20 m. Set the Blue sled mass to 100 kg and its height to 35 m.

Introduction: Burt the Yeti and his friend Karen are having a contest called a “sled war.” The winner is the sled that pushes the other sled backward when the two sleds collide.

Question: Who will win the sled war?

1. Predict: Burt’s red sled is a bit heavier than Karen’s blue sled, but she is higher on the slope.

What is the potential energy of each sled?	Red sled: Blue sled:
Which sled do you think will push the other back when they collide? Explain.

2. Test: Check that Sleds stick together is selected. Click Play. Click Pause after the two sleds collide with each other.

Which sled won the sled war? Did this surprise you?
After the collision, what is the kinetic energy of each sled?
What happened to the combined energy of the two sleds when they collided?
Because energy is conserved, the “lost” energy has actually been changed into other forms. Looking at the two sleds, what effect did some of this energy cause?

3. Calculate: Click Reset. The momentum (p) of an object is the product of its mass and speed: p = m • v. The units of momentum are kilograms-meters per second, or kg•m/s. Click Play, and then click Pause just before the collision.

A. What is the speed of the red sled?
B. What is the momentum of the red sled?
C. What is the speed of the blue sled?
D. What is the momentum of the blue sled?

4. For each combination of sled masses and starting heights below, click Play, and then click Pause just before the sleds collide. Record the speed and momentum of each sled. Then, click Play again and record the winning sled.

Trial	1	2	3
Red sled height	50.0 m	25.0 m	5.0 m
Red sled mass	20 kg	120 kg	100 kg
Red sled speed
Red sled momentum
Blue sled height	10.0 m	20.0 m	45.0 m
Blue sled mass	100 kg	130 kg	40 kg
Blue sled speed
Blue sled momentum
Winning sled

In each trial, circle the greater momentum value. What is true about the momentum of the winning sled in each contest?
If you knew each sled’s mass and speed, how could you determine which sled will be pushed backward in a collision?

5. Explore: Click Reset. Select Sleds bounce. In this collision, large springs cause the sleds to bounce off each other. Using the same settings from the last trial above, click Play.

What do you see?

6. Extension: Click Reset. Without changing any of the sled masses or heights, record the potential and kinetic energy of each sled and find the total energy before the collision. Click Play and then Pause after the collision. Record the potential and kinetic energy of each sled after the collision, and find the total energy.

	Red PE	Red KE	Blue PE	Blue KE	Total energy
Before collision
After collision

Is energy conserved in this type of collision? Explain.