A fan cart with the fan set to High rolled across a floor. The cart's speeds are shown below. If the fan were set to Medium instead, what could the cart's speed be at 5 seconds? A. 10.9 cm per second
B. 31.0 cm per second
C. 40.1 cm per second
D. 48.3 cm per second
Correct A - 10.9 cm per second
Explanation: The fan supplies a force to the cart. If a lower fan speed were used, less force would be applied. This would cause a slower change in the cart's speed. So, the cart would be rolling more slowly than 31.0 cm per second after 5 seconds. The only choice that is less than 31.0 cm per second is 10.9 cm per second.

Force is needed
A. for a moving object to keep moving at the same speed and direction
B. for a moving object to change its speed
C. for a motionless object to remain still
D. to prevent a moving object from turning
Correct B - for a moving object to change its speed
Explanation: An object will change its speed only if a force acts on it. In the Gizmo, forces can speed the cart up (fan) or slow the cart down (friction). Answer choice A is incorrect because a moving object will keep moving at the same speed if there is no force acting on it (like the cart after the fan was turned off). Answer choice C is incorrect because no force is required for a motionless object to remain at rest. Answer choice D is not correct because a moving object will naturally move in a straight line if there is no force.

Rebecca puts her backpack on a fan cart and turns the fan on. She then measures the speed each second afterward. At 10 seconds the cart has a speed of 20 cm per second. How fast will the cart be moving at 12 seconds?
A. 18 cm per second
B. 20 cm per second
C. 22 cm per secnod
D. 24 cm per second
Correct D - 24 cm per second
Explanation: The cart started at 0 cm per second. Ten seconds later it was moving 20 cm per second. Therefore, every second the cart is gaining 2 cm per second. So, after 11 seconds the speed will be 22 cm per second. After 12 seconds the speed will be 24 cm per second.

A fan cart is moving forward on a frictionless surface. If you change the fan speed from High to Low, what will occur?
A. The cart will stop moving.
B. The cart will slow down.
C. The cart will keep moving at the same speed.
D. The cart will continue to speed up, but not as quickly as before.
Correct D - The cart will continue to speed up, but not as quickly as before.
Explanation: The cart will continue to speed up because there is still force acting on it. However, less force is acting on it now because the fan speed is lower.

The speed-versus-time bar graph for a fan cart experiment is shown below. What could have happened?
A. The track was frictionless, and the fan was changed from High to Low after 8 seconds.
B. The track was frictionless, and the fan was changed from High to Off after 8 seconds.
C. The track was wooden, and the fan was changed from High to Off after 8 seconds.
D. The track was wooden, and the fan was changed from Off to Low after 8 seconds.
Correct B - The track was frictionless, and the fan was changed from High to Off after 8 seconds.
Explanation: The bar graph shows that the speed stayed the same after 8 seconds. This means there is no total force on the cart. This matches the situation in which the fan is off and the surface is frictionless.