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Your lab group has a fan cart that is able to roll on an aluminum track. You turn on the fan and observe that as the air blows to the left, the cart accelerates to the right.

The next step is to put a plastic shield on the back of the fan cart so that the air catches it. There are many different ideas of what may happen when the fan is turned on with the shield in place. These include that the shield will act as a sail so the cart will accelerate to the left and the cart will continue to accelerate to the right, but slower because of the increased total mass. You state that the cart will stay at rest.

Which of the following explanations is correct so that you can convince your lab partners that the cart remains at rest?

A

The fan air will hit the shield as an action force. The shield pushes the air the opposite direction as a reaction force. These forces are equal and opposite according to Newton's Third Law. Now apply Newton's First Law. These equal and opposite forces cancel out and the net force is zero. The cart remains at rest.

B

An internal force cannot change the momentum or velocity of a system. The fan, cart, and shield are part of the same system so that the force acting on the shield will not change the motion. Forces acting on the cart are the force of the air hitting the shield (pushing left) and the force of the air pushing back on the fan (pushing right). These forces are equal and opposite, the net force is zero. The cart stays at rest.

C

Adding the shield to the fan cart adds so much mass that the force of the fan is unable to accelerate the cart. The small force from the air cannot move such a large object. The cart stays at rest.

D

When the shield is attached to the cart there is additional friction between the wheels and the track and between the wheels and the axles because of the increased normal force. This force acts opposite to this motion because friction always opposes the force. Until the cart starts moving, you must consider static friction which is larger than kinetic friction. If the frictional force and the applied force of the air pushing the fan and cart forward are equal and opposite, the forces are not balanced and the cart will not move.

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