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?