In this paper, I describe an activity in which students measure the forces acting on a fan cart using two different methods. Students are first asked to visualize a fan cart moving against the force caused by the fan so that the fan cart slows down, stops, and accelerates back in the other direction. They are then challenged to find the force exerted because of the fan and the force of friction acting on the cart using the acceleration of the cart as it slows down and then as it speeds up. This experiment works well as an interactive classroom demonstration or a lab experience, reviewing many of the big ideas at the end of an introductory unit on Newton’s laws.

To get them started, we ask the students to imagine a fan cart being given a push toward the left (opposite the direction of the fan force) and then...

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