Chapter 4, Problem 19P

* EST Finn and Hazel are using a battery-powered fan fixed on a low-friction cart. As the fan blades rotate, they exert a force on the air and therefore the air exerts an equal and opposite force on the blades, causing the cart to move at constant acceleration. They decide to investigate how the acceleration of the cart depends on the tilt of the fan. They conduct two experiments one with the fan oriented horizontally (see Figure P4.19a) and one by tilting the axis of the fan by a certain angle (Figure P4.19b). In both experiments, they hold the cart still, turn on the fan, then let the cart move for some time before stopping the cart. During the experiment they measure the acceleration of the cart using a motion detector. The graphs in Figure P4.19c show their measurements.

(a) Which graph (A or B) belongs to the cart with the horizontal fan and which to the cart with the tilted fan? Explain. (b) Based on the graph, estimate the angle between the axis of the fan and the horizontal in the second experiment Indicate any assumptions that you made. Check how your result compares with the angle on the photo (Figure P4.19c) of the actual experimental setup (you will need a protractor). Indicate any assumptions that you made.