A shallow cart of negligible friction rests at the origin on a horizontal track
PHYSICS · Multi-part · AP · Physics 1: Algebra-based · Force and Translational Dynamics · Force and Translational Dynamics
Problem
A shallow cart of negligible friction rests at the origin on a horizontal track. Beginning at t=0, an overhead hopper releases dry sand that falls vertically (zero horizontal speed relative to Earth) into the cart at a constant rate of 0.40kg/s for the next 5s. At the instant just before the sand begins to fall, the cart and the small amount of sand already inside it have a combined mass of 8.0kg and zero horizontal velocity. No external horizontal forces act on the cart--sand system during the 5s interval.
(a) A student chooses to model the system as only the cart (not the incoming sand). Describe the horizontal motion the student will predict for the cart during the 5s interval, and state the physical reasoning that leads to this prediction.
(b) A second student chooses to model the system as the cart plus all sand currently in the cart (including sand that lands during the interval). Describe how this student’s prediction for the cart’s horizontal motion differs from the first student’s prediction, and explain the reasoning.
(c) Without performing any calculations, indicate which student’s prediction agrees with the actual motion of the cart observed in the laboratory. Justify your selection by referring specifically to the behavior of the center of mass and the presence or absence of external forces.