A student pushes a crate with a constant horizontal force of so that it moves across…
PHYSICS · Single correct · AP · Physics 1: Algebra-based · Force and Translational Dynamics · Force and Translational Dynamics
Problem
A student pushes a 2kg crate with a constant horizontal force of 15N so that it moves across a level floor at constant velocity. In the free-body diagram for the crate, the arrow representing kinetic friction is drawn the same length as the arrow representing the student’s push. Which statement best justifies this choice?
(a) Because Newton’s third law states that every force has an equal and opposite reaction, the force of friction must equal the student’s push.
(b) Equal-length arrows are required whenever two forces act in opposite directions on a free-body diagram.
(c) The forces must be equal in magnitude because the crate’s acceleration is zero.
(d) The normal force equals the weight, so the horizontal forces must also be equal.