SAT Subject Physics Practice Question 193: Answer and Explanation
39. A box of mass 40 kg is pushed in a straight line across a horizontal floor by an 80 N force. If the force of kinetic friction acting on the box has a magnitude of 60 N, what is the acceleration of the box?
A. 0.25 m/s2
B. 0.5 m/s2
C. 1.0 m/s2
D. 2.0 m/s2
E. 3.5 m/s2
Correct Answer: B
The net force acting on the block is 80 N – 60 N = 20 N. Dividing the net force by the object’s mass gives the acceleration (Newton’s second law), so we find that a = Fnet/m = (20 N)/(40 kg) = 0.5 m/s2.