Bob is pushing a box across the floor at a constant speed of 1.7 m/s, applying a
ID: 2232038 • Letter: B
Question
Bob is pushing a box across the floor at a constant speed of 1.7 m/s, applying a horizontal force whose magnitude is 70 N. Alice is pushing an identical box across the floor at a constant speed of 3.4 m/s, applying a horizontal force. What is the magnitude of the force that Alice is applying to the box? F = N With the two boxes starting from rest, explain qualitatively what Alice and Bob did to get their boxes moving at different constant speeds. In order to keep the box moving twice as fast, Alice had to apply a constant force that was twice as large as the force that Bob applied. Each initially applied a force bigger than static friction to get the box moving and accelerating, then when the desired final speed was achieved they reduced the force to make the net force zero.Explanation / Answer
since, the two boxes are identical => they will be facing the same frictional force Hence, the forces on the box are : friction and force applied by the person for constant velocity (no acceleration motion), net force = 0 => in both cases same force will be applied to balance the friction, as the friction is same. SO, Alice is applying 70 N only. statement b is correct. The reason for above solution is the reason for statement a to be wrong, as force of double magnitude means acceleration of double magnitude, not velocity. And, for the motion to start, the initial force has to exceed static friction, and further for constant motion, force has to decrease to be equal to the static friction.