AS Daily A Level Physics question
On a level warehouse floor, a 30 kg crate is pulled with a steady horizontal force of 60 N and moves at constant speed. A worker quickly loads an extra 25% mass onto the crate, but the pulling force remains 60 N. Frictional resistance is proportional to the normal force and independent of speed. What is the horizontal acceleration of the loaded crate immediately after the load is added?
Answer
The correct answer is C.
Correct: C — -0.40 m/s^2. Initially friction equals 60 N (constant speed). With 25% more mass, friction rises by 25% to 75 N, so net force = 60 − 75 = −15 N and the new mass is 30 × 1.25 = 37.5 kg, giving a = −15/37.5 = −0.40 m/s^2. A uses the original 30 kg when calculating a = −15/30, giving −0.50 m/s^2. B assumes constant pulling force means constant speed, ignoring that friction increased with weight. C is correct as shown by the net-force and mass change calculation. D has the right magnitude but wrong sign; friction now exceeds the pull so the acceleration is backward, not forward.