AS Daily A Level Physics question
Two carts, A and B, start from rest on a smooth horizontal track. The same motor pulls each cart with the same constant horizontal force for 4.0 s. Cart B has 50% greater mass than cart A. At 4.0 s, cart A is moving at 3.0 m/s. What is the speed of cart B?
Answer
The correct answer is B.
Correct: B — 2.0 m/s. With the same constant pull for the same time from rest, speed after 4.0 s is inversely proportional to mass, so v_B = v_A × (m_A/m_B) = 3.0 × (1/1.5) = 2.0 m/s. A says 3.0 m/s, which ignores that a heavier cart accelerates less under the same force, so it cannot reach the same speed in the same time. B is correct because a 50% increase in mass reduces the speed by a factor of 1.5, not by 50%. C gives 4.5 m/s, mistakenly implying a heavier cart speeds up more under the same force or confusing force with power. D gives 1.5 m/s, a common error of subtracting 50% rather than using the inverse ratio 1/1.5.