AS Daily A Level Physics question
In a lab, a 0.80 kg trolley moving at 1.5 m/s on a near-frictionless track collides with a 1.2 kg trolley at rest and they stick together. Immediately after impact, what is the speed of the joined trolleys?
Answer
The correct answer is D.
Correct: D — 0.60 m/s. Total momentum before is 0.80 × 1.5 = 1.2 kg m/s; after they couple the total mass is 2.0 kg, so speed = 1.2 / 2.0 = 0.60 m/s. A assumes the speed is unchanged, ignoring that the same momentum is shared by a larger mass. B is the simple average of the two initial speeds, which does not conserve momentum. C comes from wrongly assuming kinetic energy is conserved in a sticking (inelastic) collision; KE is not conserved here. D uses conservation of momentum correctly for a 1D inelastic collision.