PHYSICS · Multi-part · A Level · Physics · Mechanics and materials · Mechanics and materials
Problem
A freight train of mass 2.0×105kg moves on a straight, horizontal track with a steady speed of 54km h−1. The locomotive supplies a horizontal pulling force while resistive forces due to air and track act in the opposite direction.
(i) Define dynamic equilibrium in the context of the motion of this train.
(ii) Draw a clearly labelled free-body diagram for the train and identify all horizontal forces acting on it.
(iii) Explain, without performing any numerical calculation, why the speed of the train remains constant even though several forces act on it simultaneously.
(iv) The driver suddenly closes the regulator so that the pulling force becomes zero while the resistive forces remain unchanged. State and explain qualitatively what will happen to the motion of the train immediately after this action, naming the physical law that governs the behaviour.