PHYSICS · Single correct · A Level · Physics · Mechanics and materials · Mechanics and materials
Problem
Which of the following best explains why, in a perfectly inelastic collision between two frictionless carts, momentum is conserved while the system’s kinetic energy decreases?
(a) If friction is absent, both momentum and kinetic energy must be conserved; any kinetic-energy loss indicates an unaccounted external force.
(b) Momentum is conserved because the combined mass after sticking offsets the reduced speed, whereas kinetic energy falls since it is proportional to the square of that speed.
(c) Momentum is a vector quantity conserved by Newton’s third-law internal forces, whereas kinetic energy can transform into internal energy such as deformation or sound.
(d) Momentum is conserved because it is a scalar that cannot cancel out, but kinetic energy decreases because it is a vector whose components can cancel during the collision.