Physics · Mathematical methods and mechanics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Newton's second law says the net force on a body equals its rate of change of momentum, which is F = ma for constant mass. Always draw the forces on one body first. Third-law pairs are equal and opposite but act on different bodies, so they never cancel each other.
Friction acts along the surface and opposes sliding. A body moving in a circle needs a net force towards the centre; this centripetal force is supplied by tension, friction, gravity or a normal force.
Work done by the net force equals the change in kinetic energy. Total energy is always conserved. Momentum is conserved in every collision if no external force acts, but kinetic energy is conserved only in elastic ones.
Assuming kinetic energy is conserved in every collision. Use conservation of momentum first; use conservation of kinetic energy only if the collision is stated to be elastic.
Answer: 6 m s−1; 24 J of kinetic energy is lost.
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