Physics · Motion and force. 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 first law says a body keeps its state of rest or uniform velocity unless a net force acts (inertia). The second law says a net force gives an acceleration in its own direction. The third law says forces come in equal and opposite pairs that act on two different bodies.
Momentum is mass × velocity. Force is the rate of change of momentum, and impulse (force × time) equals the change in momentum. In an isolated system the total momentum stays constant, which explains recoil, collisions and rocket motion.
In an elastic collision kinetic energy is also conserved. In an inelastic collision momentum is conserved but some kinetic energy is changed to other forms.
Students ignore direction when a body rebounds. Momentum is a vector, so a ball that comes back has a velocity of opposite sign and the change in momentum is m(v + u), not m(v − u).
Answer: The 2.0 kg ball moves on at 2.0 m s−1 and the 1.0 kg ball moves at 8.0 m s−1, both in the original direction
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