Momentum: p = mv and conservation of momentum

Key ideas

1. p = m v: momentum (kg m/s) = mass (kg) × velocity (m/s).

2. In a closed system, total momentum before = total momentum after.

3. In a collision where objects stick together, add the momenta then divide by the total mass.

4. When objects push apart from rest, their momenta are equal and opposite, so the total stays zero.