Example 1: Two cars collide and stick together (inelastic)
Car B: mass = 1500 kg, velocity = 0 m/s (stationary)
Momentum before = (1000 × 20) + (1500 × 0) = 20,000 kg m/s
Momentum after = (1000 + 1500) × v = 2500v
2500v = 20,000
v = 8 m/s (combined, moving in original direction)
Example 2: Explosion (person jumps off skateboard)
Person (60 kg) jumps left at 3 m/s
Momentum of person = 60 × (-3) = -180 kg m/s
Momentum of skateboard = +180 kg m/s
Skateboard (5 kg) → v = 180/5 = 36 m/s right
Quick Check: A 3 kg ball moves at 4 m/s and collides with a stationary 5 kg ball. They stick together. What is their combined velocity?
Momentum before = 3 × 4 = 12 kg m/s. Momentum after = (3 + 5) × v = 8v. 8v = 12, so v = 1.5 m/s in the original direction. Total momentum is conserved.
This key facts covers Conservation of Momentum — Worked Examples within Momentum for GCSE Physics. Revise Momentum in Forces for GCSE Physics with 15 exam-style questions and 6 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 4 of 12 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Momentum
What is the equation for momentum?
Explain what is meant by saying momentum is a vector quantity.