Momentum, Impulse, and Collisions
Momentum and Impulse
Momentum
Momentum is defined as . Because it depends on both mass and velocity, it is especially useful for describing collisions and recoil events.
Important relations
Which quantity is conserved in an isolated collision?
Total momentum is conserved when no external net force acts on the system.
Correct answer: Momentum
Why does increasing collision time reduce injury risk?
For a given change in momentum, spreading the impact over a longer time reduces average force.
Correct answer: It lowers the average force for the same impulse
Collision Types
Elastic vs inelastic collisions
Elastic
- Momentum conserved
- Kinetic energy conserved
Inelastic
- Momentum conserved
- Kinetic energy not fully conserved
Reality check
Most real collisions are partially inelastic because some kinetic energy becomes heat, sound, or deformation.
Practical applications
Airbags, crumple zones, and sports padding all increase collision time or distribute force over larger areas, reducing peak force on occupants or athletes.
What is the main purpose of a car crumple zone?
A crumple zone lengthens the time of impact, reducing peak force.
Correct answer: Decrease collision time
What does a large impulse indicate?
Impulse equals change in momentum, so large impulse means the momentum changes substantially.
Correct answer: A large change in momentum