5.21 - Momentum 2

Overview

This lecture explains how forces change an object's momentum and discusses practical applications, especially in vehicle safety systems.

Momentum (p) is calculated as mass × velocity (p = m × v).
To increase momentum, a force must be applied in the direction of motion, raising the object’s velocity.
The force required to change momentum can be calculated with: Force = change in momentum / time taken.
Change in momentum = final momentum − initial momentum.

For a 2 kg object accelerating from 5 m/s to 8 m/s in 0.5 s:
- Initial momentum: 2 kg × 5 m/s = 10 kg·m/s
- Final momentum: 2 kg × 8 m/s = 16 kg·m/s
- Change: 16 − 10 = 6 kg·m/s
- Force: 6 kg·m/s ÷ 0.5 s = 12 N
For a 0.6 kg ball accelerating from 0 to 14 m/s in 0.1 s:
- Initial momentum: 0
- Final momentum: 0.6 kg × 14 m/s = 8.4 kg·m/s
- Force: 8.4 kg·m/s ÷ 0.1 s = 84 N

During a car crash, a sudden stop causes a rapid loss of momentum, resulting in large forces on passengers.
Spreading the momentum change over a longer time reduces the experienced force and risk of injury.
Safety features in cars:
- Crumple zones increase the time to stop by deforming on impact.
- Seat belts restrain passengers and stretch slightly to slow deceleration.
- Airbags cushion impact, prolonging the momentum change and lowering the force.
Other safety devices like helmets and crash mats work similarly by increasing time for loss of momentum.

Momentum (p) — the product of an object's mass and velocity (p = m × v).
Force (F) — a push or pull that changes momentum; calculated as change in momentum divided by time.
Crumple Zone — sections of a car designed to deform upon impact and increase stopping time.
Airbag — a safety device that inflates to slow down the passenger’s impact.