Newton’s Three Laws of Motion, Explained With Everyday Examples
Newton’s first, second and third laws of motion explained simply, with everyday examples, the F = ma formula, worked problems and common misconceptions.
Momentum Explained With Examples
Momentum explained simply: p = mv, conservation of momentum in collisions and explosions, impulse and why crumple zones and airbags work, with worked examples.
Key takeaways
- Momentum is mass times velocity: p = mv, measured in kg·m/s.
- In a closed system, total momentum before a collision equals total momentum after.
- Impulse (force × time) changes momentum, which is why longer impact times reduce injury.
On this page
Momentum describes how hard it is to stop a moving object. A slow truck and a fast baseball can both be hard to stop, for different reasons.
The formula
p = m × v
- p = momentum (kg·m/s)
- m = mass (kg)
- v = velocity (m/s), including direction
Because velocity has direction, momentum does too: it’s a vector.
Conservation of momentum
In a closed system with no external forces, total momentum stays the same. This lets you predict what happens in collisions and explosions.
Worked example: collision
A 1,000 kg car at 10 m/s hits a stationary 1,500 kg car and they move together. Before: 1,000 × 10 = 10,000 kg·m/s. After: (1,000 + 1,500) × v = 10,000, so v = 4 m/s.
Worked example: recoil
When a skater throws a heavy ball forwards, they move backwards: the momenta are equal and opposite. This is Newton’s third law in action; see Newton’s laws of motion.
Impulse
Impulse = force × time = change in momentum. For the same change in momentum, a longer impact time means a smaller force. That’s why:
- Crumple zones and airbags reduce injuries
- You bend your knees when you land a jump
- Catching a ball means moving your hands back with it
Elastic and inelastic collisions
- Elastic: kinetic energy is conserved (close to true for billiard balls).
- Inelastic: some kinetic energy becomes heat, sound or deformation (car crashes).
Momentum is conserved in both. See kinetic vs potential energy for the energy side.
Frequently asked questions
What is the unit of momentum?
Kilogram metres per second (kg·m/s), equivalent to newton-seconds (N·s).
Is momentum the same as inertia?
No. Inertia is resistance to changes in motion (related to mass); momentum depends on both mass and velocity.
Is momentum always conserved?
In a closed system with no external forces, yes.
Sources
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