Momentum
Momentum (Higher tier only)
Every moving object has . It is a , so its direction matters. You must recall this equation:
momentum = mass × velocity, or p = mv
| Quantity | Symbol | Unit |
|---|---|---|
| Momentum | p | kilogram metres per second (kg m/s) |
| Mass | m | kilograms (kg) |
| Velocity | v | metres per second (m/s) |
Example: a 70 kg runner moves at 3 m/s.
- p = mv = 70 × 3
- p = 210 kg m/s
Common mistake
Momentum has a direction. If one object moves left and another right, give one of them a negative velocity.
Key terms
Conservation of momentum (Higher tier only)
In a , the an event equals the total momentum after it. This is the .
Example: a 2 kg trolley moving at 3 m/s hits a stationary 1 kg trolley. They stick together. How fast do they move?
- Momentum before = (2 × 3) + (1 × 0) = 6 kg m/s
- Momentum after = 6 kg m/s, with a total mass of 3 kg
- v = 6 ÷ 3 = 2 m/s
In an explosion, the total momentum before is zero, so afterwards the pieces have equal and opposite momentum. That's why a gun recoils backwards when it fires.
Key terms
Force and change in momentum (Higher tier only) (Separate Physics only)
A force changes an object's momentum. The force equals the . This equation is on the equation sheet:
F = mΔv/Δt
| Quantity | Symbol | Unit |
|---|---|---|
| Force | F | newtons (N) |
| Mass | m | kilograms (kg) |
| Change in velocity | Δv | m/s |
| Time taken | Δt | seconds (s) |
Example: in a crash, a 60 kg passenger stops from 15 m/s. Compare stopping in 0.1 s and in 0.5 s.
- In 0.1 s: F = 60 × 15 ÷ 0.1 = 9000 N
- In 0.5 s: F = 60 × 15 ÷ 0.5 = 1800 N
Taking longer to stop means a much .
Key terms
Safety features (Higher tier only) (Separate Physics only)
Many safety features work by increasing the time it takes for a person's momentum to change. This lowers the , so the on the person is smaller and injuries are less serious.
- Seat belts stretch a little, so the passenger slows down over a longer time.
- Airbags inflate and then squash as the passenger hits them, slowing their head and chest gradually.
- Crumple zones at the front and back of a car crush in a crash, making the collision last longer.
- Cycle helmets, gym mats and soft playground surfaces squash on impact for the same reason.