9709 · 4.3
Momentum
Momentum is simply 'mass in motion' and has a direction. In any interaction like a collision, the total amount of this 'mass in motion' before and after is the same, provided no external forces interfere.
Need to know
What you need to know
- Units: kg m s⁻¹.
- Vector Quantity: Direction is crucial. Always define a positive direction for one-dimensional problems.
- A stationary object ($v=0$) has zero momentum.
Explanation
The Rules of Crashing
- Momentum, $p = mv$, is a vector quantity. In a closed system with no external forces, total momentum is always conserved.
- Impulse is the change in momentum, $I = \Delta p = F \Delta t$. It's also the area under a force-time graph.
- In elastic collisions, both momentum and kinetic energy are conserved. In inelastic collisions, only momentum is conserved; some KE is lost.
- In explosions from rest, the system's initial momentum is zero. By conservation, the total momentum of all fragments must also be zero.