運動量

Hard to stop

• An empty shopping trolley is easy to stop. A loaded truck at the same speed is not.
• The truck has much more momentum 动量.
• Momentum links mass, velocity, force and collisions 碰撞 — and it is always conserved.

Momentum

• Momentum is mass times velocity:

• It is a vector — it has a direction (the direction of motion).
• The unit is the kilogram metre per second ($\text{kg}\,\text{m/s}$).

The water an irregular solid pushes up gives its volume

Force changes momentum

• A resultant force changes momentum. The force is the change in momentum per second:

• So a given change in momentum over a longer time needs a smaller force.
• This is why crumple zones, airbags and crash mats help: they make the stop take longer, so the force is gentler.

Conservation of momentum 动量守恒

• In a collision with no outside force, the total momentum before = total momentum after:

• Here $u$ is a velocity before and $v$ a velocity after.
• Momentum lost by one object is gained by the other.

You've got it