Why passengers lurch
The place the first law is felt most clearly in everyday life is on a bus.
When the brakes go on
Imagine a passenger standing inside a moving bus, holding on to nothing at all. If the brakes are suddenly applied and the bus is stopped, they fall forwards.
Why
Their feet were in contact with the bus, so the bus applies a force to the feet and brings them to rest.
But no such force is applied to the upper part of the body, so by the first law it carries on forwards at the velocity it already had. That is why the passenger falls forwards.
When it pulls away
Now imagine that same passenger inside a bus that is standing still. If the bus is started without their noticing and pulls away, this time the passenger falls backwards.
Why
As the motion begins, the bus applies a force to the feet that are in contact with it, so the lower part of the body gains a velocity.
But the upper part of the body is still at rest. The lower half moves off while the upper half stays put, so the passenger is pushed backwards.
| What the bus does | The upper body | Passenger falls |
|---|---|---|
| Moving fast, then stops | Keeps its velocity | Forwards |
| At rest, then pulls away | Keeps being at rest | Backwards |
What this shows
In both cases the upper body is doing the same one thing — carrying on exactly as it was.