What causes relative stillness?

Question

I learnt this relative stillness theory from "general relativity" 10 yrs ago in China, however the teacher never explained what causes this.

Using earth as "frame of reference", a fly in a moving bus acquires the same speed the bus does, because, the teacher said "relative stillness", yeah, we all know a fly in a moving bus is said to be still within the "bus system". Like passengers in this system, they wouldn't crash their heads to the rear wall just because the bus is moving ahead.

All human beings on earth automatically acquire the same earth rotating speed as the earth does.

But please, tell me why, why in the world do we and any other substances in the universe have this sort of parasiting ability to inherit speed from speed initiator???

The failure of Chinese education is it never explains why.

Answer 1

If you would stand above the bus, or jump just when it leaves, it would pass right under you.

If the bus is driving at a constant speed and you jump, you would land on the exact same spot on the bus, because you already had its speed.

Lets say you're standing on the bus, the bus is driving and suddenly stops. You fly forwards unless you hold on.

You inherit speed by friction with the object. Your feet on the earth make you move along, your body in the seat makes you move with the bus.

If you sit in the bus you have more friction (contact area) and its easy to move along the bus. Yet when you're standing, as you might have experienced, its quite hard to move along when the bus suddenly stops!

Answer 2

Well, people take on the same rotational velocity of the earth because of gravity and friction.  If you were to place a person on a large rotating sphere where gravity was negligible, he would of course be hurled off.

To be a 'parasite', in your terms, you simply must be moving at the same velocity or accelerating along with the 'host' body.  The two ways to do that in an accelerating frame of reference are to be physically coupled (be inside or attached in some way) to the body, or be force-coupled (gravitationally or electromagnetic) to an accelerating body.

Answer 3

It comes from Newton's first law

"A particle will stay at rest or continue at a constant velocity unless acted upon by an external unbalanced force."

Two objects which are moving in the same direction at the same speed will continue to move together unless a force acts on one or both of them.

When you are in the bus, and it is accelerating away from the stop, your body is acted on by a force from the seat, pushing you forwards. Once the bus is moving at a constant speed, your body is also moving forward at the same speed, so you experience no force or motion inside the bus. The bus and passengers (and flies) are in the same "inertial frame of reference".

Maybe they did not teach it because Newton was not Chinese!

Edit:

> what about the fly? it doesn't contact anything in the bus ...

Yes it does, the air inside the bus.

A fly has a large surface area compared to its weight, so it is greatly affected by air currents. There is a volume of air inside the bus which moves along with it, and the fly gets carried along with the air. When the bus accelerates it exerts a force on the air inside it, and the air in turn exerts a force on the fly.

An analogy is a goldfish in a tank. When you move the tank, the water moves with it, and the goldfish moves with the water.