Question: If you threw a stone down a bus inside in a straight line, from the back of the bus, while the bus was turning right, would the stone hit the front of the bus or the left wall of the bus?

Ian Stephenson answered on 19 Jun 2014:

Supprised Edward hasn’t tackled this one, as its a physics question with some REALLY deep implications.

If the bus is driving at a constant velocity (speed and direction), then you don’t notice it. However if it turns, or accelerates (changes velocity), then you DO notice. It turns out this is tied up in Relativity (that Einstein stuff!).

However the simple thing to remember is that to make something CHANGE velocity you need a force. When the bus is driving forwards at constant speed then the net forces are zero. Your hand is moving at constant velocity, applying no force to the rock, so it stay at constant velocity, and everything appears to be staying still (to you sat on the bus).

When the bus turns, its technically accelerating (changing velocity). That means there must be a force. That force is the wheels gripping the road. You feel that force as you grab the handrail, you have too pull to say upright, which is applying a force to your body to match the bus.

You throw the rock by applying a force to it, but once its left your hand, the only force on it is gravity pulling it down, so it will keep going with constant horizontal velocity. One the rock has left your hand, if the bus turns theres no way the force gets from the bus to the rock, so the rock keeps going in a straight line, while the bus turns.