# Relative motion

1. Nov 19, 2015

### rakshit1992

Suppose a fly/mosquito is flying along with a moving truck. If the fly is outside the truck, then the fly has to move forward to keep up with the truck. What if the same fly was inside the truck. Will it have to move forward inside the moving vehicle just so that it is seen at the same position at all times?

2. Nov 19, 2015

### Orodruin

Staff Emeritus
The only reason the fly needs to expend energy moving forward when it is outside the truck is that there is air resistance, in the truck rest frame the air outside the truck is moving and therefore resulting in a drag force on the fly. If the fly is inside the truck, the air is moving with the truck and there is no head wind. It is a matter of how the air moves relative to the truck, not a matter of the Earth rest frame being special.

3. Nov 19, 2015

### mathman

If the fly was inside the truck when the truck started moving, then the fly doesn't have to do anything when the truck is moving at a constant speed. The only thing that the fly needs to do is accelerate while the truck is speeding up.

This has nothing to do with the theory of relativity.

4. Nov 19, 2015

### Ibix

In either case, the fly will have to move at the same speed as the truck to stay in the same position relative to the truck. Inside the truck it won't have to work to do this, because it will be carried along by the air inside the truck which is being pushed along by the back door (loosely). Outside, it'll have to flap its wings a lot to keep up with the truck because the air outside the truck is not moving along with the truck, so won't be giving it a free ride.

I see Orodruin decided to take the exact opposite perspective - our answers are equivalent.