A car is travelling in one direction at 80 km/h and a fly is coming from the opposite direction at 5km/h. (So its velocity is -5 km/h, since it's in the opposite direction.)

The fly hits the windshield of the car and is now travelling at 80km/h. In order for the fly to achieve the speed of 80km/h it's speed must have passed from -5km/h to 80km/h. (Meaning it must pass through the speed of zero) Therefore if the fly passes through a speed of 0km/h and the car is in contact with the fly the car must also pass through that speed of 0km/h.

This seems to mean that everytime a car gets hit by a fly it will completely stop. Why is this not so?

(In reply to Not a solution by E Woodford)

Are you confusing velocity (direction specific) with speed (direction non-specific)? If we assume that the car is travelling North (at a speed of 80km/h), then the fly is travelling South (at a speed of 5km/h). Considering the northerly velocities of each: the car has a velocity of 80km/h and the fly has a velocity of -5km/h (ie it is travelling away from North at 5km/h).

Therefore we have the situation that before the collision the fly was travelling at -5km/h and after the collision its remains are travelling at 80 km/h. So logic says that at some point during the collision as it accelerated from -5 to 80km/h it must have been travelling at 0km/h (ie it was stationary).

For what it's worth, I think Terry and np_rt have it right.

Posted by fwaff on 2003-01-20 00:33:17