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?

i'm no physics expert, but it seems to me that the problem lies in treating the two objects, car and fly, as both points and objects of mass. they can't be both. if we are talking about two points colliding then at the instant they collide they will share a constant speed, not passing through any other speeds to get there (fly will never pass through 0 km/h). but if we are talking about two objects of mass then the atomic repulsive forces between the two objects will cause the fly to decelarate before it collides with the car (the fly will never actually collide with the car), so the fly passes through 0 km/h but it does this as it is still approaching the car (talking nano distances here) so there is always some distance between the two which decreases until the point that the fly has reached the same speed as the car.

Posted by donnmike on 2003-12-04 23:55:44