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?

THe car most certainly does not pass through 0 MPH the cars velocity does not change more than very slightly.
The fly accelerates in the direction of the car as it comes in contact with it.

Think of it from the cars's frame of reference. In that frame of reference teh car is stationaryn and the fly in flying toward it at 85km/h. when the fly hits the car it comes to a halt (in the car's frame of reference) it does not move the car more than fractionally.

Posted by FatBoy on 2003-08-19 08:26:24