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 Mass and Time by np_rt)

your answer may be so, but consider the plight of the poor fly. Since its mass is negligible,its first contact with the windscreen will be its head(given that our fly is about 5mm long). Its head will be driven backwards at 80kph whilst its bum will still be travelling forward at 5kph. In that very short distance and time the poor fly could be considered to be not really moving anywhere within that time frame and distance other than compacting, and the vehicle won't have slowed down at all. Hopefully you won't take this comment seriously.

Posted by terry on 2003-01-19 18:49:48