Quote:
Originally Posted by pimpsticky
If the fly, or any object for that matter, is in contact with something, then don't they possess the same velocity?
I still say that in order for the fly to switch direction, it has to come to a complete stop.
Well, the train is stopping the fly from moving forward, so for a split second, (if the train is in contact with the fly) the train has to be going zero mph.
I've not found someone that can explain this and I go crosseyed thinking about it...lol

They do possess the same speed, it just isn't zero. Think of the passengers, they're all travelling at the same speed as the train.
Yes, in order for the fly to switch directions it must reach zero veolcity before it can travel the opposite direction. However the train is continuing in its original direction, its vector remains unchanged and therefore it does not need to decelerate to zero.
If you look at simple conservation of momentum (MassxVelocity)...
Mass of train x Velocity of train + Mass of fly x Velocity of fly = Mass of fly and train x Velocity of fly and train
Initially neither are travelling at zero velocity, and since the mass of the fly and train are not reduced to zero (they dont disappear), the Velocity of the two cannot be zero. Otherwise you would violate the law of conservation of momentum (not to mention conservation of energy as well)
The non mathematical way to think of it...IF the train's velocity was zero for a split second a number of things would happen; it would derail, passengers would be decelerating at a rate close to infinity (acceleration=velocity/time, time is very very small) causing all of their hearts to rip from the aorta (sp, and yes this happens in high impact crashes because of the deceleration), it would take forever to get anywhere since the train would have to accelerate back up to 100km/h about every 5 seconds.
Just something else to help shed some light. If a passenger is walking backwards on the train, does that mean they are walking faster than 100km/h?