# Thread: Is it possible to reverse the direction of travel without velocity becoming zero?

1. This has long puzzled me. I imagine a fly hitting a vertical train window dead-on. The fly ( or its remains) reverses its direction of travel but the train does not come to a stop at any point. Now, the fly does come to a stop, its velocity reaches zero ( quite rapidly) relative to the train but to an observer the fly reverses direction whilst at no time being motionless.
Or is this just a reworking of Zeno's arrow paradox?

2.

3. In order for the fly to change directions instantaneously, it would require an infinite rate of acceleration and infinite force, and that is not possible. The window glass and the fly's body would have to be infinitely stiff for that to happen. Actually there is some elasticity in the materials. The fly would make a miniscule dent in the window during the collision, during which a tiny portion of the window in contact with the fly would actually come to a stop.

4. Thanks, thats fairly satisfying as an explanation. However I use the fly and train analogy to make visualisation of of a more general situation easier.

What would happen if a small inelastic mass were to impact a very very much larger inelastic mass travelling in exactly the opposite direction?

When viewed in relativity terms though, nothing is ever really stationary relative to everything else and particles' journeys are described by their respective World Lines.

Hope that helps.

6. Originally Posted by Jeaunse23
Thanks, thats fairly satisfying as an explanation. However I use the fly and train analogy to make visualisation of of a more general situation easier.

What would happen if a small inelastic mass were to impact a very very much larger inelastic mass travelling in exactly the opposite direction?
The answer would be the same. When you say inelastic I think you probably mean stiff, but nothing is infinitely stiff.
When discussing collisions, inelastic usually means something that deforms without returning to its original shape.

7. Good and thanks! Thats put that issue to rest then...

8. Yes it is.

9. Turn on a dime. and you can never be motionless.

 Bookmarks
##### Bookmarks
 Posting Permissions
 You may not post new threads You may not post replies You may not post attachments You may not edit your posts   BB code is On Smilies are On [IMG] code is On [VIDEO] code is On HTML code is Off Trackbacks are Off Pingbacks are Off Refbacks are On Terms of Use Agreement