Does a tennis ball stop a train if it hits it?

[DaveC426913]

so, like when you throw the ball inside the train, the ball stops relative to the train but not relative to the ground - so obviously the train doesn't stop.

This doesn't address the original problem!

When the tennis ball hits the rear wall, it stops and changes direction. Somewhere in there, the ball's v is zero. If the ball's v is zero, and it is in contact with the train, one could conclude that the train's v is zero for a moment.

That's the issue that needs to be addressed.

 

[ObsessiveMathsFreak]

The ball stops the train.

But the train doesn't all stop at the same time.

 

[Cyrus]

Um....what?


So you mean to tell me that a x-hundred ton train comes to a stop in the split second of time it makes contact with the ball, and then speeds back up to its original speed an instant later, after all, we agree that the train will continue straight ahead after the collision. What, can this train withstand thousands upon thousands of g' forces to do that? Does any of this seem unreasonable to anyone else?

DaveC gave the correct anwser the first time. Whats the issue?

 

[DaveC426913]

The ball stops the train.

But the train doesn't all stop at the same time.

As Cyrus point out,

No.

 

[DaveC426913]

I'm just going to say this one last time, and then y'all can do whatever you want.

There is no such thing as a perfectly rigid body. If the tennis ball and train were both perfectly rigid, we would definitely have a problem. The energy required to reverse the direction of one perfectly rigid body by another perfectly rigid body would be infinite - as would the acceleration rates.

And with infinite energy kicking around, bringing the train to a halt would be the most trivial of concerns. Note that neither the train nor the ball would be able to vapourize, or do any other such thing that would require the transfer of energy between atoms - which they can't do since they are perfectly rigid. And, since we have things changing direction without loss or gain of energy, kinetic or otherwise, we've also eliminated inertia. But I digress...

You can see how this universe rapidly deteriorates into a fantasy.

Bodies interact (transfer kinetic energy) over a non-zero distance and a non-zero time.

 

[Farsight]

Well said Dave.

 

[DaveC426913]

Theng kew...

Now, LOCK THIS THREAD before some idiot comes up with another completely erroneous explanation!

 

[Doc Al]

Good idea!