The Train and the Ping Pong ball - this puzles me, help! :-p

[cosmicpanda]

Scenario:

 

A kiddy stands on a railway track and throws his pingpong ball at an oncoming train. The train is moving at 100 km/h.

 

Ignoring complicating factors like airflow and air resistance, what happens when the ball hits the train?

 

The train obviously gives the ball a fair old wallop and the ball goes flying away.

 

But when you think about it, the pingpong ball is going in one direction. To bounce off, it would have to stop when it hits the train's windshield and then bounce away.

 

Because the ball stops, do you agree that the train must also stop? :shock:

 

If not, what does actually happen to the train?

[Charis alAslan]

This is a job for ----

 

THE WHITE AJAH!!

 

*waits for one of the Whites to come by*

[Helike]

*writes absently in her notebook*

 

It definitely needs to be studied further...

[LordDaemonDracos]

Actually according to physics the train would slow, but very slightly due to the force that it is exerting vs the force of the ping pong ball. This slowing would not be noticeable.

 

But then again it has been about 12 years since i studied physics so I might be wrong.

[cosmicpanda]

But then, LDD, you have a pingpong ball that's stopped for an instant, and a train that's continuing to plow into it.

 

That means that the ball cannot be stopped - but it has to be, to change direction! :?

[F Horn of Valere]

maybe one or both of them are both stopped and moving at the same time :?

[Helike]

*doesn'tlook up from her note pad* Charis told you to wait for a white....

 

*replays the conversation quickly in her head, discovers an anomoly in the question.*

 

 

 

 

 

 

 

 

 

 

 

 

Why has no one asked after the welfare of the child?

[Guest Karana Majin]

Ok... *rolls up sleeves* Seeing as I AM a physics teacher (and the head brown, so therefore I know everything anyway...) I'll explain this.

 

This problem falls under the conservation of momentum (although it can also be argued to be under Newton's 2nd Law, F=ma. The two are not mutually exclusive). Momentum, being a vector quantity, cannot "disappear", it can only change forms, as in magnitude (size) and direction. Specifically, this problem involves collisions. There are two possible scenarios for collisions... an elastic collision, where the ping pong ball bounces off the train or an inelastic collision where the ping pong ball is stuck to the train and the two move off together as one unit.

 

My impression is that the problem is stated incorrectly, according to actual laws of physics. It is impossible that the train could give the ball a wallop that would cause it to actually fly forward ahead of the train. However, it is possible that the ball could hit the train at an angle, get its "wallop" and travel off sideways. The reason the first assumption is impossible is because some of the momentum from the train will be transferred to the ping pong ball, causing it to change direction. It is impossible for the train to transfer enough momentum to actually cause the ball to be travelling faster in the opposite direction than the train was originally travelling.

 

So, I'm going to say this would most logically be an inelastic collision, where the ball would stick to the train and they would move off together. If you want to find out how fast they are moving together, I could solve that for you, although for all intents and purposes, the ping pong ball is practically massless when compared to the extremely massive train. Meaning, the train would still be going 100 km/h in the same direction, with a ping pong ball stuck to the front.

[JenniferL]

I'm so glad you saw this. I haven't done any physics in awhile and I couldn't articulate an answer at ALL. My brain is all full of tides and currents and coral reefs right now.

[LordDaemonDracos]

Thanks Karana. I really need to get back to studying physics. Any suggestions on where to start again?

[TheMasterDude]

For everyone that thought that explination was confusing here is the explination for dummys:

 

The train has soo much monentum moving it in its direction that it transfers some of it's mometum to the ping pong ball therefore the ball stops while it changes direction and goes the same way the train does, but the train does not stop because its momentum is far more than the ping pong balls.

 

 

*paises last years physics teacher...I actually learned somthing.....*

[Guest Karana Majin]

You did, TMD! That was a very succinct lay explanation! If you were my student, a year later, and you still remembered that much, I would be happy. Although the most I want from my students is to learn how to THINK... I could care less if they remember actual physics.

[JenniferL]

I want my students to not attempt alligator wrestling on school property.

[TheMasterDude]

Thats a very interesting goal Kathana...Where r u? In the swamps?

[JenniferL]

Suburbs of Atlanta. Go see my other thread for an explanation. =)

[Charis alAslan]

WOW! :shock: Karana's a GENIUS!! *bows before the Brown head with even more respect than is allowed*

[Guest Karana Majin]

*blinks* You just didn't know that because I walk around all vague and muttering to myself most of the time!

 

*g*

[cosmicpanda]

OK Karana, I get that much, but what I'm wondering is, what happens right at the moment of impact?

 

I have a feeling that the ball must stop before changing direction. I also have a feeling that since it is the train that causes the ball to change direction, then the ball must stop when it hits the train, before changing direction. This makes me think that the train can't keep ploughing into the ball for that moment of time, as the ball is stopped. That then makes me think that the train's stopped just for that moment of time. :shock:

 

This then makes me think that I'm missing something, because that sounds crazy.

 

Or can the ping pong ball change direction immediately without first stopping? :? *confused BA head*

[Guest Karana Majin]

Ok... the ping pong ball DOES stop and change direction, but that does not noticeably affect the train's momentum. Technically, the train DOES slow down (but nothing like stopping), but so little in this situation as to be negligible.

[goldeneys]

hmmmm very interesting, do you do this often guys?

[cosmicpanda]

well, we could, if people dream up more strange questions.

 

 

Thanks Karana, I know all I need to know about the behaviour of ping pong balls regarding trains, now :D