# Thread: If you jump on a train....

1. ## If you jump on a train....

I couldn't believe someone asked the question at work, how you would land in the same place on a moving train if you were to jump. I said, I know all about that, but then realised I couldn't answer.

After reading lots of different explanations it still seems a little unclear.

I understand motion is all relative. And I understand what a frame of reference is. But it still doesn't really explain why the train doesn't move beneath if you were to jump.

Is it because of inertia? The train is moving at 60 mph relative to the ground. As are you. So when you jump, you continue to move (the force of friction with the air is minimal).

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chas53 (10-27-2012)

3. ## Re: If you jump on a train....

In an enclosed car, wouldnt the air be moving as well though?

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chas53 (10-27-2012)

5. ## Re: If you jump on a train....

Originally Posted by mrjaffa
I couldn't believe someone asked the question at work, how you would land in the same place on a moving train if you were to jump. I said, I know all about that, but then realised I couldn't answer.

After reading lots of different explanations it still seems a little unclear.

I understand motion is all relative. And I understand what a frame of reference is. But it still doesn't really explain why the train doesn't move beneath if you were to jump.

Is it because of inertia? The train is moving at 60 mph relative to the ground. As are you. So when you jump, you continue to move (the force of friction with the air is minimal).
First of all you need to define which coordinate system you are using. Otherwise you get a conradiction. With respect to a ground based coordinate system you DON'T land at the same place, the train is moving. With respect to a coordinate system that moves at constant velocity with respect to the train you DO land at the same place.

So useing the train's coordinate system and under your assumptions the "reason" could be either inertia or conservation of momentum. The two are equivalent in this case. If you jump perpendicularly to the train there would have to be a force acting in some direction in the plane for you to land someplace different on the train.

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chas53 (10-27-2012)

7. ## Re: If you jump on a train....

Originally Posted by mrjaffa
Is it because of inertia? The train is moving at 60 mph relative to the ground. As are you. So when you jump, you continue to move (the force of friction with the air is minimal).
Yes, exactly. An object in motion will remain in motion unless acted upon by a force. (one of Newton's Laws of Motion)

There is no relevant friction with the air, since the air is moving at 60 mph too. That's assuming you are in a closed carriage. If you were on an open flatcar, wind would be a factor. You would land slightly behind where you jumped relative to the train, due only to the wind. Relative to the ground, you would still have moved rapidly in the same direction as the train, just a tad slower.

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chas53 (10-27-2012)

9. Ok. Good stuff. Think I can go into work on Monday and explain it better now :-)

10. ## Re: If you jump on a train....

I believe you would land at the same spot relatively unless you were on top of the train and there was a strong wind going against the train and you were not wearing loose fitting clothing that might catch the air and move you backwards. Inside the train, no doubt you will land in the same place.

11. ## Re: If you jump on a train....

Originally Posted by mrjaffa
After reading lots of different explanations it still seems a little unclear.

I understand motion is all relative. And I understand what a frame of reference is. But it still doesn't really explain why the train doesn't move beneath if you were to jump.
Imagine someone sitting near the tracks, looking at the train passing by and at you doing silly gymnastics inside. What does that person see?

Does he see you jumping straight up? No. You do jump up relative to the train, but the train also launches you forward at great speed, because it imparts you its own speed. So, from the p.o.v. of the bystander lounging on the fields nearby, you jump a little up and a HECK OF A LOT forward. You describe a curved trajectory and then finally you meet the train again in the same spot on the floor from where you jumped.

Why do you meet the train in the same spot? Because both yourself and the train are moving forward at the same speed. You do have some vertical speed as well, but that doesn't matter. The fact is, if the train goes forward at 100 km/h, SO DO YOU. Horizontally, your speeds are the same.

Of course, all of this assumes the train car is closed, so there's no air drag that would pull you back. Perhaps the car is made of glass, like an aquarium, so the dude outside can see you jump.

This is the key to all problems of relative motion. First and foremost, before doing anything else, decide what is the point of view from where you observe the whole thing. In this case, we agreed to look at the problem from the perspective of someone sitting on the ground. Once we agree on that, the entire reasoning was done from that perspective only.

If you're studying relative motion and you're having trouble figuring out what happens and why, it's because you're unconsciously shifting back and forth between different perspectives (different frames of reference). Pin the frame of reference down and stick with it, and then the problem becomes simple.

Just one of the basic tricks in physics.

12. ## The Following 2 Users Say Thank You to Florin Andrei For This Useful Post:

chas53 (10-28-2012),not_Fritz_Argelander (10-28-2012)

13. ## Re: If you jump on a train....

Originally Posted by Florin Andrei
Imagine someone sitting near the tracks, looking at the train passing by and at you doing silly gymnastics inside. What does that person see?

Does he see you jumping straight up? No. You do jump up relative to the train, but the train also launches you forward at great speed, because it imparts you its own speed. So, from the p.o.v. of the bystander lounging on the fields nearby, you jump a little up and a HECK OF A LOT forward. You describe a curved trajectory and then finally you meet the train again in the same spot on the floor from where you jumped.

Why do you meet the train in the same spot? Because both yourself and the train are moving forward at the same speed. You do have some vertical speed as well, but that doesn't matter. The fact is, if the train goes forward at 100 km/h, SO DO YOU. Horizontally, your speeds are the same.

Of course, all of this assumes the train car is closed, so there's no air drag that would pull you back. Perhaps the car is made of glass, like an aquarium, so the dude outside can see you jump.
From the point of view of the stationary outside observer, does the person jumping on the train stay in the air longer then for the person on the train? I would think that would be the case as the clock for the person on the train should run slower from the point of view of the outside observer. Correct? I know that this is getting off track (Pun?) a little.

14. ## Re: If you jump on a train....

One thing that might not have been mentioned is, is the train braking, coasting or accelerating during this jump?

15. ## Re: If you jump on a train....

Or going around a curve (lateral acceleration). In this case the jumper would go in a straight line and the side of the car would move towards him. I hope the window is closed!

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