Hello world. I’m hoping that someone can help me get to an answer to a problem that I think I know the answer to but just can’t seem to wrap my head around.
Picture a train. A regular passenger train is fine. I know that everything in the moving train is moving relative to everything else in the train. That’s why if you jump in a moving train you will land in the same spot on the train and not get smashed into the end of the train car.
If you were on the top of the same moving train and jumped, I feel like you would land further towards the rear of the train because once you left contact with the train you would (rapidly) decelerate as the train maintained its speed. (Right?)
Now, say you were on a platform waiting for the train. The train comes into the station with its doors open and you jump from the platform into the train. Assuming you could make a clean jump into the train through the open doors would you land in a spot opposite of your jump off point in a different part of the train, or would you land just inside the doors of the train at some point further away from your jump off point? I’m thinking my jumping velocity, perpendicular to the train’s velocity, would make me land in a spot opposite my jump off point. (Huh??)
Now, just suppose, I jumped into the train but kind of just hovered just inside the door, never landing. Would I get smashed into the back of the train? Or would I instantaneously accelerate to match the train’s velocity and end up…well, where on the train would I be hovering?
Please disregard things like wind resistance and the chances of actually jumping into a moving train is unlikely, if not impossible. And stupid. I know this has everything to do with velocity and the relative motion of one thing to another. I just can’t seem to make the ideas stick in my brain. What do you think?
Yes but only due to the air resistance, because you are moving and the surrounding air is not. Edit: If you were in a vacuum, you would land on exactly the same spot on the train where you jumped from.
You would land farther towards the rear of the train because you were not yet moving at the same speed as the train. As soon as you land the friction between you and the train floor would eventually cause you to stick to one place in the train as you accelerate to its speed.
Yes. Because you are still stationary relative to the ground and the train is moving “past” you. The reason that everybody else is relaxing comfortably in their seats is that they accelerated along with the train when it started so they are going the same speed as the train.
Imagine this scenario in a different way. Imagine the train is stationary and you are running like mad from the front of the train to the rear. Then you jump through a doorway. You would not expect to be magically stopped dead on the spot just because you are now inside the train. The physics is exactly the same as your question.
In the first case, both you and the train are moving at the same speed, just jumping up and falling down doesn’t change your horizontal speed, so you land on the same spot …
Jumping from the stationary platform onto a moving train, you and the train are moving at different speeds … so the time you are in the air, the train is moving passed you … thus you will land further back in the train …
If you want to land on a specific spot on the train, you’ll have to jump sooner and time it so that spot is there when you land …
Perhaps the better example is a midfielder (quarterback) passing the ball to a running forward (wide receiver) … the midfielder has to 'lead" the forward to make the pass successful
Let’s say the train is moving left to right. It has no forward/backward motion at all
Let’s say you are moving forward. If the train never caught you, you would simply jump across the tracks. You have no left to right velocity.
Can you see what happens in your scenario now? You jump and magically shed all forward/backward motion once inside the train. The train however is still moving left to right while you are simply hanging motionless. The back of the train continues to move left to right, intercepting your body and losing a small amount of energy as it brings your mass up to whatever left to right speed it originally had.
For you to match velocities with the train you would have to be accelerated in the left to right direction by some force. The only force available in this scenario is the rear wall of the train.
If you disregard wind resistance, then if you were on top of a moving train and jump up, you would come down on the same spot (same part of the train).
And in your second scenario, the train moves forward between the time you pass through the door and when you land on the floor of the train. So you would land some distance behind the door.
World, or SD portion thereof: Hello JavaMan. Welcome aboard.
Sometimes it seems like every other day questions get asked and answered on relativity: trains and elevators to near-incomprehensible stuff when the professional physicists around here occasionally duke it out, while everyone else passes around the popcorn.
Make sure you’re in a place where you can tell the exact spot, where I’ll let you define how exact do you want to be.
Jump up.
Have you landed in a different spot?
The Earth is moving pretty quick, if you use a frame of reference centered on… Mars, for example. Or the Sun. But it’s stationary if you center the frame of reference on it, and you were stationary with reference to it when you jumped (in reference to those external points, you were moving at exactly the same speeds and directions as Earth). Same for the train: if you’re standing on top of it you’re stationary with respect to it, whether it’s moving with respect to the surrounding landscape or not. When two objects are stationary with respect to each other, they are that way regardless of the frame of reference.
Yes, but if frames of reference are involved, and thought experiments with them in trains were good enough for Einstein to add just a little…something…to the classical issues, it’s good enough to point out the initiating thought conditions.
Should have been called Einstein’s theory of Invariance, because calling it relativity just confuses people.
In any case, if you’re jumping around on or between trains, all you have to remember is that an object in motion will stay in motion unless acted upon by an outside force.
So if you’re inside, what forces are acting on you? You and the train are moving together, you jump up and then gravity pulls you down. So you stay in the same place relative to the train.
On the roof it’s the same thing, but there’s another force here, the force of the wind. So you jump up and land farther back, just like if you were standing on the ground in a heavy wind and jumped up the wind would blow you back and you wouldn’t land in the same spot.
If you’re jumping onto a moving train, before you jump the train is moving relative to you and after you jump the train is moving relative to you. Or, if you’re on a moving train and jump off, you were moving relative to the ground before you jump, and you’re still moving relative to the ground after you jump. Then friction with the ground slows you down, and you skid to a stop. Same thing when jumping onto a moving train, except it’s friction with the train, not the ground.
A lot of relatively good answers, but I think I have a more practical one. As in how to wrap your head around it.
Recognize that each object in motion is independent of the others. Imagine each object without the others. Then combine the results.
Picture the person jumping from the platform. That person will land in the same spot whether the train is there or not. Imagine the train moving through the station. It will move in the same way whether the person is there or not. Then imagine both occurring at the same time, and you will “see” the person jump, manage to get through the open doorway, and then land on the floor of the train, which is moving as they land. Their body will land and remain in the immobile spot they would have, if the train were standing still, but that spot in space isn’t available anymore, because the moving train floor has superseded it. Don’t try to think about the person having to “catch up” to the moving train, just imagine them landing on a “spot” that is being pulled sideways out from under them.
The primary physical law of motion you need, in order to handle this imagining, is just the one. That is, that an object in motion, tends to remain in motion unless acted upon by another force or object.
You can’t disregard wind resistance. In your first example the air inside the train is moving at the same velocity as everything else in the carriage and is therefore static in relation to the carriage. However, the air outside the carriage is moving at a different velocity.
As for the rest of it, think about a treadmill instead of a train. If you throw a coin onto a treadmill, where does it land? Etc.