# Jumping off a moving object

1. Mar 27, 2009

### david91

i wondered what would happen if you took f.eks. your dirtbike on top of a moving train, drove against the rear end of the train (backwards), and made sure you had 0 velocity compared to the ground. if the train is moving north at 12m/s and you are moving south at 12m/s, will your dirtbike "stand still" when you jump/fall of the train?

2. Mar 27, 2009

### Archosaur

Exactly. You would fall straight down, just as if someone had simply dropped you from "train-height" (not recommended).

3. Mar 27, 2009

### david91

thank you!

4. Mar 27, 2009

### Danger

I'd stock up on the Preparation H first...

5. Mar 27, 2009

### skeptic2

An intrepid young student named Raleigh
Deserted a fast moving traleigh
To discover a flaw
In Newton's first law
but his efforts were faleigh, by galeigh.

6. Mar 27, 2009

### Danger

:rofl:

7. Mar 27, 2009

### skeptic2

Your wheels however would still be spinning at 12 m/s. You'd fall straight down but immediately lurch forward.

8. Mar 29, 2009

### LURCH

Was that a dig?!

9. Mar 29, 2009

### skeptic2

Not in the least. A lurch backwards, maybe.

10. Mar 29, 2009

### tiny-tim

Also, your front wheel would start falling first, because it would come off the train first, and so the bike would also rotate slowly, back-over-front.

11. Mar 29, 2009

### Archosaur

skeptic2 and tiny-tim are both right, of course.

I was just talking about relative velocities.

12. Mar 29, 2009

### danb

jumping off a FAST object

Not if the rider leans back at that point. What would really be neat is if the train is going faster than the bike, so the jumper needs to do a 180 in midair and lands on the ground chasing the train

13. Mar 31, 2009

### NWH

Interesting concept, that made me smile...

If this is the inevitable future of Freestyle Motorcross stunts I'll be damn happy. They already have such great senses to things like rotation and velocity, it's actually quite beautiful to watch. It would be wildly pleasing to see such a vast inprovement in skill due to the knowledge and reliability of such physics because it's sports like these that I feel show the absolute peak of human ability. Certain things are only possible if you risk your life and it's amazing to see that these guys can do them.

Please enjoy, for the physics. Calculation at it's best...

Last edited by a moderator: Sep 25, 2014
14. Mar 31, 2009

### DaveC426913

You would not lurch. The wheels have too little momentum to cause any appreciable lurch.

I think a cyclist would be quite hard-pressed to not automatically compensate for this.