What Happens When a Dirtbike Jumps off a Moving Train?

  • Context: Undergrad 
  • Thread starter Thread starter david91
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Discussion Overview

The discussion explores the physics of a dirtbike jumping off a moving train, specifically considering the effects of relative velocities and the dynamics of the jump. Participants examine the implications of jumping off while the train is in motion, including the behavior of the bike and rider during the fall.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that if a rider on a dirtbike jumps off a moving train while maintaining a velocity equal and opposite to the train's, they would fall straight down as if dropped from the height of the train.
  • Others argue that while the rider would fall straight down, the bike's wheels would continue spinning at the train's speed, leading to a forward lurch upon falling.
  • A participant suggests that the front wheel of the bike would start to fall first, potentially causing the bike to rotate back-over-front during the fall.
  • Another participant counters that the momentum of the wheels would not cause a significant lurch and that a rider could compensate for the bike's rotation.
  • One participant introduces a hypothetical scenario where the train moves faster than the bike, suggesting the rider would need to perform a 180-degree turn midair to land while chasing the train.
  • There is a creative expression about the potential future of freestyle motocross stunts, highlighting the beauty of physics in sports and the skill involved in such maneuvers.

Areas of Agreement / Disagreement

Participants express differing views on the dynamics of the jump, particularly regarding the effects of the bike's momentum and the rider's ability to control the bike's rotation. No consensus is reached on these points.

Contextual Notes

Discussions involve assumptions about relative velocities and the physical behavior of the bike and rider during the jump, which may depend on specific conditions not fully explored in the conversation.

david91
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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?
 
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Exactly. You would fall straight down, just as if someone had simply dropped you from "train-height" (not recommended).
 
thank you!
 
I'd stock up on the Preparation H first...
 
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.
 
:smile:
 
Archosaur said:
Exactly. You would fall straight down, just as if someone had simply dropped you from "train-height" (not recommended).

Your wheels however would still be spinning at 12 m/s. You'd fall straight down but immediately lurch forward.
 
Was that a dig?!
 
Not in the least. A lurch backwards, maybe.
 
  • #10
skeptic2 said:
Your wheels however would still be spinning at 12 m/s. You'd fall straight down but immediately lurch forward.

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. :smile:
 
  • #11
skeptic2 and tiny-tim are both right, of course.

I was just talking about relative velocities.
 
  • #12
jumping off a FAST object

tiny-tim said:
your front wheel would start falling first, because it would come off the train first
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 :smile:
 
  • #13
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:
  • #14
skeptic2 said:
Your wheels however would still be spinning at 12 m/s. You'd fall straight down but immediately lurch forward.
You would not lurch. The wheels have too little momentum to cause any appreciable lurch.

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

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