Why Don't We Feel the Earth's Movement When We Jump?

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Discussion Overview

The discussion revolves around the question of why a person does not feel the Earth's movement when jumping straight up, considering the Earth's rotation and orbit. Participants explore concepts related to inertia, reference frames, and the effects of Earth's motion on jumping, with some comparisons to jumping on a moving train.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that when jumping, a person shares the Earth's velocity, which explains why they land in the same spot.
  • Another participant notes that at higher latitudes, the trajectory of a jump differs from the ground's path, but the deflection is negligible for human jumps.
  • A comparison is made to jumping on a moving train, emphasizing that in both cases, the jumper maintains a constant horizontal velocity relative to the moving object.
  • Inertia is mentioned as a key factor in understanding why the ground does not move beneath a jumper's feet.
  • One participant introduces the concept of the Coriolis force and its dependence on latitude, suggesting it affects the trajectory of objects in motion.
  • Participants speculate about the effects of sudden changes in the Earth's motion, questioning whether such changes would be felt immediately.

Areas of Agreement / Disagreement

Participants express various viewpoints on the mechanics of jumping in relation to Earth's motion, with some agreeing on the role of inertia and reference frames, while others introduce additional complexities such as latitude effects and hypothetical scenarios. The discussion remains unresolved regarding the implications of sudden changes in Earth's motion.

Contextual Notes

Some assumptions about the constancy of velocity and the effects of latitude on jumping trajectories are discussed, but these are not fully resolved. The discussion also touches on the Coriolis effect without a consensus on its significance in everyday jumping scenarios.

Fuz
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So after swim practice I was waiting for my ride and pondering the universe. I was thinking about the Earths orbit around the sun and rotation around its axis. I then came across a very interesting question...

If I were to simply jump straight up into the air, why doesn't the Earth move right under my feet? Assuming that there is no wind and stuff like that, why do I land in the exact same place I jumped from? Wouldn't the Earth just move really fast under me until I hit the ground?

Hope my question made sense :)
 
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It's the same reason you can jump on an airplane and not hit the back wall. You share the Earth's velocity. At the equator, Earth rotates at about 1000 mph. If you stood on the equator, you also share this velocity, so when you jump, the Earth does move, but so do you, so you land in the same place.

At higher latitudes, you actually don't land on the same spot. After jumping, while in the air, you have a great circle trajectory, while the ground does not. But the amount of deflection for a human jump is negligible. Missiles and other objects on ballistic trajectories need to take this into account.
 
Let's change the experiment a little to make the understanding easier. Imagine you're on a train running straight ahead at an even speed. If you jump, why doesn't the train move forward under your feet?

Cheers,
Mike
 
Inertia.
 
If you jump straight upward on a moving train that has a constant velocity, in your frame of reference you go straight up and down. In the frame of reference of someone standing on the ground and watching you, your path was a parabola.

why doesn't the train move forward under your feet?

Your parabolic path has a constant horizontal component of velocity that is the same as the constant velocity of the train.
 
tony873004 said:
At higher latitudes, you actually don't land on the same spot. After jumping, while in the air, you have a great circle trajectory, while the ground does not. But the amount of deflection for a human jump is negligible. Missiles and other objects on ballistic trajectories need to take this into account.

The Coriolis force if you jump up is actually proportial to cos(latitude), greatest at the equator and 0 at the poles. 2 \Omega \times v has a magnitude of 2 \pi v cos(latitude) / T
 
Try and think of what would happen if just after you jumped the train locked up its breaks.

OUCH!
 
Well what would happen if the Earth were to suddenly decelerate or accelerate? Would we feel the effects immediately?
 
Fuz said:
Well what would happen if the Earth were to suddenly decelerate or accelerate? Would we feel the effects immediately?
If you were on a train that suddenly accelerated, would you feel it? Same thing.
 

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