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Could it be possible for an object to bounce for a very long time.

  1. Jul 14, 2012 #1
    A ball in free-fall (meaning no forces are acting on it other than gravity), upon hitting the ground, will never bounce back up to the spot where it first started to descend. This is due to the Laws of Thermodynamics, which state that, in every action, some energy being used in that action escapes into the enviroment.
    What could happen if you had a super material which was very good at bouncing it need not be 100% efficient would the Earth's rotation add more energy than the energy which escapes into the enviroment when it bounces.
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  3. Jul 14, 2012 #2
    If you had zero friction acting on the ball whilst travelling and the collision was perfectly inelastic then the ball would bounce back and forth forever. But realistically this wouldn't happen.
  4. Jul 15, 2012 #3
    Forever is a very long time.I suppose the Earth will eventualy stop turning in the meantime there does not seem to be any apparent manifestation of added energy to Earth collideing objects.A cannonball must impact the ground with more energy than was supplied by the cannon.Is there any way to make this usefull.
  5. Jul 15, 2012 #4


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    The ball would be moving with the Earth, including it's rotation, so it would not receive any extra energy from it.

    This is incorrect. A cannonball will usually impact the ground with less energy than it started with due to air resistance. Ignoring that, the cannonball would have equal energy if the height of the impact point was equal to the height of the firing point. It would have more energy if the height of the impact point was less than the firing point, and less energy if they impact point was higher than the firing point. (Due to gravity)
  6. Jul 15, 2012 #5


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    How so? A cannon ball fired straight up with initial KE=0.5mv2 climbs until all the KE has turned to PE. It then falls until all the PE has been turned back to KE. If you ignore air resistance it should have the same energy it was fired with.

    What do you mean by..

    Perhaps see..

  7. Jul 15, 2012 #6


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    I believe you meant "perfectly elastic". if the collision were perfectly inelastic, the ball would not bounce at all!
  8. Jul 15, 2012 #7
    Yes I did. Thank you :)
  9. Jul 15, 2012 #8
    If the ball started from a position where it was not moving with the Earth.You gave it some velocity.

    Ignoring air resistance would it not depend on the location you fired your cannonball from rather than if the impact point was equal to the height of the fireing point.
    If it were higher it would have less energy because of the climb at some point.
    As for location if the ball was fired near the poles of the Earth would it not have less impact than at the equator.
  10. Jul 15, 2012 #9
    Ignoring air resistence it should have the same energy it was fired with untill the moment of impact.Depending on the direction and where the cannonball is fired from the earth rotation should add some extra energy on impact due to the coriolis effect.
  11. Jul 15, 2012 #10


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    The Coriolis effect does not add energy. The Coriolis [pseudo]-force acts at right angles to the direction of travel and therefore does no work. Centrifugal force, on the other hand can add or subtract energy [as viewed in the rotating frame].

    Applying conservation of energy arguments in a rotating frame is a tricky business.
    Last edited: Jul 15, 2012
  12. Jul 15, 2012 #11


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    You can modify the potential in the rotating frame to account for centrifugal force due to coordinate rotation. In fact, this is usually done on earth, and it is relevant for the shape of the earth.
  13. Jul 15, 2012 #12


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    The equatorial velocity of the Earth is around 465 m/s, or about 1500 ft/s. Assuming you were near the equator you would have to fire it in the opposite direction at near that speed to get it to a point where it is not moving with the Earth's rotation.

    Yes, this is exactly what I said. If your cannon ball falls further than it climbs it will have more energy upon impact. (ignoring air resistance)

    IF we ignored wind resistance AND we fire it a huge distance, say 10,000 km, then we could see a noticeable increase in the impact energy, as the cannonball would be traveling at whatever velocity it was launched with PLUS an increase due to the Earth moving underneath it at around 1500 ft/s.

    In reality there would be negligible difference, as we cannot launch a cannonball that far and it would have to travel through the air.
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