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Conservation of energy and gravity

  1. May 13, 2013 #1
    I am a bit confuse about gravity and work
    Suppose a ball falls from one height to another, hi to hf

    When talking about the system BE the ball AND the earth, all the work done is internal, and no external work, change in KE = change in PE, right?

    But when talking about the system B of just the ball only,
    total energy of the system B increase? because external force gravity is doing work on the ball?

    Change in KE increase, I know that, .5mv^2, velocity increase
    But what about PE? if gravity is external force, then the ball doesn't have potential energy to start with at hi ? So potential energy are only for a system that includes earth?

    Am I looking at these... gravitational potential energy and work stuff all wrong?
  2. jcsd
  3. May 13, 2013 #2

    Simon Bridge

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    Falling in gravity, the gravitational field does work (loss in PE) on the ball (which gains KE).

    The ball still has PE in the case where it is treated by itself because there is a gravitational force acting on it.
    Doesn't matter if the forces are internal or external. The PE comes from the field.
  4. May 14, 2013 #3
    What's the difference between internal and external forces??
    I don't know if this will help but I remember gravity is a negative potential as at infinity away from mass potential is zero. I don't know what you mean by system B and A are as I imagine they are the same system because the Ball has equal pull on the earth as the earth does on the ball; as Simon Said "The PE comes from the field."
  5. May 14, 2013 #4
    It may also be helpful to remember that potential energy is entirely arbitrary. It doesn't matter what's in your snapshot of a system, all that matters is that you have a conservative force (e.g. gravity) and two points of reference. As a matter of fact, you could put the ball on the ground and say it has potential energy, it just won't do you a lot of good, because it isn't likely to fall then.

    Toneboy, in my view, internal and external forces are almost the same thing, it's just a matter of where you put the boundaries of your system.

    As a real humdinger to get you exploring these concepts, take Newton's Third Law of action/reaction pairs. How, if this is true, can there ever be an external force?
  6. May 15, 2013 #5


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    Looks ok to me.

    What Denver said. You can't talk about the energy of the ball without reference to a system boundary. When you talk about the ball having KE = 0.5mV^2 the velocity term implies a system boundary. For example one that allowed you to ignore the fact that the earth is spinning and orbiting the sun.

    We draw system boundaries all the time without really thinking about it. For example when looking at conservation of energy for a light bulb we worry about the amount of electricity going in and light and heat coming out. We ignore the fact that the light bulb has gained PE when we fixed it to the ceiling.
    Last edited: May 15, 2013
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