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I'm having trouble understanding exactly what energy is.

  1. Mar 18, 2012 #1
    I know the definition of energy mathematically and how to work simple physics problems using energy considerations.However, I'm trying to look at energy from an intuitive standpoint.So far, this is what I have...Energy is essentially the ability to make things move.Saying that something has energy is saying that it possesses some trait or characteristic that gives it the ability to alter the physical condition of something else. So saying that something has kinetic energy is saying is just saying that because this object is moving, it has the ability to make other objects move.But I'm not sure if this intuitive definition directly translates to the formal definition of the capacity to do work.If energy is the ability to be a force acting through some displacement then when a moving billiard ball collides with a stationary ball and makes it move is it correct to say that the moving ball did work on the stationary ball to get it to move?Since energy is transferred from the moving ball to the stationary ball work must have been done on the stationary ball right?The part where I'm confused is all the examples of work and energy transfers in my textbook involve some constant force pushing or pulling on something over a distance and all the examples that involve the transfer of motion by collision deal with momentum and not work?But that moving ball colliding with the stationary ball is a force which is doing work on the stationary ball right?so why is only momentum talked about for these examples and not energy?
  2. jcsd
  3. Mar 18, 2012 #2
    You can look at problems like that either through conservation of momentum or conservation of energy, if you want accurate results you usually have to model problems with both.

    Your understanding of energy is pretty good, yes in situations like a billiard ball collision, even though you usually talk about momentum there, you are right in saying one ball is doing work to the other.
  4. Mar 18, 2012 #3
    Energy is fundamentally a mysterious concept. Energy is closely related to changes in time. E = hf is one example. How fast a quantum wave oscillates (frequency) is proportional to how much energy it has.

    Another example is E = mc^2 from relativity. When a particle is stationary, it's still moving through the time dimension. The "velocity" it moves at is c, the speed of light.
  5. Mar 18, 2012 #4
    Your understanding of energy is quite good.
    Energy like you said is defined as the measure of capacity to do work.(or to put in simpler terms to change physicak conditions around it )

    The reason we usually talk of momentum in cases like billiards balls is because it makes problem solving easy.During the collision the momentum remains conserved .
    However, in terms of energy some energy maybe converted into sound waves and it is very difficult to account how much useful energy was lost in this process.
    Thats why we use momentum as in momentum conservation no such problem occurs.
  6. Mar 18, 2012 #5


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    I think this is a good description:
  7. Mar 18, 2012 #6
    Fantastic post A.T.

    Till now the only plausible explanation of conservation of energy and momentum has been attributed to symmetry of space time.

    To be precise energy conservation law is based on tenporal symmetry.

    An experiment perfomed today should yield exactly the same result as an experiment performed a day later.
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