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Questions about kinetic energy, thermal energy and momentum

  1. Nov 29, 2011 #1
    Hello, kinetic energy is in some cases a bit of a mystery to me, I've made several assumptions about these things. But some of those assumptions seem to conflict a bit with what seems to be happening, so I thought I'd throw these assumptions out here to maybe hear where I'm wrong. Any comments would be greatly appreciated.

    - Thermal energy is kinetic energy in that it is kinetic energy which differs from the total objects reference frame. You can't speak of thermal energy in single particles because there won't be a difference.

    On single particles:

    - It takes work to both accelerate and decelerate a particle.
    - Force applied from the front of the particle slows it down, force applied from the back speeds it up. Force applied directly from the side changes the direction. Force applied from any other side changes both direction and speed.
    - The amount of work that a particle could do on another particles speed depend on their momentum's. Higher momentum particles would do more work on other particles by spending more of their energy than low momentum particles would. As both particles act on each other, the speeds and directions of both particles may change. The total momentum always remains the same.
    - Work can also be done on particles by other means such as magnetic fields. These can change the momentum and direction of particles. The particles also work on the magnetic fields source.


    For larger objects:
    - If two large objects collide, any kinetic energy that does not change the direction or momentum of the objects can dissipate away through sound, thermal energy in the objects or even parts of the object being flung away. If two unbreakable objects would collide in a vacuum, all energy that gets transformed gets transformed into thermal energy. The total momentum would not have to stay the same.
    - As thermal energy is kinetic energy, and because the particles within that behave more randomly have more momentum it should technically be possible to speed an object up by expending thermal energy. It should also technically be possible to slow an object down by increasing the randomness to heat it up. Of course you're dealing with entropy and such here, so no actual device might ever be capable of doing this.
    - Slowing down a larger object generally warms it up if the energy has no where else to go.
    - A process in which all particles are pushed/pulled towards the same direction will speed the object up. Would it also cool it down if the push/pull is constant because all the randomness seems to get evened out?
    - A random process in which all particles are continually pushed/puled from random directions will heat the entire object up.
    - Acceleration and deceleration itself do not necessarily cost energy for a different reference frame. Earth doesn't stand still in space, yet it rotates around its axis so the side of the earth that turns towards the side earth is moving to appears to be accelerating in some reference frames, while the side moving towards the side earth came from appears to be decelerating. Only within the reference frame of the individual particle would momentum changes cost work.
     
  2. jcsd
  3. May 1, 2012 #2
  4. May 2, 2012 #3

    haruspex

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    "It takes work to both accelerate and decelerate a particle."

    No. Work done is ∫F.ds, where F and s are both vectors. If in exactly the same direction then it's the product of the magnitudes. If at right angles there is no work done - e.g. a planet in a circular orbit. If opposing the work done is negative, which means the decelerating object is doing work on the source of the force.

    "The amount of work that a particle could do on another particles speed depend on their momentum"

    It depends on energy and momentum. Consider the simple example of a straight line collision. Each particle has a speed afterwards, so there are two unknowns, and you need two laws to pin down what will happen.

    "If two large objects collide, any kinetic energy that does not change the direction or momentum of the objects can dissipate away ... The total momentum would not have to stay the same."

    Momentum is conserved. It won't be dissipated unless there are fragments breaking off that you're not including in the calculation.

    "it should technically be possible to speed an object up by expending thermal energy"
    Again, momentum is conserved, so you need to send something in the opposite direction. Also, because of the laws of thermodynamics, you won't be able to use thermal energy unless you have a heat sink (a lower temperature body).

    "Would it also cool it down if the push/pull is constant because all the randomness seems to get evened out?"

    I don't see why a general tug on a collection of particles would alter their motions relative to each other.

    "Acceleration and deceleration itself do not necessarily cost energy for a different reference frame"
    If you have inertial frames (constant velocity) then all the rules still work. If you take as reference an accelerating/rotating frame things can look very odd.
     
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