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.