Thread: Energy Question

Hi, first-time-poster here.

I was wondering, when a particle is actually set into motion, or given kinetic energy, what actual force (I'm hoping for a specific particle or force) is transfered from one body to another? It can't be electrons, as most high school teachers will tell you is the energy that drives anything, because 1) protons are stripped of their electrons and are propelled to great speeds when in an atom smasher, 2) electrons themselves are sped up in atom smashers, 3) photons are sped up in atom smashers.

If you're still confused of what I'm asking, here's an example:

When I punch a punching back, what am I actually transfering over to the punching bag? Since all quantum particles can be put into motion or are capable of moving, aka given kinetic energy, what actually does this?

Thanks.

Just kinetic energy. Some people (and I am not one of them) like to call it relativistic mass.

Force is not something which is transfered from one body to another.

I suppose you could be thinking about the fact that many fundamental forces have an exchange particle associated with them.

When you punch the puching bag it is the electrical repulsion between the electrons around the atoms than can be considered responsible for the transfer of energy and photons are the exchange particle associated with the electrical force. But no physicist thinks of the transfer of kinetic energy as a transfer of photons, but just as a transfer of kinetic energy.

But on the quantum level, how is kinetic energy transfered (sorry for the term - how does it come about?) if some particles are not receptive to the EM force?

Why do you want to change forces? The EM force is the simplest with a repulsion force, and that is the force by which normal matter interacts in collisions. So let me elaborate with the EM force first before I consider other forces.

If a Helium atom with velocity 1.0 m/s collides head on with a Hydrogen atom which is stationary then the Hydrogen atom will carry off 64% of the kinetic energy that was in the Helium atom at a velocity of 1.6 m/s, while the Helium atom will have slowed down to a velocity of 0.6 m/s.

In more detail, as the Helium atom approaches the Hydrogen atom, the Hydrogen atom begins to experience an electrostatic force (due to the repulsion of electrons) away from the Helium atom causing it to start moving away from the Helium atom (which happens to be in the same direction that the Helium atom is moving) and the Helium atom experiences an electrostatic force away from the Hydrogen atom which is opposite its motion and therefore causes it to slow down. The electrostatic force acts on the Hydrogen atom much like the spring in pinball machine acts on the ball pushing it away from the Helium atom at a speed greater that the Helium had to begin with.

As I said before, if you like, you can think of photons carrying the energy from the Helium atom to the Hydrogen atom, but it is not like there are now little photons attached to the Hydrogen atom. When a photon hits a charged particle it simply accelerates the charged particle and vanishes and all that is left is an increase in the charged particle's kinetic energy.


If you want to involve other forces then you have to spend a couple of years learning Quantum Field Theory, because then the particles are quite likely to exchange a bit more than just kinetic energy. For example if the Helium atom is move at a velocity near the speed of light when it hits the Hydrogen atom then it will push right through the electron shells and then over come the repulsion of the positively charged nuclei and then the much more complex strong and weak nuclear forces will come into play and depending on just what the velocity of the Helium atom was you are likely to get all sorts of particle interactions. The end result is that in addition to smashing the Helium atom in to component particles you will convert a lot of the kinetic energy of the Helium atom into massive particles and antiparticles speeding away from the collision in all directions. This is the sort of thing they do in particle accelerators.

Ok. Well, I guess my yearn can't be satisifed on this one. If it's EM force that causes the repulsion and movement of an object, how can a fundamental particle such as a photon change its motion or a gluon be set into motion if they are not receptive to the EM force?