# Collisions in classical mechanics

1. Aug 27, 2014

### stroustroup

The only force really considered in classical mechanics is gravity. And yet, we often have problems involving collisions and friction, which are intrinsically electrical phenomena, and thus outside the scope of classical mechanics. We have laws such as conservation of momentum which is used for collision problems, but in the end this is all based on the assumption that collisions obey Newton's laws (the 3rd in particular). Also, including collisions and friction basically means accepting the electrical force, and thus Maxwell's equations, which have been shown to contradict classical mechanics (galilean invariance vs lorentz invariance).

Including friction and collisons also causes the systems to be hard to describe using Lagrangian or Hamiltonian approaches (discontinuous potentials), which is another argument that they might be "out of place" in mechanics.

And yet, any course on mechanics has a section on collision problems and friction force. Can we really consider them as classical mechanics phenomena? How would the forces causing collisons be described in a consistent way with Newton's laws?

Would mechanics be consistent if we assumed collisions did not exist (i.e. objects pass through each other) and there was no friction? Even rigid bodies would not exist then...

2. Aug 27, 2014

### Staff: Mentor

I think you're reading too much into things... Classical Mechanics studies forces acting on everyday objects at slow speeds. Under these circumstances friction can be treated as a force that is velocity dependent and that's all you need to have theory match experiment.

Of course in a more advanced treatment can bring in EM theory but that now becomes an EM problem.

3. Aug 27, 2014

### Delta²

You are right that the friction and collisions are (deep down) due to the EM force. The same holds for pressure and viscosity in fluids. But we care for the lorentz invariance only when the speeds are big enough and comparable to the speed of light. For low speeds there is nothing wrong to encompass these forces into the regime of classical mechanics.