- #1

Soren4

- 128

- 2

Linear and angular momentum of the system are always conserved, while the kinetic energy of the system is conserved only if internal forces acting in the collision are conservative. This last point is not clear to me.

During the time interval [itex]\tau[/itex] of the collision we assume that the position of the two particles does not change, i.e. [itex]\vec{r_1}=\vec{r_2}=\vec{r}[/itex]. But the velocity of each particle (precisely its momentum) does change during the collision, and, from WE theorem, that means that

*is done on each particle, equal to the change in kinetic energy of the particle itself.*

**a work**Nevertheless work is force times a displacement, and we said that the position of particle is constant during the collision. So where is the displacement?

Supposing that I didn't made mistake in this reasoning, if there is no displacement there is no work, so how can the velocity of particles change actually?

And then if there is no work how can kinetic energy of each particle (and of the system) not be conserved?