- #1
fog37
- 1,569
- 108
Hello,
Trivial question: a system is isolated and all its internal forces are conservative. Because of Newton's 3rd law, all internal forces are pairwise and the net internal force is always zero (regardless of the forces being conservative or not) hence the system's total momentum is conserved. If all the internal forces are conservative, then also the system's mechanical energy is conserved. But it is possible for some kinetic energy to convert into potential energy and vice versa. How do the internal conservative forces manage to do work to transform kinetic energy into potential energy and vice versa? Shouldn't the net work done by the internal forces be zero?
Example: Isolated system = Earth + apple.
##F_{Ea}=## force of Earth on apple.
##F_{aE}=## force of apple on Earth ##=-F_{Ea}##.
##F_{net}=0##.
##E_{mech_{total}}=KE_{tot}+U_{g_{tot}}=constant##
Net work ##W_{net}=F_{net}\dot \Delta s=0##. The work done by ##F_{Ea}## is equal and opposite to the work done by ##F_{aE}##.
Thanks!
Trivial question: a system is isolated and all its internal forces are conservative. Because of Newton's 3rd law, all internal forces are pairwise and the net internal force is always zero (regardless of the forces being conservative or not) hence the system's total momentum is conserved. If all the internal forces are conservative, then also the system's mechanical energy is conserved. But it is possible for some kinetic energy to convert into potential energy and vice versa. How do the internal conservative forces manage to do work to transform kinetic energy into potential energy and vice versa? Shouldn't the net work done by the internal forces be zero?
Example: Isolated system = Earth + apple.
##F_{Ea}=## force of Earth on apple.
##F_{aE}=## force of apple on Earth ##=-F_{Ea}##.
##F_{net}=0##.
##E_{mech_{total}}=KE_{tot}+U_{g_{tot}}=constant##
Net work ##W_{net}=F_{net}\dot \Delta s=0##. The work done by ##F_{Ea}## is equal and opposite to the work done by ##F_{aE}##.
Thanks!