JEE Advanced · 2009 · 12 Apr 2009 · Paper 1 · Conservation of Momentum

When the net external force on a system is zero, by Newton's second law, $\frac{d\vec{p}}{dt}=0$, so the linear momentum $\vec{p}$ is conserved. However, internal forces can still do work, changing kinetic energy. Angular momentum is conserved only if the net external torque is zero, which is not guaranteed by zero net force alone. Potential energy depends on the configuration and can change due to internal forces. Therefore, only the linear momentum is surely conserved.