A bullet of mass 0.01kg is fired horizontally into a 4kg wooden block at rest on a horizontal surface. The coefficient of friction between the block and the surface is 0.25. The bullet remains embedded in the block and the combination moves 20m before coming to rest. With what speed did the bullet strike the block?
To get the answer, we are supposed to apply the conservation of momentum.
Why? Isn't the frictional force acting as an external force in the given situation?