this question is from the book AP Physics C A mass m1 initially moving at a speed v0 collides with and sticks to a spring attached to a second, initially stationary mass m2. the two masses continue to move to the right on a frictionless surface as the length of the spring oscillates. at the instant that the spring is maximally extended, the velocity of the first mass is ------------ regardless to the answer because it's not the problem i want to figure out, it's the conceptual problem.so the answer on the back is the momentum is conserved, but i wonder if the kinetic energy is conserved too(since it's spring related)? and i don't understand why they are saying" the spring force is an internal force, which does not change the net linear momentum." but i think if you throw a rock on the wall the linear momentum on the x-axis of the rock would lose, but there is no internal force on the rock on the x-axis(ignore the air resistance). in exactly, when and how would an object's linear momentum be conserved?