# Problem understanding the conceptual solution to a deformable system problem

A particle is suspended from a post on top of a cart by a light string of length L as shown in (a). The cart and particle are initially moving to the right at some constant speed, with the string vertical. The cart suddenly comes to rest when it runs into and sticks to a bumper as shown in (b). The suspended particle then swings through an angle θ. If the bumper is still exerting a horizontal force on the cart when the hanging particle is at its maximum angle forward from the vertical, at what moment does the bumper stop exerting a horizontal force?

According to the book, the bumper stops applying a force when the particle swings back to its lowest point. This leftward force is necessary to reverse the rightward motion of the particle and accelerate it to the left. However, I'm having trouble understanding why would the bumper keep exerting a force even after the particle has reached its maximum angle? I just don't see how that's possible. Wouldn't the bumper stop exerting a force the moment the total momentum of the cart+particle system becomes zero?