A particle is suspended from a post on top of a cart by a light string of length L.  The cart...

A particle is suspended from a post on top of a cart by a light string of length L.  The cart and the particle are initially moving to the right with constant speed Vi with the string vertical.  The cart suddenly comes to rest when it runs into and sticks to a bumper.  The suspended particle swings through an angle Θ.  Is the bumper still exerting a horizontal force on the cart when the hanging particle is at it maximum angle from the vertical? At what moment in the observable motion does the bumper stop exerting a horizontal force on the cart?