To demonstrate that the horizontal motion of an object is unaffected by vertical motion
Locking pin with string
First place the metal ball into the cart. Press down on the ball and spring and then use the pin with the string attached to lock the spring in place. Take the string in your hand and push the cart with the same hand. Pull on the string with a quick hard jerk.
Before you start this demonstration, make sure that the surface you are going to use is completely flat. Also make sure that you have some way of stopping the cart before it either falls off the table or collides with a wall because the cart usually will just keep going. Also when you pull the string out make sure that you pull directly back along the line of motion or there may be a sideways force that will cause the ball to miss. Some practice may be needed.