Demos: 1D-21 Independence of Vertical and Horizontal Motions (Ballistic Cart)

A ball is pressed down against a spring in a vertical tube on a cart, the ball being held in this position by an electromagnet. The cart is then given a horizontal velocity and it passes by an electric eye circuit that releases the electromagnet and fires the ball vertically upward RELATIVE TO THE CART. The cart is low-friction so it travels at constant velocity over the length of the track. The ball drops back into the cup, illustrating that the horizontal motion of the ball is unaffected by its vertical motion.

Directions: Be sure that the cart is behind the electric-eye slot. Press the ball down into the cup. It will stay in that position. Give the cart a firm push (be careful here—a forceful push may make it leave the track).

Suggestions for Presentation: Before loading the ball, you might push the cart across the ramp to give the sense that it is approximately constant speed. When suggesting that the ball is fired "straight upward", get the students to think about "straight upward" with respect to what? In the classroom frame, the ball is not fired upward, but rather at an angle.

Applications: Remind the students that if they were on a plane moving along at constant speed, they could flip a coin or toss a peanut straight up and it would return to their hand.

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