Demos: 1D-20 Independence of Vertical & Horizontal Motions (Drop-Kick)

This device demonstrates the independence of vertical and horizontal motion

Directions: Two large bocce balls are placed on a horizontal wooden platform. When the spring on the projecting device is released, one ball is projected horizontally while the other is dropped to the floor. By listening to the "clicks" as the balls hit the floor, it is evident that they reach the floor simultaneously.

Note: There are two different apparatuses available to demonstrate this phenomenon. The older one consists of two small steel balls, one of which has a hole drilled through its center. One ball is placed on a horizontal rack in front of the spring-loaded rod, whereas the other is pushed onto the backend of the rod by means of the hole in the ball. The spring-loaded rod is released, causing the front ball to be projected forward while the rear ball drops off the rod to the floor. The rod can be "loaded" in two different positions to give two different horizontal velocities, illustrating that horizontal velocity is not a factor in the time of fall.

Students usually find the newer one to be more dramatic.

Suggestions for Presentation: Point out to the students that the projected ball has a greater travel distance (suggesting that maybe this one will require a longer time to reach the floor.). You might ask for a vote on how it will turn out.

After presenting the demo, place on the Overhead projector the transparency which contains a strobe photo of a similar demonstration. Note that the strobe picture (included in almost all texts) illustrates nicely that the vertical fall time is the same. However, it is not as evident that the horizontal motion is uniform. A grid overlay can show this nicely.

Applications: A popular extension of this demo is to ask which would hit the ground first—a bullet fired horizontally from a rifle or a bullet dropped simultaneously from the same position. Students often will not believe that the two will fall at the same rate.

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