The Falling Ball Demo

P115/P116 Laboratory Demonstration #1

Summary:

* A given mass falls 2-8 meters.
* Its motion is recorded with a video camera.
* The motion is observed during playback on a VCR using the Stop Action and Frame Advance features.

You can click on the image to download it in gif format.

The purpose of this demo is to help students visualize and quantify the acceleration of gravity by watching a given mass fall. Further information on Theory, Apparatus, Procedure, and Helpful Hints is available. The equipment for this demo is stored in Rockerfeller rooms 302 and 302B.

Theory

If a mass is dropped from a given height, it follows the equations of motion given by the acceleration due to gravity (9.8 m/s*s). Snapshots taken at regular intervals should match up with the theoretical path of the ball. Because a video camera films at 30 frames per second, it is capable of capturing the motion of the mass falling past measuring marks. The data collected by the camera can then be analyzed quantitatively.

Apparatus

The equipment needed for this demo is:

* A video camera
* TV and VCR
* A long strip of paper (there is a ten foot ruled board with 1" markings under Rockefeller 301)
* A marker
* A ball of some sort. The ball must be large enough to view in a still frame (2" to 3" diameter).

Procedure

The video camera is set to view a sheet of paper 2 to 8 meters long with black marker lines set every ten centimeters. When choosing the length of the paper, there are a few important points to keep in mind. If the height is short (near 2 m), fall time is sacrificed and the acceleration may not be very clear. Taller heights, on the other hand, require a big room, a tall ladder and a video camera with an extra-wide lens.

The ball is held at the top black line and released. The camera records the ball's motion. The tape can be played on a VCR using the FRAME ADVANCE option to page through the images. The position of the ball in each frame is denoted by the black lines and the time between each frame is 1/30th of a second.

Helpful Hints

1) Tracking: The tracking knobs must be adjusted when working in the FRAME ADVANCE mode on the VCR to eliminate the noise and interference which causes much of the image to be obscured. A four-head VCR is highly recommended.

2) Timing: There will be an extra frame between the time the ball is dropped and the time it is first seen moving. This is because the first frame constitutes a movement of half a centimeter. Other subsequent frames constitute more significant displacements within the realm of many centimeters. This can be seen on the screen if you look carefully at the first few frames. A dropping device is currently under development to minimize the uncertainty in the first frame of motion.

3) Finding t=0: You are sure that the velocity "v" equals zero at some point. A plot of velocity versus time (remembering that the velocity is calculated halfway between frames) will give you the velocity at an arbitrary "zero frame," or let you find the time when the velocity was truly zero.

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