There are many ways of demonstrating an elementary science unit on force and motion.

The physics of motion is important in and of itself, but the principles of force and motion also underlie some of the most fundamental physical interactions. A few examples will provide a starting point for your own demonstration. In one such example, a few billiard balls and a board can help demonstrate force and motion.

Level the laminated board. Place a billiard ball in the center of the board, and have the students describe the motion - or lack thereof - of the ball. Ask them why they think it's not moving. This is a demonstration of Newton's first law: a body at rest tends to remain at rest.

Slightly lift one edge of the board. Have the students observe the motion of the ball, and ask them why it is moving. It is likely someone will say "because the board is tilted," but this is not an explanation. The board is moving because a portion of the force of gravity is now along the direction of the board, and is not opposed by the strength of the board - as it was when the board was level. This is the other part of Newton's first law: a body remains at rest unless a force acts on it.

Level the board with the grooved side up. Place a wooden wedge at one end so the ball will slide down into the groove. Put the ball just 1 or 2 inches up the wooden wedge.

Place the timer where the students can read the elapsed time. Release the ball and have the students count the seconds out loud. With each second, mark the location of the ball on the paper. Have the students observe the distance between each of the marks. The distance between each pair of points will be the same. Ask the students why. The answer is that there is no force acting on the ball on a level board, and there is nothing to change its speed.

Set up a wedge on each end of the board and have two students place the ball at the same mark on the wedge. Line up the two wedges so the balls will enter the groove from each end. Have the students release the balls simultaneously and observe the motion. The balls will meet in the middle and stop. This demonstrates the conservation of linear momentum.

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  • It can be difficult to level the board in both directions. If you use the grooved side of the board then the leveling isn't quite as critical.
  • You can do a variety of experiments with the same apparatus: collide two balls when one is released from higher on a wedge. You can also release the balls from the same height but at different times. Have one stationary ball in the middle and release another ball from a wedge - there are many variations that illustrate important physical principles.

Things Needed

  • Billiard balls
  • Smooth laminated board, grooved lengthways on one side
  • Wooden wedges, marked at several heights
  • Construction level
  • Paper taped along the edge of the board
  • Pencil
  • Timer

About the Author

First published in 1998, Richard Gaughan has contributed to publications such as "Photonics Spectra," "The Scientist" and other magazines. He is the author of "Accidental Genius: The World's Greatest By-Chance Discoveries." Gaughan holds a Bachelor of Science in physics from the University of Chicago.