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Law of Conservation of Momentum: Marble Collisions

By — John Wiley & Sons, Inc.
Updated on Dec 13, 2010

If you have ever played pool or marbles, you have probably noticed that when a moving ball strikes a resting ball, the moving ball stops and the resting ball is set into motion. This phenomenon is due to the law of conservation of momentum, which states that the total momentum of objects that collide with each other is the same before and after the collision. Momentum affects how hard it is to stop an object that is already in motion. Momentum depends on the mass and velocity of the object and is affected by friction. If friction were completely eliminated, the total momentum would be completely conserved in collisions. In this activity you will observe the conservation of momentum in marble collisions.

Materials

Ruler with a grooved top

Four marbles of equal size

Book

Law of Conservation of Momentum

Activity

  1. Place the ruler flat on a table with the grooved top facing up. The ruler should be placed with the zero end near you and the other end away from you.
  2. Place one marble in the grooved section about halfway down the ruler.
  3. Place a book at the far end of the ruler to keep the marble from rolling off the table.
  4. Put the second marble on the zero end of the ruler. Flick the marble on the zero end with your finger so it rolls down the groove and strikes the marble at the center of the ruler. Observe what happens.
  5. Repeat these steps, but this time place two marbles at the center of the ruler.
  6. Continue to vary the number and arrangement of marbles and observe what happens.

Follow-Up Questions

  1. What happened to the single center marble when it was struck by one marble? What happened to the marble that struck the one marble at rest?
  2. What happened in the other collisions? How does this activity confirm the law of conservation of momentum?

Answers

  1. It moved forward. The marble that was flicked with your finger came to a stop once it set the other marble into motion.
  2. Answers will vary. When two marbles are moved forward together and they strike two resting marbles, the two resting marbles roll off together and the two you flicked with your finger stop. The law of conservation of momentum says that momentum before a collision equals momentum after the collision, so the marbles set into motion by your finger transferred their momentum to the marbles being struck farther down the ruler.

Extension

Get some large marbles and use them to strike smaller marbles. Observe what happens. Try using the small marbles to strike the large ones. Describe how the law of conservation of momentum explains the behavior of the marbles.

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