3.5. LAW OF CONSERVATION OF MOMENTUM
Marble Collisions
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
Activity
Book
One
marble
Two
marbles
1. Place the ruler flat on a table with
Table
the grooved top facing up. The
Ruler
ruler should be placed with
End
Flick
the zero end near you and the
here
other end away from you.
2. Place one marble in the grooved
section about halfway down the
FIGURE 3.2. Law of Conservation of Momentum
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?
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.
Copyright  2010 by John Wiley & Sons, Inc., Energy of Motion
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