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Discuss the information in the Teachers Text with the students about
Newton's Third Law of Motion.
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Place the two rulers on a surface that the coins will slide on
easily. A plastic counter or table is ideal for this. Make sure the
rulers are parallel to each other. Use double-sided tape or a piece of
modeling clay to keep each ruler firmly in position.
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Lay five identical coins in line between the two rulers, making sure
that each one touches the next. Arrange them so that they are about one
inch in from one end of the rulers.
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Now place another identical coin at the entrance to the gap between
the rulers.
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Carefully flick that coin toward the coins between the rulers, making
sure it hits them dead center. The whole group will move a little, but
the end result will be that the coin on the other end flies off.
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Repeat the experiment several times, flicking the coin harder or
softer and watching the result. The coin on the end will fly off more or
less depending on how hard the first coin is flicked.
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In theory, the row of coins shouldn't move when hit. But in practice
this isn't so, as you will notice.
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But the coins do not have to move in order to pass on the force. Ask
a friend to hold down one of the coins in the row (but not the one on the
end) and try flicking the coins again. You'll find that the one at the
end flies off as before - the force is still passed on.
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Now try flicking two identical coins against the row to see what
happens. This time two coins will fly off the end. No matter how many
identical coins you knock into the row, the number of coins flying off
will be exactly the same.
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Also, you can do this experiment with coins of different weights. If
you flick a heavy coin against a row of identical coins which has a light
coin at the end, you will find the one at the end flies off very quickly,
followed by one of the heavier coins.
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