New trends in physics teaching, v.4; The ... - unesdoc - Unesco

Dropping a string of marbles
String and tape experiments
This technique for examining acceleration may be applied in a slightly different fashion. You
require five marbles, a piece of string and some sticky tape. The string should be as high as the
room, which we wil suppose to be 2.5 m. The marbles are taped to the string at intervals proportional
to the squares of the whole numbers, i.e.,
Number 0 1 2 3 4
Square 0 1 4 9 16
Distance 0 15 cm 60 cm 135 cm 240 cm
Difference 15 cm 45 cm 75 cm 105 cm
Now stand on a chair, holding the string as shown in figure 5. The bottom marble should not
quite touch the floor. Drop the string and listen to the clicks. They are more audible if you drop
the string into a trash can, or onto a metal plate.
You can repeat the experiment with a string having marbles spaced at uniform 50 cm intervals.
Do you hear the time between clicks get shorter as the higher marbles from this last string strike
the floor? Qualitatively, the higher marbles have been accelerated for a longer time, and are
travelling faster, covering the same distance in a shorter time as they approach the floor than do
the marbles starting near the floor.
Quantitively, we have the familiar formula
1
distance = 3 g (time)2
We spaced the marbles on the nonuniform string so the square roots of successive distances are
proportional to whole numbers. The time taken between successive clicks should then be constant,
about 0.175 s. Shift one of the marbles up or down the string to test the sensitivity of your ear
to the time between clicks. A change of 20 per cent is easily detectable.
The thermal expansion of a drinking straw
This experiment on expansion requires three plastic drinking straws, sticky tape, very hot water,
a pencil, a piece of card (or paper) and a cup. Bind two straws together very tightly along their
hot water
taDe
hot straw
bl
Figure 6.
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