There are two long rods hanging from a shorter cross bar. Spread these long rods about five inches apart. There are marks on the cross bar showing where to position them.
One long rod is marked with a single line at the bottom, and the other is marked with a double line at the bottom. These allow you to distinguish between them.
Quiet each of the long rods so that they are hanging straight down and not moving.
Now draw one of the long rods straight back, as shown by the arrow on the table, about half the distance to the edge of the table and release it.
Watch what happens to the two rods over several minutes.
A pendulum is an object that hangs from one end and can swing forward and backward. The two long rods in this exhibit are examples of pendulums. When one of the long rods is started swinging it has momentum that wants to make it keep swinging. When it reaches the end of its travel its momentum tugs on the cross bar, which causes it to move. This momentum in the cross bar in turn tugs on the other long rod, causing it to move. In review, at each end of its swing the long rod you started in motion gives some if its energy to the second rod.
Almost every playground has a number of pendulums side by side on a swing set. There are two significant differences, however. The swing pendulums are flexible chain rather than stiff rods, and they do not have a moveable connecting cross bar.
To make a simplified coupled pendulums, stretch a string between two table legs. Bend two paper clips into an S shape. Tape the lower half of each paper clip to the top of a pencil, then hang the pencils from the string. To make a moveable cross bar, bend another two paper clips into S shapes and use them to suspend a third pencil horizontally from the string.
You can conduct a number of experiments by:
When you started one of the long rods swinging back and forth, it slowly gave up its moving or kinetic energy to second long rod. Based upon your observations, explain why one of the rods could be called the "leader" and the other the "follower." Do these labels change over time?
Start one rod moving. When it reaches the end of its swing, it gently pulls the other rod in that direction. But the first rod is already moving in the opposite direction. When the first rod then reaches the other end of its swing, it gently pulls the second rod in that direction. The first rod is always changing direction before the second rod, so it could be considered to be "leading" the second rod. Eventually the first rod has given all its energy up and is stationary. Now the process repeats, but the roles of leader and follower are reversed.
Is energy conserved in this exhibit? Why or why not?
When you start one rod in motion you have given it energy. Over time the two rods exchange that energy back and forth, but if you wait long enough eventually they both are stopped.
If you think just in terms of this exhibit, energy appears to be lost. However, energy is never created or destroyed - it merely changes form. When you started a rod in motion you brought energy into the exhibit. The rods eventually stop as a result of friction:
A two hundred pound adult is sitting in one of the swings in a playground swing set. A ten year child old wants to make the adult swing back and forth, but isn't strong enough to lift the adult. Can you suggest two different ways the child might over time make the adult swing back and forth?
The weights at the bottoms of the long rods might or might not be magnets. What can you observe and what can you observe in this regard?
By holding the bottoms of the two rods together, you can observe that the two sets of weights do not exhibit any magnetic effect on each other. You might want to infer that therefore they are not magnets, but can you be sure they are not magnets? Perhaps they simply do not exhibit any observable magnetic effect, and might be very weak magnets.
How many different ways could you add energy to offset the friction losses? Where in the exhibit could energy be added?
The easiest way to add energy would be to give the bottom of one of the long rods a slight push each time it reached the end of its swing. Over time, however, that would effect the action of the exhibit, because the roles of leader and follower change. Another place to add the energy would be to give the cross bar a gentle push. Would you suggest the push be given to an end of the cross bar or the middle? The best way to give a regular push would be to use a motor and revolving arm that is timed to give a slight push each time at just the right instant.
If starting the exhibit consists of drawing back one of the long rods a distance of ten inches and releasing it, would you expect the exhibit to behave differently if it were placed at the equator or at the North Pole? What if it were placed on the surface of the moon?
When you started one of the long rods in motion, you drew it back, in effect lifting it. When it was released, gravity pulled it back towards its resting position.
Gravity is a constant force directed towards the center of the earth. To an observer on the moon, the direction of Earth's gravity is different at the equator than at the North Pole, but the exhibit will behave the same in both locations.
The force of gravity is less on the moon, so when a long rod is drawn back and released it will swing more slowly, but otherwise the action of the exhibit would basically be the same. The one significant difference is that the moon does not have an atmosphere, so there would be no air friction and the exhibit action would continue for a longer time than on the Earth.
challenge question 7 needed
challenge question 7 answer
Friction, gravity, pendulum, resonance, period, energy transformation, potential energy, kinetic energy, momentum
further information listing needed
This exhibit is included in the 1982 NOVA broadcast on the Exploratorium.
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