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This week's LabZone activity
Feb. 11, 2004
"Stopping" a Spinning Fan
Things you will need:
- electric fan with a protective screen
- television screen or computer monitor
Turn on an electric fan. Be sure the fan has a protective screen so there is no danger of your fingers being cut by the fan blades. Notice that you cannot see the blades because they are spinning too fast. Now place the fan in front of a television screen or a computer monitor and turn on the set or monitor in a darkened room. You'll find that you can see the fan blades. They appear to be either stopped or spinning slowly forward or backward.
If the speed of the fan blades can be adjusted, change the speed until the blades appear to be stopped or very nearly so. What happens if you slowly increase the fan's speed? If you decrease the fan's speed?
If the speed cannot be changed, turn off the fan and watch the blades in front of the screen as they slow down from top speed to zero. You will see the blades appear to stop, rotate slowly forward or backward, stop again, rotate slowly again, and so on, until the blades finally stop.
Although we cannot it see it because it happens so fast, a television screen or monitor emits light intermittently like a strobe light at intervals, say, 1/50 second apart. If the light comes on 50 times per second and the fan is rotating at 50 turns per second, the fan blades will always appear to be at the same place. If the fan makes 100 rotations per second, it will still appear stopped but will make two turns between light flashes. Assuming the fan blades are symmetrical, the fan will also appear to be stopped if it makes 25 or 12.5 turns per second because it will have made half or a quarter of a turn between light flashes.
Suppose, however, that the fan is making 51 turns per second. It will then appear to be turning slowly forward because it will complete slightly more than one turn between light flashes. Similarly, if it is rotating 49 times per second, it will appear to move slowly backward because it makes a little less that one turn between light flashes.
Why do you think the images of a rotating fan seen in front of a television screen or computer monitor have a wavy appearance?
If you have, or can borrow, an adjustable-rate strobe light, you can measure the rotational speed of a great variety of objects.
Exploring on Your Own
How can you use a strobe light and any additional equipment you may need to measure the speed of a toy train moving along a straight track?
Reprinted with permission from Science Projects About Physics in the Home by Robert Gardner. © 1999 by Enslow Publishers (www.enslow.com).
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