To investigate the concept of tension in a string:
The concept of tension in a string can be very difficult for beginning physics students to grasp. (Actually, it can be a very difficult concept for advanced physics students to grasp...)
If you hold a length of string taut between your hands, any force that you exert on one end of the string is transmitted through the string to the other hand. This "force" transmitted by the string is called the tension in the string. How does the tension in different parts of the string compare?
Suppose you hang a 5 Newton weight from a string, and hold the other end of the string in your hand. If the weight (and the string and your hand) is at rest, then the weight exerts a 5 Newton downward force on the lower end of the string, and you exert a 5 Newton upward force on the upper end of the string. What is the tension in the string? It is possible to build very plausible arguments that the tension in the string is 10 Newtons, or that it is 0 Newtons, or that it is 5 Newtons - but what is it, really, and why?
A key to the puzzle is the realization that tension is not force. A force always has a very definite direction - up, to the left, North, etc. The tension in a string or rope must follow the rope! The tension may have to extend around corners, over and under pulleys, etc. So, tension transmits a force through a string or rope, but tension is not force. Tension doesn't work exactly the way force does.
In this activity, you will use rubber bands as "miniature spring
scales". The rubber bands can be inserted at various points in the
string to measure the force at that point - without adding a lot of
extra weight, as a "regular" spring scale would do.
1/2 meter of string |
ruler or meter stick |
three identical rubber bands |
pen |
small mass - 0.5 to 1.0 kg. |
spring scale |
ring stand |
ring stand clamp |
c-clamp |
four paper clips |
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Set up each situation in the table below, and record the spring
scale reading, as well as the stretch of each rubber band, where
appropriate. In the table, "band1 + string + band2" means "Hang a
rubber band from the spring scale (which is always attached to the
ring stand). Use a paper-clip hook to suspend a string from this
rubber band. Use another paper-clip hook to suspend a second rubber
band from the string. Attach the mass to the lower rubber band." Get
it?
Trial |
Setup |
Spring Scale |
Band 1 Stretch |
Band 2 Stretch |
Band 3 Stretch |
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Scale (only) |
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Scale + string |
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Scale + band1 |
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Scale + band1 + band2 |
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Scale + band1 + string + band2 |
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Scale + band1 + band2 + band3 |
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Scale + band1 + string + band2 |
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