You should already have completed Creating your Calibration Graphs.
(This experiment uses rubber bands, calibration graphs, plastic bag, paper clips, measuring strip, which you should have received with your CD purchase; if not pleas email instructor right away)
Important: Never stretch a rubber band to more than 140% of its original length. If you do all calibrations will be invalidated.
Note for Spring 2002 Students: If you have already done the Introductory Force Experiment in Assignment 2, you need not complete this activity.
See the Introductory Force Experiment video clip on the GEN 1 CD, then proceed as follows:
Set up the experiment and collect your initial data:
1. Use rubber band #4. Under no circumstances allow the rubber band to stretch out to more than about 140% of its original length.
2. Hang the rubber band from a support (e.g., a table edge) as demonstrated on the video clip, using a paper clip bent into an appropriate shape. As shown on the video clip, suspend the plastic bag from the other end of the rubber band using another paper clip. Measure the length of the rubber band.
3. Carefully add 1/2 cup of water to the plastic bag, being careful not to spill any, and measure the length of the rubber band from clip to clip. If you don't have a 1/2-cup measuring cup you may use eight full tablespoons for 1/2 cup. Be sure to specify if you use something other than a measuring cup.
Determine the weight of the water:
1. A cup of water has a weight of eight ounces. It takes 16 ounces to make a pound. A kg of water would have a weight of 2.2 pounds, and each kg has a weight of 9.8 Newtons. What therefore is the weight of a half-cup of water? (Your answer should be reasonably close to but not exactly equal to 1 Newton; if you can't get the accurate result use 1 Newton for the remaining questions, but note that if you do it will be necessary later to slightly modify your results).
2. Use your calibration graph or your calibration function to determine what therefore would be the length of rubber band #4 if exactly 1/2 cup of water was suspended from the rubber band in a very light plastic bag suspended from a rubber band.
3. What would be the weight of 2, 3 and 4 half-cups of water?
4. What would be the length of the rubber band if it supported 2, 3, or 4 half-cups of water?
Obtain additional data:
1. Now suspend each of the weights, 1, 2, 3 and 4 half-cups from the rubber band and measure the length corresponding to each of these weights.
2. What force is indicated by your calibration graph or calibration function for each of these lengths?
3. What is the percent difference between the actual weight hung from the rubber band and the force indicated by your calibration graph or function?
Hang weight from a chain of rubber bands and obtain data:
Rubber bands acting in parallel: If we attach two rubber bands between the same pair of paper clips, then if we suspend a weight from the system or in some other manner pull the paper clip apart so that the rubber bands are stretched, then the two rubber bands are said to be working in parallel.
Rubber bands acting in series: If we attach two rubber bands so that they form the chain, for example stretching one rubber band between two paper clips, then adding another rubber band to the end of one of the paper clips and stretching it out to another paper clip, or however we can word this, then the bands are said to be working in series. We wish to investigate the difference between series and parallel combinations of rubber band. The main question is, if a system of rubber bands is in series or parallel supports a certain way to or in some way exerts a restoring force then how do they split the force between or among them?
Hang a plastic bag containing half a cup of water from the first rubber band, and using another paperclip bent into a hook suspend this system from the second rubber band, and suspend this system from the third rubber band, etc., so that the water is suspended from a chain consisting of rubber bands #1 - #4. This chain is said to be a series combination of rubber bands.
1. Measure the length of each rubber band and put your data into a table.
2. How much force does each rubber band exert, according to your calibration information and to the lengths you measured?
3. How much does the water weigh?
4. How do you think the force on each rubber band depends on the weight of the water?
Add 2 more half-cups to the bag. Hang the bag from rubber bands #2 and #3, with the rubber bands stretched in parallel between a single pair of paper-clip hooks.
1. What is the length of the two rubber bands (being hung from the same hook they should have identical lengths)?
2. How much force does each rubber band exert, according to your calibration information and to the length you measured?
3. How much does the water weigh?
4. Which rubber band would you say is the stiffer of the two?
5. How do you think the forces exerted by the rubber bands depends on the weight of the water and on the stiffness of the rubber bands?
Think about what's going on here:
1. What makes the rubber band stretch?
2. Why does the rubber band stretch more when more water is added to the bag?