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PASPORT Force Sensor
(PS-2104)
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ScienceWorkshop
Force Sensor
(CI-6746)
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Purpose:
Explore the connection between the area of contact and the friction force.
Background Information:
In this third installment on friction from Edward Nodding, we investigate the relationship between friction and the area of contact between an object and a surface. In the lab, students will observe the result of doubling the area of contact without changing the mass of the object.
Students will rediscover that the friction force depends on the normal force (See also Lab 2 from the June, 2002 Newsletter) but does NOT depend on the area of surface in contact nor on the pressure (force per unit area) between the two surfaces.
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Equipment and Supplies:
For each lab group:
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Software and Probeware Setup:
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Tie a short string from the hook of the Force Sensor to the eyelet of the friction block.
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Set up your work space so that you have a clear area on your table or lab bench to drag the friction block with the sensor for several centimeters.
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Ensure that your USB Link, Xplorer or ScienceWorkshop 500 interface is connected to the computer and connect the Force Sensor to your computer interface.
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Data Collection & Recording
Students will follow the on-screen instructions. Hint: to move through the pages in the Electronic Workbook, use the navigation buttons at the bottom of the page: 
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Data Analysis:
Students will compare data from two trials: the first trial with one unit of area (skinny side of friction block touching table) and the second trial with two units of area (flat side of friction block touching table).
The key concept for students to recognize is that, although the area and pressure differs in each trial, the normal force is the same in each case. Students may view pressure as "how hard the surfaces press together." Be sure to define pressure as equal to force per unit area. For the first trial there is a pressure of 20 newtons per one unit of area. For the second trial there is a pressure of 20 newtons per two units of area. Tell students to imagine the block of two units of area as being cut in half, as if it was really two one-unit blocks. Each of the one-unit blocks has half of the total weight (half of the normal force): a force per unit area of 10 newtons. Recall from Lab 2 (June, 2002 Newsletter) that the friction force depends directly on the normal force. If each of the blocks has half of the normal force, then each will have half the friction of the first trial, and since there are two of them the frictional force in trial #2 will be equal to that of trial #1.
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Conclusions and Extensions:
- Using the smooth side of the friction block, we have investigated how things work in an idealized world. Try the experiment again with the fuzzy side of the friction block. How do the results differ?
- At the microscopic level, what could account for the existence of friction between two different surfaces?
- Do the results of this experiment support the use of wide tires on vehicles as a safety feature? Or are there other reasons a car manufacturer might decide to use wider tires?
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