Name: _______________________
If you try to slide a heavy box resting on the floor, you may find it difficult to get the box moving. Static friction is the force that counters your force on the box. As you increase the force you apply, eventually the box will move. The friction acting on the box while it is moving is called kinetic friction. In order to slide the box with a constant velocity, a force equivalent to the force of kinetic friction must be applied. Kinetic friction is sometimes referred to as sliding friction.
Both static and kinetic friction depend on the surfaces of the box and the floor, and on how hard the box and floor are pressed together. In this experiment, you will use a Force Sensor to measure the peak static friction force and the kinetic friction force as a function of the normal force on the block. In each run, you will pull the block as described here, but by changing the masses on the block, you will vary the normal force on the block.
1.
Distinguish between the peak static friction force and the kinetic friction force.
2.
Determine the relationship between the maximum value of the static friction force and the normal force applied on an object.
3.
Determine the relationship between the value of the kinetic friction force and the normal force applied on an object.
4.
Determine the coefficients of static and kinetic friction for a particular block and surface.
Computer, string, Vernier interface, block of wood with hook, logger pro, four 0.5kg blocks,
Force sensor set to 50 N range
1. In pushing a heavy box across the floor, is the force you need to apply to start the box moving greater than, less than, or the same as the force needed to keep the box moving? On what are you basing your choice?
2. How do you think the force of friction is related to the normal force applied to the box?
3. Draw and label a FBD of a book as it is sitting at rest on a table and you are pushing it gently to the right (gently enough that it isn’t even moving at all).
1. Open Logger Pro. Open File 12a Static Kinetic Frict.
Connect the Force sensor to channel 1 of the Lab Pro.
2. Tie one end of a string to the hook on the Force Sensor and the other end to the hook on the wooden block. Place a total of 1 kg in mass on top of the block, fastened so the masses cannot shift. Practice pulling the block and masses with the Force Sensor using this straightline motion: Slowly and gently pull horizontally with a small force. Very gradually, taking one full second, increase the force until the block starts to slide, and then keep the block moving at a constant speed for another second.
Mass
Wooden block
Pull
Physics with Computers 12 - 1

Experiment 12
Figure 1
3. Hold the Force Sensor in position, ready to pull the block, but with no tension in the string.
Click to set the Force Sensor to zero.
4. Click to begin collecting data. Pull the block as before, taking care to increase the force gradually. Repeat the process as needed until you have a graph that reflects the desired motion, including pulling the block at constant speed once it begins moving.
5. Using LoggerPro tools, determine the maximum value of the force while the block was still at rest. Record this value of the maximum force of static friction.
6. Using LoggerPro tools, determine the average value of the force required to keep the block moving at a constant velocity. Record this value as the force of kinetic friction.
7. Repeat these steps for two more trials at this mass and average the results to improve the accuracy of data. In your lab journal, include a graph of one trial with all analysis clearly shown. Label the portions of the graph corresponding to the block at rest, the time when the block just started to move, and the time when the block was moving at constant speed.
8. Increase the mass on the block and repeat the experiment several times.
Mass of block kg
Total mass
(m)
Normal force
(N)
Trial 1
Peak static friction
Trial 2 Trial 3
Average peak static friction
(N)
Total mass
(m)
Normal force
(N)
Trial 1
Kinetic friction
Trial 2 Trial 3
Average kinetic friction
(N)
12 - 2 Physics with Computers

Static and Kinetic Friction
1.
Draw free body diagrams showing the forces acting on the block at each point listed below: a.
while the block is being pulled but before the block starts to slide b.
at the moment when the block just started to move c.
as the block is moving at a constant velocity to the right.
2.
Determine the normal force of the table on the block alone and with each combination of added masses. Show your work for an example and fill in the Normal Force entries in the data tables.
3.
Compare the Normal Force values to the Frictional Force values. According to the results, what effect does the Normal Force have on the Frictional Force? Does there need to be a
Normal Force for Friction to be present?
4.
Is the relationship between Frictional Force and Normal directly proportional? What should you do to determine and prove the relationship?
5.
Plot a graph of the maximum static friction force vs . the normal force. Since F maximum static
 s
F norm
, the slope of this graph is the coefficient of static friction
 s
=
. Find the numeric value of the slope, including any units. You should include this graph and curve fit information in your lab journal.
4.
Plot a graph of the average kinetic friction force vs . the normal force. Since F the slope of this graph is the coefficient of kinetic friction
 k .
kinetic
=
 k
F norm
. Find the numeric value of the slope, including any units. You should include this graph and curve fit information in your
, lab journal.
5.
Compare the force necessary to keep the block sliding compared to the force necessary to start the slide. Compare your values for the coefficients of static and kinetic friction. How do your observations relate to your answer to Preliminary Question 1?
6.
From your results, does the coefficient of kinetic or static friction depend on the weight of the block? Explain clearly.
7.
How could you make changes to the experimental procedure to improve your results? What equipment could you employ to further refine the experimental results? How would you use the equipment to further refine your experimental result?
Physics with Vernier 12 - 3