Download FRICTION

Name: ______________________________
Friction, p. 2/7
FRICTION
Topic: Friction
Objectives:
•
To determine the relationship between friction force and normal force.
•
To measure the coefficient of kinetic friction between two surfaces
•
To measure the coefficient of static friction between two surfaces
• To estimate the reliability of a measurement from repeated trials
Overview:
In this lab you will investigate friction. In the first investigation, you will get a “feel” for friction and
make some preliminary measurements and predictions. In the second investigation, you will
investigate the numerical relationship between friction force and normal force. Since you can’t
measure friction force or normal force directly, you will measure each of these quantities indirectly.
There are several educational purposes for this lab. The most obvious purpose is to give you some
intuitive sense about the relationship between friction force and normal force. Another is to get
some practice applying Newton’s laws. Another, far less obvious, educational goal is acquaint you
with some of the issues involved in developing/testing ideas about the world. Throughout this lab,
you will critically examine what claims can be substantiated by the experimental evidence you collect.
Safety/Equipment Notes:
Be careful when handling heavy weights: The masses you will be handling can be as large as
a few kilograms. Dropping these masses can damage toes, equipment, etc.
Strong pushes and pulls may damage the force probes. The probes are rated for a
maximum of 50 N.
Writing it up:
In this handout, you will be asked to perform calculations, analyze graphs and answer questions. It is
strongly recommended that you do all the calculations and answer all the questions as you go through
the experiment. This record is for your purposes only. You will not be graded on it. You will be graded
on how well you understand the material covered in this lab, so you should do the entire activity,
discuss the questions with your partners and take careful notes.
Written by DSA for P3
Revised: 7/2004
Name: ______________________________
Friction, p. 3/7
Investigation 1: Getting a “feel” for friction
Activity 1-1: Pulling a load
1. Obtain a tray, drawer or similar container that can hold large masses. Note what material is
on the bottom surface of the container. Measure the mass of the container. Fill it about half
full with known masses.
2. Record the total mass of the container and masses: ______________ (Don’t forget units)
3. Set the force probe and motion detector to collect data at the fastest rates possible. Set the
force probe for pull positive. Set up axes for force-time and velocity-time graphs. Zero the
force probe. Use the force probe to slowly pull on the container with the masses, but not so
hard that the container moves. Note the graph. Gradually increase your pull until the
container moves with constant velocity.
velocity (m/s)
4. Sketch the graphs below. Note the maximum force reading recorded during this run. Note the
force reading during the time interval when the container was moving at constant speed and
record them below. Leave your data displayed on the screen for later use.
0
force (N)
+2
0
-2
1
2
3
4
5
time (s)
Prediction 1-1: Suppose you doubled the amount of mass in the container. How would the forcetime graph change? Explain the basis for your prediction.
5. Test your prediction.
6. Use your data to estimate the coefficient of static friction and the coefficient of kinetic
friction between the bottom surface of the container and the table. Show your work here.
Written by DSA for P3
Revised: 7/2004
Name: ______________________________
Friction, p. 4/7
Activity 1-2: Making a more careful measurement
The calculation you just performed relies on (at least) two assumptions:
Assumption 1: The force probe reading equals the force of friction acting on the container.
Assumption 2: The normal force acting on the container equals the total weight of the
container and contents.
How valid are these assumptions? What precautions can you take to make sure these assumptions
are valid? Consider the following questions:
Q1-1: Once the cart is moving, is it important to keep the cart moving at constant velocity?
If the cart speeds up as you pull it, would the force probe reading be (larger than,
smaller than, or equal to) the force of friction? Explain.
Q1-2: Is it important to pull the container in the horizontal direction (rather than at an
angle)? Explain, using a force diagram.
Q1-3: Is it important to pull the cart at the same constant speed each time you do the
experiment? Explain.
Q1-4: Does the force probe always read zero when there is nothing pulling on it?
Î Check your answers with an instructor.
1. Fill the container about half full with masses and repeat the measurements you took in the
previous section using the appropriate precautions you determined above. Don’t forget to
zero the force probe!
2. Repeat the measurement at least four more times with the same mass, re-zeroing the probe each
time. Doing the measurement several times allows you to estimate how consistent your
measurements are and to calculate an average.
Record the total mass of container and masses: ____________
Trial
1
2
3
4
5
Ave.
Spread
Max force probe
reading during no
motion (N)
Probe reading
during constant
velocity (N)
Written by DSA for P3
Revised: 7/2004
Name: ______________________________
Friction, p. 5/7
3. Calculate the average values and estimate the spread in the data. Place the results in the table
above.
Q1-5: Suppose you did the same measurements again. What ranges do you think the next
measurement for the max probe reading would likely fall within? the constant speed
probe reading?
Q1-6: Does one set of measurements (static or kinetic) seem more reliable than the other?
If so, which one?
4. Use the values from the table above to get your best estimate of the values of µ s and µ k . Also
estimate how close you might expect another group would get to these values if they used your
procedure.
Î Check your answers with an instructor.
Investigation 2: Are friction force and normal force proportional?
It seems that the larger the normal force is, the larger friction will be. Is the relationship a
proportional one? In this investigation you will investigate whether the relationship between normal
force and friction force is a proportional one.
Activity 2-1: Changing the normal force
In this activity you will change the normal force by placing different amounts of mass in the
container. You will repeat the measurements of the previous section for several different values of
normal force.
Prediction 2-1: What do you expect a graph of maximum probe reading versus normal force to
look like? Sketch your prediction on the graph below. Explain the basis for your prediction. Is the
relationship you predict a proportional one? Explain.
Probe reading (N)
Prediction 2-2: What do you expect a graph of constant speed probe reading versus normal force
to look like? Sketch your prediction on the same set of axes you used for the previous prediction.
Explain the basis for your prediction. Is the relationship you predict a proportional one? Explain.
Normal Force (N)
Î Check your predictions with an instructor.
Written by DSA for P3
Revised: 7/2004
Name: ______________________________
Friction, p. 6/7
5. Check your predictions. Repeat the basic measurement for several different values of normal
force. Try to use as wide a range of normal forces as possible. You only need to get one good
trial for each value of normal force. Record the results in the table below.
Mass of
container and
contents (kg)
Max force probe
reading during no
motion (N)
Probe reading during
constant velocity (N)
6. Make a graph to test your prediction. Do the data support your prediction?
Investigation 3: How do different surfaces compare?
Find a group that has a container with a different surface on the bottom than your container than
yours. Record their data here:
velocity (m/s)
From Activity 1-1:
0
force (N)
+2
0
-2
1
2
3
4
5
time (s)
Written by DSA for P3
Revised: 7/2004
Name: ______________________________
Friction, p. 7/7
From Activity 2-1:
Record the total mass of container and masses: ____________
Trial
1
2
3
4
5
Ave.
Spread
Max force probe
reading during no
motion (N)
Probe reading (N)
Probe reading
during constant
velocity (N)
Normal Force (N)
1. Compare their data with your own. How is it similar? How is different? Is this what you
might expect, given the difference in the types of surfaces? Explain.
Written by DSA for P3
Revised: 7/2004