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Friction
Introduction
Newton’s genius lay in studying systems in which friction was not present, such as planetary motion.
In our world friction is not only present, but is very often the most important force. So, in the spirit of
“know your enemy,” in this laboratory you will study friction.
Textbook Reading
Physics 150: Halliday, Resnick, and Walker: Sections 6-1 through 6-3 (pages 108-115).
Physics 125: Cutnell & Johnson: Section 4-9 (pages 106-109).
Overview
You will concentrate your exploration on two kinds of friction. One is the friction that occurs when two
solids are in contact. Friction like this is important when you’re pushing a box across the floor, or
accelerating or braking your car. The second kind results when objects move through the air. While
parachute jumpers want this kind of friction, designers of autos and planes try to reduce it.
The exploration part of the laboratory will concern the motion of falling bodies. You’ll test the
predictions of Aristotle and measure the terminal velocity of a coffee filter.
In the second part, you’ll explore the textbook model of sliding friction between two objects.
Objectives
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To observe how light and heavy bodies fall.
·
To understand how a falling body reaches a terminal velocity.
·
To explore the dependence of an object’s terminal velocity on its weight.
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To measure the force of friction between two objects in contact and in relative motion.
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To explore how the frictional force depends on the area of contact, relative speed, and normal force.
·
To determine whether there exists a coefficient of kinetic friction for two surfaces in contact.
Pre-lab Question
1.
Object 1, mass m, falls through the air at a constant velocity (acceleration is zero). What is the upward
force on it? Object 2, mass 4m, also falls at a constant velocity (not necessarily the same). What is the
ratio of the upward force on object 2 to that on object 1?
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Fupward on 1 = _____
Fupward on 2 = _____
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F upward on 2
= ________
Fupward on 1
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Moving Through Air
Name______________________________
Equipment
· Rubber ball
· Six coffee filters
Partner _____________________________
Introduction
In 330 BC, the Greek scientist, Aristotle, formulated laws describing how bodies fall though air and water. He
argued that the speed with which they fall is proportional to their weight. Thus, if an object is ten times
heavier, it should fall ten times faster. Was Aristotle right? How do objects fall through air?
Experimental Activities
Activity 1
·
Hold the ball at arm’s length in front of you.
·
Remove the force that you are applying to the ball (i.e., let it go).
1.
What happened? Describe its motion (its acceleration, velocity, and/or position as a function of time).
You learned in the last laboratory that if an object is accelerated, then there
must be a net force acting on it. That is, Fnet = ma.
2.
What force(s) act on the ball? What object(s) in the environment cause the force(s)?
Activity 2
·
Place the ball in one hand and a single coffee filter in the other. Hold both horizontally at arm’s length in
front of you, the same height above the floor. Simultaneously remove all forces that you are applying to
the two objects.
3.
What happened? Describe the filter’s motion (its acceleration, velocity, and/or position as a function of
time).
4.
Write down the similarities and differences you observed about the motion of the filter and the ball.
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5.
Based on your observation in this activity, and the differences in weights of the two objects that you
could sense, was Aristotle’s description of how things fall right?
Activity 3
·
Place a stack of four coffee filters in one hand and a single coffee filter in the other. Hold them
horizontally at arm’s length in front of you, both at the same distance above the floor. Simultaneously
remove all forces that you are applying to the two objects.
6.
What happened? Describe the motion of the single filter and the stack of four filters (acceleration,
velocity, and/or position as a function of time).
7.
Based on your observations, does Aristotle still seem to be correct?
Activity 4
·
Wad up a single coffee filter. Repeat Activity 2 with the wadded coffee filter and ball.
8.
What happened?
9.
Based on your observations, was Aristotle right?
10.
Based on Activities 2-4, what can you conclude about the forces on objects falling in air near Earth’s
surface?
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Experimental Study of Falling Coffee Filters
Equipment
·
·
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Five coffee filters
Computer with ULI, motion detector, FilterFall.
Support rods for motion detector
Introduction
How does an object like a coffee filter (or parachute, leaf, or even a raindrop) fall? It starts from rest,
and, at least according to the textbook, soon reaches a constant speed, called its terminal velocity.
Let’s investigate the relationship between force and motion for this system.
Predictions
On the sketches of a coffee filter below draw arrows showing the forces on the object at three stages
of its descent. Label the arrows with the causes of the forces and make their lengths proportional to
the magnitude of the forces.
Then, on the axes predict the
velocity and acceleration of a
falling filter. Explain the sign
convention you are using.
Identify on your graphs the
three times at which you drew
the forces on the filter.
Experiment
1.
Use the ULI and the motion detector to make distance-, velocityand acceleration-time graphs of the descent of a single filter. Because the
filter cannot be closer than 50 cm from the detector, the detector will be
mounted on a rod near the ceiling. Use LoggerPro and the file FilterFall.
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2.
Make sure the falling filter is “seen” by the detector as it falls. Holding the filter, click on the Collect
button and lower the filter by hand. This procedure will allow you to figure out where to release it so
that it remains in view of the detector.
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3.
Try it! Drop the filter. Accurately sketch or print out the graphs you obtained on the axes above. Use a
different color or kind of line to distinguish the results from the predictions. Were your predictions of
the velocity-time and acceleration-time behavior accurate? If not, explain how to correct them.
4.
Store the data by moving Latest Run into Run 1. Repeat the experiment with a stack of four filters.
Questions
1.
Describe in words how the velocity and acceleration of a falling filter change during its fall.
2.
If the acceleration changes, then Newton’s second law says that the force must change. Which force(s),
if any, remain(s) constant, and which change(s)? Explain why.
3.
How do the forces on one filter compare with the forces on four filters when they are moving at their
terminal velocity?
4.
What are the measured terminal velocities of one and four filters?
5.
According to the textbook, the force of air resistance is proportional to the square of the velocity. Do
your results support this model? Be quantitative in your assessment.
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Sliding Friction
Equipment
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·
·
·
·
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Aluminum track
Hardwood board with felt surfaces, hardwood block with cork surface
String
1-N and 5-N Spring scales, meter stick
50, 100, 200, 500, 1000-g masses
Computer with ULI, motion detector, force probe, file Friction
Introduction
If you shove a piece of wood and release it, it doesn’t keep moving at a constant speed, rather it
slows and comes to rest. The changing velocity means it accelerates, and thus there must be a force
on it. You're well acquainted with that force—it’s called friction. In this part of the lab, you’ll explore
this pervasive force.
The Textbook Model
According to the textbook, when an object like the wood block slides on another object like the
aluminum track, the frictional force on the block is in the direction opposite the velocity of the block and
a)
b)
c)
d)
is independent of the speed of motion (as long as the speed is not zero),
is independent of the area of contact,
depends on the nature of the surfaces in contact (i.e., wood, cork, cloth, etc.)
is proportional to the normal force of the track on the block. The coefficient of kinetic friction, μk, is
defined by the equation μk = Ffriction/Fnormal .
Does the textbook model work in real life? Your task is to design and carry out experiments to test the
four predictions of the model.
Experimental Suggestions
You may measure the frictional force with either a spring scale or the force probe.
Be sure the surface of the track is clean. Finger prints, dust, or dirt will make your results unreliable.
The motion detector can be used to find the speed, and the LoggerPro file Friction plots force and
velocity. If you use it, don’t forget to zero the force probe.
The drawing below shows how the probes may be set up. It is important to keep the force probe close
to the table so that the string is horizontal. The motion detector is upside-down so that it “sees” the flat
board.
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Neatly record your data in tables or on graphs and attach them to this report. Be sure to describe the
experiments you have done and link the table and/or graph to the experiment.
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Experimental Planning
The model predicts that friction will not depend on two quantities, will depend on the type of surface,
and will be proportional to a third quantity. All experimental results have uncertainties. In the
procedures you describe below, be sure to indicate how you’ll determine the uncertainties.
1.
Does the frictional force depend on the speed of motion? Briefly describe the experimental procedures
you’ll use and how you could create and use a graph of frictional force versus velocity.
2.
Does the frictional force depend on the area of contact? Briefly describe the experimental procedures
you’ll use.
3.
Describe which pairs of surfaces you’ll test.
4.
Does the frictional force depend on the normal force on the block? Briefly describe the experimental
procedures you’ll use and how you will create a graph of frictional force versus normal force. You must
use at least four different normal forces. Use Graphical Analysis for Windows to plot and analyze the
data.
Experimental Data
Neatly record your data in tables or on graphs and attach them to this report. Be sure to describe the
experiments you have done and link the table and/or graph to the experiment. Refer to these results in
summarizing your conclusions below.
Experimental Conclusions
1. Does the frictional force depend on the area of contact?
2. Does the frictional force depend on the speed of motion?
3. Does the frictional force depend on the type of surface?
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4. Is the frictional force between two surfaces described by a coefficient of friction, μk? What is the value of
μk? Be sure to specify the surfaces for this coefficient.
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