Friction Forces Lab
Finding an object’s
through the use of tension force
Victor Jeung, Cathy Liu, Jason Feng, Terry Tong
2011 October 14
1
Friction Force Lab
Abstract
The purpose of this lab is to help better understand what friction is and how it affects an object’s
movement close to the surface of the earth. In our experiment, a box with leather padding on the
bottom is pulled by a hanging weight over a ledge. The mass of the hanging weight is used to find the
coefficient of static friction.
Introduction
Static friction is the force responsible for holding an object in place. In order for an object to move, the
applied force must overcome the force of static friction. Our lab uses the force of gravity in order to find
the maximum static friction force on an object before it turns into kinetic friction. This will help in the
understanding of how static friction will either increase or decrease due to the mass of the object.
Hypothesis
Our group has concluded that the mass of an object will affect the static friction by a relationship of MG.
Theory
Equation
The controlled variable within this experiment is the contact surface between the box and the wooden
plank. The independent variables are the mass of the whole object and the time it takes the box to
accelerate. The dependent variables are the tension need to reach the maximum static friction and the
kinetic friction which can be controlled by adjusting the weight being held at the end of the string. We
start off the calculations of the static max by letting the force of friction equal the
Static Friction
The net force is zero because there is no acceleration in the
system.
Linearizing the equation
( )
( )
From this equation you can see that it is a linear relationship
between the mass and the static friction
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Friction Force Lab
Theoretical Acceleration vs. Actual Acceleration 12
Theoretical (no friction surface)
(
=
(
)
)
(
)
(Theoretical net force on box)
Actual (with friction)
(
)
(
1
2
)
(Actual net force on box)
Please refer to the Figure 1 for the FBDs.
refers to the box on the horizontal surface,
refers to the dangling mass
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Friction Force Lab
Experiment
Apparatus
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a long wooden plank (approximately 0.75 m long)
a simple pulley
a wooden box with leather padding on the bottom
a long string
a set of masses
a meter stick
Figure 1 Diagram of our apparatus setup along with the FBD’s of both of the objects in the system.
Setup
The setup of the apparatus is very simple. The long wooden plank was laid on a table. 0.1 m of the plank
hung over the table’s edge, and at the edge of the plank there was a pulley placed there. We then
placed a wooden box with leather padding on the bottom as seen in Figure 1 onto the plank. Then we
tied a string to it. The string was pulled across the pulley and attached to a dangling mass which
represents the force of friction when the acceleration of the system is zero. If the wooden box began
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Friction Force Lab
moving after adding additional masses to the dangling mass, it meant we have surpassed the force of
static friction and now had kinetic friction.
Collected Data
During the experiment, the wooden box was measured to be 0.188 kilograms. We increased the mass of
the hanging object to find the static friction.
Fstatic friction max = 1.457N
When we doubled the boxes mass by stacking a second box with the same mass on top of the other it
yielded a total mass of 0.376 kg.
Fstatic friction max = 2.989N
d= 0.525 m
Horizontal displacement.
Measured time for Fkinetic friction
Trial 1: 0.56s
Trial 2: 0.48s
Trial 3: 0.73s
Trial 4: 0.69s
Average: 0.615s
0.16
0.79
Data Analysis
The data that we have collected for this lab confirms the force of static friction on to the wooden box. As the mass
of the box increased, the frictional force increased at a proportional rate, which we have concluded to be a linear
relationship. With only enough time to test for 2 data plots we are only able to make assumptions based on our
equation that we have derived, which shows is a linear relationship. However the kinetic friction data collected
could have not been more wrong, as our group had done the procedure for the kinetic friction improperly. The
acceleration times that we calculated where completed with the dangling mass helping the box accelerate instead
of using the ramp method which lets the box slide on its critical angle. We determined this mistake and can only
state that the static friction is valid as leather on oak wood has an approximate coefficient of static friction of 0.70,
3
and while our kinetic friction’s coefficient is wrong, from online data bases to be around 0.5
3
Beardmore, Roy. "Friction Factors." Coefficients of Friction. Royal Mechanical. Web. 22 Oct. 2011.
<http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm>.
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Friction Force Lab
Conclusion
After analysis of the data, we found that our hypothesis is true in which the static and kinetic friction is
affected by the mass of the object. The coefficient of the kinetic and static friction depends on the
materials used for both contact surfaces. The coefficients will never be bigger than 1 and the coefficient
of kinetic friction is always smaller than the one of static friction for the same scenario. The lab however,
we did make a critical mistake which has made our kinetic friction value unfeasible, as the value for
we got was the result of the tension pulling on the block.
There were some random and systematic errors in our lab. These were:

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The systematic error of the hanging masses at the end of the pulley swinging or moving. This
caused the box to pull forward, even though kinetic friction had not been reached since it
resulted in an additional force pulling down.
The systematic error of the string not being perfectly straight (parallel to the lab bench). This
resulted in a smaller force of tension.
Kinetic friction not being calculated with the box’s own mass and acceleration through the use
of critical angle.
The systematic error of not having ideal circumstances for the lab. Ropes and pulleys were not
massless or frictionless and had forces acting on them.
The systematic error of reaction time of the timer. This resulted in times that were either
smaller or greater than the actual time.
The systematic error of having limited masses. For example, as we got close to determining the
maximum static friction there were no masses smaller than 0.01 kg that could be used to
accurately determine what amount of mass caused acceleration.
The systematic error of the wooden box and countertop not having a uniform surface resulting
in a non-uniform friction coefficient.
If we were to complete the friction lab again, we would:

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

Try to eliminate the systematic error of the string not being perfectly straight by realigning the
eyehook on the wooden box to be perfectly straight with the pulley system.
Try to ensure the hanging masses do not move before adding additional masses to the hanging
mass.
Try to determine a different object’s
value such as a rubber or steel object.
Try to make the contact area of both table and box harder so the contact area won't increase as
the weight increase, as the leather contact surface would deform due to weight.
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Friction Force Lab