Download Friction is a force between two objects in contact

Physics – Friction Lab Procedure
Kowinsky
Introduction:
Friction is a force between two objects in contact. This force opposes an applied force.
There are two types of friction, static and kinetic. When an object is pushed, it requires
more force to initially set the object in motion. This initial applied force must overcome
the objects static friction before it begins to move. After it is moving, a constant force
must be applied for it to continue its motion. This constant applied force must be equal to
the force of kinetic friction in order to maintain a constant velocity. Kinetic friction is
always less than static friction between two surfaces. Two surfaces in contact will also
always have the same coefficient of friction, regardless of the objects mass and weight.
In this lab we are going to determine if static friction is indeed greater than kinetic
friction, and verify the coefficients of friction remain constant for objects of different
masses in contact with the same surfaces. Also we will compare an contrast the
coefficients of friction between an object and two different surfaces. In order to do this,
we will use 3 different masses (bricks) and drag them along a surface to determine the
coefficients of static and kinetic friction.
Procedure:
Materials needed are 3 bricks, 1 20N force reader, 1 large force reader, String, 2 surfaces
to drag bricks on.
1.) To start, determine the mass of each brick. Most bricks have the mass written on
them, if one doesn’t, use the force reader to determine the force of gravity on the brick,
then calculate the mass.
2.) Next, determine the coefficient of kinetic friction for 3 masses.
To do this, start with 1 brick. Attach the 20 N force reader to the brick using the string.
Carefully drag the brick using the force reader along a surface. It is very important that
you drag the force meter perpendicular to the surface, and drag the brick at a constant
velocity. You may want to practice to ensure you are applying a force such that the brick
moves at a constant velocity, and the force reader is parallel to the surface.
Since the object is being dragged at a constant velocity:
Fxtotal = Fa – Fk = 0 ,
Therefore Fa = Fk
So, your applied force read from the force reader is also the force of kinetic friction.
Record this data in a data table.
Also, since you are not pulling the brick in the y-direction,
Fytotal = Fn – Fg = 0 ,
Therefore Fg = FN
Physics – Friction Lab Procedure
Kowinsky
So, the force of gravity acting on the brick is also the normal force. Record this data in a
data table.
Once you have your data, calculate k , and record its value in the table.
Repeat the above steps for the next two trials, where you increase the mass by 1 brick
each additional trial. When dragging all 3 bricks, use the large force reader to determine
Fk.
Table for Kinetic Friction for Brick on Surface 1
Normal Force
Kinetic Friction Force
Trial
Mass
(F
)
(Fk)
N
(m)
Coefficient
of Kinetic
Friction
(k)
1
2
3
3.) Next, determine the coefficient of static friction for 3 masses. The procedure is the
same as in step #2, however, instead of pulling the bricks at a constant velocity, slowly
apply a force to the bricks until they just start to move. Record the force required to
initially set the bricks in motion.
Table for Static Friction for Brick on Surface 1
Normal Force
Kinetic Friction
Trial
Mass
Force
(F
)
N
(m)
(Fs)
1
2
3
Coefficient of
Static Friction
(s)
Physics – Friction Lab Procedure
Kowinsky
4) Repeat all of these steps for the bricks on another surface.
Table for Kinetic Friction for Brick on Surface 2
Normal Force
Kinetic Friction Force
Trial
Mass
(F
)
(Fk)
N
(m)
Coefficient
of Kinetic
Friction
(k)
1
2
3
Table for Static Friction for Brick on Surface 2
Normal Force
Kinetic Friction
Trial
Mass
Force
(F
)
N
(m)
(Fs)
1
Coefficient of
Static Friction
(s)
2
3
5.) In your lab report, be sure to address the following:
* Draw a freebody diagram showing all of the forces acting ON the bricks while they are
being dragged. Use this diagram to discuss where the equations for the total forces in the
x and y direction came from.
* Did k and s remain constant between the bricks and the same surfaces, regardless of
mass?
* Was s was larger or smaller than k for the same surface. Is this what you expected?
* Which surface did you expect to have a larger s and k? Where you correct? Why or
why not?
Physics – Friction Lab Procedure
Kowinsky
Rubric
Friction Lab Write Up
20 points total
______ 2 pts. Typed – The Lab Write up should be typed (equations, calculations,
diagrams and graphs can be hand written)
______ 2 pt. Title Page - should include the title of the lab and group member names
______ 3 pts. Introduction – This should contain the purpose (what you are trying to
find, verify, examine, etc..) and a general description of the lab. Also introduce the
concepts in this section (Introduce and define the concepts of static and kinetic friction).
The introduction should not be 1 or 2 sentences; it should be a well developed
paragraph or two.
______ 4 pts. Procedure – This should include the materials needed, and a detailed, step
by step, explanation of the materials used, how they were set up, and how the lab was
done. This should be clear enough so the experiment could be reproduced by
anyone who reads your procedure. Explain all steps done in this lab.
_______ 5 pts. Analysis – This section should contain all of the data and calculations
needed for the lab. The data measurements need to be recorded in proper units. For the
calculations, all applicable formulas and data tables need to be listed, and the
calculations should be worked out. Your free body diagrams and equations for the total
forces should be included and explained here. The equations for Fg, k, and s should
also be placed here and explained. Do not staple my lab procedure sheet into your lab
for your analysis section.
_______ 4 pts. Conclusion – The conclusion ties into the purpose of the lab and tells the
reader what was found. Experimental data from the analysis needs to be included to
support the conclusion. Sources of error need to be listed. The error needs to be
calculated if applicable. The sources of error need to reflect involved thought processes
and deep understanding of the experiment. It should also contain an explanation of how
the error was caused, and how the error could be minimized in the future.
Also, the questions need to be addressed in sufficient detail.
TOTAL:________ out of 20 pts