Friction
By
Todd May
Lab Partners
Ryan Condon
Mike Moore
Mason O’Lennick
Victor McBrooks
October 10, 2008
Physics 201
Lab Section 2
Introduction
The purpose of this lab was to find the coefficient of kinetic friction and determine
whether it is really a constant between wood and aluminum and between felt and aluminum.
Then we verified that friction is independent of the speed the two surfaces are sliding against
each other.
Materials
1 wood friction block
1 track with bumper
1 “smart pulley” with interface cable
1 small hanging mass set
1 large hanging mass set
Paper clips
Foam padding
Computer
Procedure
The first thing we did was to set up the track that the block would slide on. We made sure
the track was level and had one end of it hanging off the edge of the counter with the smart
pulley at that end. Then we placed a wooden block on the other end of the track. We attached a
thread to the block over the pulley hanging off the edge of the counter. Then we hung weights on
the end of the thread. The goal was to get the wooden block to slide along the track at a constant
speed. We had to play around with the weights quite a bit but eventually got close. When we got
it working we recorded the mass hanging from the thread and the mass of the wooden block.
Then we put a 100 gram mass on top of the block and found the new mass that needed to be
hanging on the end of the thread. We did this two more times, each time adding 100 more grams
on top of the block. In the end we had 5 different measurements. The last one had a total of 400
grams on top of the block. From all the collected data we were able to calculate the coefficient of
kinetic friction for wood on the aluminum track. For the next part we kept the mass on the block
constant at 300 grams and varied the mass on the end of the thread. We got three different runs
and used the computer and smart pulley to record the slope of the velocity vs time graph line
which is equal to the acceleration of the block. We averaged the results and then used the data to
calculate what the acceleration should have been with the coefficient of friction that we
calculated. Next we compared the measured to the calculated accelerations. That finished the
first half of the lab and then we did it all over again but this time we flipped the block over. Now
the side sliding across the aluminum was felt instead of wood. The block had felt glued to one
side. This time we came up with different coefficients of kinetic friction.
Measurements
The measurements we took are all recorded in the Excel table attached.
Equations
m1*a = m1*g – T
T = m1*g – m1*a
m2*a = T – μ*m2*g
m2*a = m1*g – m1*a – μ*m2*g
m2*a + m1*a = m1*g – μ*m2*g
a = (m1*g – μ*m2*g)/(m1 + m2)
=
g * (m1 – μ*m2)/(m1 + m2)
Normal force N = (Mass of block g + Mass on block g) * 1 kg/1000 g * 9.81 m/s^2
-Finds the normal force of the block on the track
Hanging force N = Hanging mass g * 1 kg/1000 g * 9.81 m/s^2
-Finds the force hanging from the thread which equals the tension in the thread
Coefficient of friction = Hanging force N / Normal force N
-Uses the forces to calculate the coefficient of kinetic friction
Predicted A m/s^2 = 9.81 m/s^2 * (hanging mass g - average μ * block mass g)/(block mass g +
hanging mass g)
-This is the acceleration expected of a system
Calc Hanging mass g = Normal force N * Average μ / 9.81 m/s^2 * 1000 g / 1 kg
-This was used to get a rough estimate of what we should hang on the string to get it
moving at a constant velocity
%diff = (Predicted A m/s^2 - Average m/s^2) / Predicted A m/s^2
-The difference in the measured to the calculated acceleration
Sample Calculations
Mass of
block
(g)
200.4
Mass on
block (g)
0
Hanging
mass (g)
62
Calc
Hanging
mass (g)
-
Normal
force (N)
1.97
Hanging
force (N)
0.608
Coefficient
of friction
0.309
Normal force N = (200.4 g + 0 g) * 1 kg/1000 g * 9.81 m/s^2 = 1.97 N
Hanging force N = 62 g * 1 kg/1000 g * 9.81 m/s^2 = 0.608 N
Coefficient of friction = 0.6082 N / 1.97 N = 0.309
block
mass (g)
500.4
hanging
mass(g)
200
V vs T slope 1
(m/s^2)
0.589
V vs T slope 2
(m/s^2)
0.542
V vs T slope 3
(m/s^2)
0.558
Average
(m/s^2)
0.56
Predicted A
(m/s^2)
0.687
%diff
18.06%
Average = (0.589 + 0.542 + 0.558)/3 = 0.56 m/s2
Predicted A m/s^2 = 9.81 m/s^2 * (200 g – 0.302 * 500.4 g)/(500.4 g + 200 g) = 0.687 m/s2
%diff = (0.687 m/s^2 – 0.56 m/s^2) / 0.687 m/s^2 = 18.06%
Results
From our data and calculations we can conclude that velocity does not affect frictional
forces. In the first half with wood on aluminum our calculated values were all off by about 0.12
m/s2 and for felt on aluminum they were all off by about 0.1 m/s2. They were off by the same
amount each time which indicates our calculated coefficient of friction was a little off. We had a
hard time getting the weight on the end of the thread to calculate it though. The block/felt and
aluminum track did not have a constant friction. The block would consistently go faster in certain
spots and slower in others. Both the wood and felt had problems in the same spots so it was the
track that must not have been clean or smooth all the way. We tried two different tracks and both
had problems but one was much better and we used that one. We did our best to average it out
though.
Conclusion and Summary
The lab successfully showed what we were trying to find. The frictional force is equal to
the normal force times the coefficient. Velocity does not affect it. In this lab we varied both the
normal force and relative velocity of the two surfaces. Every time we came out with numbers
that were really close. When we varied the normal force but kept the velocity constant we got the
same calculated coefficient each time. Then we tried varying the speed which gave us a constant
coefficient of friction again. This time we were off by a little bit but each trial was off by the
same amount which means the friction force should still be constant. Both accelerations were
linear. For our felt we had a lower coefficient since felt on aluminum moves easier than wood on
aluminum. Frictional forces only depend on the normal force between the surfaces and the
coefficient of friction. Nothing else affects them.