# Friction of Solid Surfaces

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Queensland University of Technology

ENB205

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Semester 1
2012
ENB 130 M7 – Friction of Solid Surfaces
Tim Starkey n8588163
26th of April 2012 10-12am Session Group: Justin, Gerald and Knox
S c i e n c e a n d E n g i n e e r i n g - Q U T Aim
To investigate the association between solid surfaces influenced by weight, the
applied force and surface area of a timber block in relation to the coefficient of
friction along with the relationship between velocity and kinetic friction.
Introduction
The coefficient of friction (μ) between two surfaces is a critical element when it
comes physics in motion. The investigations in this report look at the relation
between different factors that are often associated with friction, such as weight
and surface area. By using the following equation for coefficient of kinetic and
static friction, these relationships can be analysed.
and
Where:
μ = Coefficient of Friction between two surfaces
FFR = Frictional Force (N)
N = Normal Force (N)
m 1 Mass of Object 1 (kg)
m 2 Mass of Object 2 (kg)
g = Gravity (9.8m/s/s)
t = Time (s)
s = Displacement (m)
The original equation for coefficient of friction can be used for both kinetic and
static friction, however for this report, kinetic friction was found by using the
second formula.
Looking at the formula, theoretically, surface area should not play a big part in
the investigation because neither equation requires its quantity to calculate the
coefficient.
Method
There were three different forms of experiments that were performed to find the
relationship between weight, the applied force and surface area in relation to the
coefficient of friction. The first investigation looked at the relation between static
friction and normal force. The steps were as follows:
1. Measure the mass of Block A with a 1kg mass on top of it.
2. Using Block A with the 1kg mass, connect the wire to the block so that it
will run freely over the pulley. Softly attach a hook onto the wire and
apply a load so that the wire runs close to parallel to the friction track.
3. Gradually increase the load hanging from the hook by 50g until the block
begins to slide. 4. Record the largest load that the block held without sliding. Repeat
process three times for each effective block mass so that an average can
be generated.
Using this data collected, a relation between static friction and normal force
became present. The second investigation looked at the relation between the
surface area of the block and static friction. To do this, the surface area of the
block was changed rather than its effective mass. This was done by calculating
the mass required so that blocks B and C had the same mass as block A. Once the
blocks were of equal mass, the same process as the first investigation was used.
This recorded data then allowed a relation between surface area and static
friction could be found.
The third investigation looked at the coefficient of kinetic friction. This
investigation was completed by having equal masses on either end of the wire,
1.5kg total of block A and added weights and 1.5kg of mass hanging of the table.
Block A was then pulled 1.25m away from the pulley, making the hanging
weights to rise. The weights were then released so that the block accelerated
towards the pulley. The time it took the block to cover the 1.25m was measured
and recorded. After repeating the process so that there were three sets of data,
50g was removed from block A so that the hanging weights were greater than
the mass of the block. This whole process was then repeated so that data for five
different block masses was recorded. Using this data, the relation between
velocity and kinetic friction could be calculated.
Results
Below are the results for the three investigations:
Investigation 1: Static Friction and Normal Friction
Block Effective Trial Maximum Mass Held (kg) Maximum Coefficient
Block Mass of Friction
Mass (kg) Held μs
(Average
of trials)
(kg)
Block A + 1.472 0.75 0.85 0.80 0.80 0.54
1kg
Block A + 2.473 1.50 1.75 1.90 1.71 0.69
2kg
Block A + 3.475 1.85 2.00 2.15 2.00 0.57
3kg Investigation 2: Surface Area and Static Friction
Block Adjusted Trial Max Mass Held (kg) Maximum Coefficient
Block Mass of Friction
Mass (kg) Held μs
(Average
of trials)
(kg)
Block A 1.472 0.75 0.85 0.80 0.80 0.54
Block B 1.435 0.70 0.65 0.75 0.70 0.48
Block C 1.472 0.80 0.90 0.90 0.86 0.58
Block C 1.472 0.70 0.70 0.70 0.70 0.47
(grooved)
Investigation 3: Coefficient of Kinetic Friction
Block Added Block Trial Time (s) Average Average Coefficient
Mass Mass Mass + Time Velocity of Kinetic
(kg) (kg) Added (s) (m/s) Friction k
mass
=m (2g)
0.471 1.05 1.521 1.81 1.88 1.63 1.77 0.70 0.203
0.471 1.00 1.471 1.63 1.72 1.62 1.65 0.75 0.197
0.471 0.95 1.421 1.56 1.57 1.66 1.59 0.78 0.198
0.471 0.90 1.371 1.50 1.50 1.47 1.49 0.83 0.189
0.471 0.85 1.321 1.53 1.43 1.40 1.45 0.86 0.193
Data Analysis
From the data that was collected in the second and third investigations, the
average coefficient of friction and that greatest percentage of difference in the
data from the average can be calculated. To first find the average coefficient from
each data, all of the coefficients are added together. From this total, the number
of coefficients is divided. This gives the following averages for the second and
third investigations:
By now finding the percentage of difference, the accuracy of the data can be
justified. To calculate this value, the largest difference between the average and a
data point is used to find out the percentage difference from the average.
Therefore, the calculations for the two investigations are: Now that the percentage difference of the two investigations has been calculated,
the accuracy of the collected data to be justified.
Discussion
Investigation 1 – Static Friction and Normal Friction
The first investigation for Friction of Solid Surfaces looked at the relationship
between friction force and the coefficient of friction. Before starting the trial,
predications

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