Basharat Akhlaq
Frictional Forces
Aim: The aim of this investigation is to find out about a factor that affects the size of frictional force between two surfaces.
Introduction
Friction is a contact force. It is a force that opposes the sliding motion of two surfaces in contact with each other. This means that when two objects slide over each other there is a frictional force present that tries to stop any sliding movement. Friction is a very important force. Most of the things we do during our everyday life involves friction: from driving the car to walking to holding a cup of tea.
No matter how smooth an object appears to look it still has friction. For example, the slide in a park is smooth and shiny and may look like there is no friction present, however once looked under a microscope, you can see that there are uneven projections on the surface; this tells us that no matter how smooth an object looks, in reality it is not and it still offers friction. When two surfaces are in contact, these projections or un-evenness must get in to each other’s way that tries to stop sliding motion and hence causes friction.
The factors which will affect the size of the maximum frictional force are: area of contact, the pair of surfaces temperature and the normal contact force.
If the area is larger, there will be more of those bumps getting on each other’s way and hence it should increase the frictional force.
The pair of the surface quality will decide how much wi ll the bumps get into each other’s way. So, for the same surface area of contact, the maximum frictional force will be different between different pairs of surfaces.
Temperature can also affect the friction as it can affect how the bumps of the two surfaces interact with each other.
If the two surfaces are pushed against each other with a harder force, it should increase the size of the frictional force. This is because, as the surfaces are pushed more towards each other, their uneven projections will get more into each other and hence should make the frictional force bigger. This push is called the normal contact foce.
I would like to investigate how does the size of the maximum frictional force depends on the normal contact force. I predict that if two surfaces are pushed harder it will increase the maximum frictional force because of the reason explained above.
Maximum frictional force is the highest amount of frictional force between a given pair of surfaces for a given size of force that is pushing the surfaces against each other. For example, if we pull a table along the floor with a small force of 1 N, the table will not move, as the opposing frictional force is also 1N. If we keep on increasing this pull, the

Basharat Akhlaq friction will also keep on increasing up to a maximum value. When we will pull the table harder than that maximum value, the friction will not increase any more and the table will start to slide on the floor. The weight of the table has been pushing the table and the floor together. So, this maximum frictional force will change if we were to put another object on the table to change the weight.
Preliminary Experiment
Before doing my main experiment I would like to do a preliminary experiment to ensure I had accurate and reliable results. I did this so I knew if I had to change anything in my actual experiment.
I spoke to the technician at school to find out about appropriate surfaces and equipment for this investigation. After carefully considering the resources we have, I have decided to use a hooked wooden block as one of the surfaces. I would like to use the carpeted floor, tile floor or table-top as the second surface. I will make a decision after my preliminary experiment.
Equipment
Wooden Block With Hook
Table-top
Tile Floor
Carpet Floor
Reason Why I Used It
1 st Surface
Possible 2 nd Surface
Possible 2 nd Surface
Possible 2 nd Surface
To measure the maximum frictional force Newton Meter
To begin with I needed to place a wooden block on a surface, attach a string to it and hook it into a Newton meter. I chose my first surface to be the carpet, I placed the wooden block on the carpet and pulled the Newton meter attached to the string when the object began to move I took the reading on the Newton meter and noted it. I then repeated this three times.
I repeated the above experiment for tiled floor and table-top as well.
Preliminary Experiment Results:
Surface
Table-top
Tile floor
Carpet floor
Frictional force 1/N
1.32
1.20
1.80
Frictional force 2/N
1.35
1.23
2.56
Frictional force 3/N
1.34
1.19
2.17
From my results I found out that the carpet was an unreliable source as the size of the frictional force varied significantly during the repeat experiments. And the table-top was the most convenient for my second surface. So, I will do my main experiment on the table –top.

Basharat Akhlaq
During my preliminary experiment, I have found out that adding a 100g mass makes a significant difference in the size of the frictional force. So, I will us 100g masses in my main experiment. I will use 10 of them to have a good range of results.
During my preliminary experiment I came across a problem when taking a reading for maximum friction. When I was pulling my Newton meter I realised that the object suddenly moved quite quickly and therefore I was unable to take an accurate reading on the Newton meter that was shown at the exact moment when the object first moved.
To avoid this inaccuracy, in my main experiment I will pull the Newton meter till the wooden block begins to move and when it does I will move my hand at a steady velocity so there is no acceleration. If I move my hand with a steady velocity then the the pulling force will be cancelled by the maximum frictional force so the resultant force on the wooden block would be zero. This means the reading on the newton meter will be exactly the same as the maximum frictional force
As I am carefully pulling the block at a steady velocity it will be impossible to take the newton meter reading. So I got a friend to take the reading from the newton meter when
I will pull the wooden block with a steady velocity.
In my main experiment I will put different masses on the wooden block on the table because the weight of the mass will change the force that is pressing to surfaces against each other.
Fair Test
I will keep the surface area, the pair of surface and the temperature the same to make it a fair test.
Main Experiment
1. First set up the equipment
2. Then place a 100g mass on the wooden block.
3. Attach the newton meter to the wooden block
4. Pull the newton meter so that the car moves with a slow and constant velocity
5. While I pull the car, my friend will take the reading.
6. Repeat the experiment three times.
7. Repeat all the above steps with different masses. (Increasing masses by 100g each time)
Equipment
Wooden Block With Hook
Table Top
Newton Meter
100g Masses
Reason Why I Used It
To be tested on the surfaces
The surface it is going to be tested on
Measure the maximum frictional force
Change the force which pushes the surfaces together
In this investigation the type of surfaces and contact area are kept the same so that it is reliable and accurate. Measurements for each of the masses are going to be done three

Basharat Akhlaq
3
4
5
6
7 times so that it is reliable. I am also going to find the mean frictional force so that I have so that I have a smaller percentage error.
The weight of the mass can be shown in the following equation:
W = M x g where g is the gravitational strength (On earth g = 10N/Kg). So, a 100g mass has an weight of 1N.
My independent variable is the weight and my dependant variable is the Maximum frictional force.
Results Table
Weight/N
1
2
Frictional force
1/N
1.4
1.6
Frictional force
2/N
1.4
1.6
Frictional force
3/N
1.5
1.5
Mean frictional force/N
1.4
1.6
1.9
2.2
2.4
2.6
2.8
1.8
2.1
3.5
2.7
3.0
1.9
2.2
2.4
2.7
3.0
1.9
2.2
2.4
2.7
2.9
8
9
10
3.1
3.3
3.7
3.2
3.5
3.6
3.2
3.4
3.7
3.2
3.4
3.7
Conclusion
When I look at my results table it is very clear that the size of the frictional force is increasing when I increase the weight. Increasing the weigh causes the normal contact force between the surfaces to increase. This must push the uneven projections of the two surfaces into each other. As a result, when the surfaces are trying to slide against each other the projections get stuck to each other even more. Hence the size of the frictional force increases. So my results make sense when I try to analyse it using scientific understanding.
I plotted a graph of maximum frictional force against the weight of the masses. This is a straight line showing that as the normal contact force increases the size of the frictional force also increases in the same proportion. There is a very strong positive correlation between the two quantities.
I have also added the error bars in my graph. The size of the error bar is relatively small compared to the difference between frictional force when I add extra mass. So, I can conclude confidently that my conclusion of very strong positive correlation is reliable.
My graph is not going through the origin so I cannot say that frictional force is directly proportional to the normal contact force. This could be different if I measured the weight of the wooden block and added the value to my independent variable as that is also adding to the normal contact force.

Basharat Akhlaq
If I repeat this investigation then I could use the total weight of the wooden block and added mass as my independent variable. This would allow me to make a better conclusion. Probably it will go through the origin. I might also be able to find out the coefficient of friction from that new experiment.
Evaluation:
Evaluation of procedure :
I have used a good method to collect a reliable set of data. As my data agrees with my scientific idea, my procedure must have been reliable. Also, my repeat results were very close to each other. Hence the procedure must have been good. However, my graph does not go through the origin. I think if I measured and added the weight of the wooden block, my results would be more meaningful. Also, keeping the wooden block moving with a steady velocity was challenging. So, if
I was to repeat the experiment, I could use a data logging system to improve my procedure. This will take away the human error and make my results more accurate.
Reliability of evidence:
I have one outlier, which has been omitted from the mean. This must have been a simple case of human error. Rest of my results look reliable. My error bars are very small compared to the difference between results for different masses. Hence, I can say that my evidence for the conclusion is reliable.
Reliability of conclusion:
My results are in agreement with my scientific explanation. Given the strength of my positive correlation, I am very confident about my conclusion from the results. I
My graph did not go through the origin. I believe this is due to not taking the weight of the wooden block in account. I could improve the quality by including this in a future repetition. However, the line is showing very good positive correlation with very small difference in repeat readings compared to readings for different weight. Hence, I am very confident about my conclusion.
However, my conclusion is only for this particular pair of surfaces. So, the conclusion cannot be a general one for all combination of surfaces. I would need to extend my investigation to other surfaces if I am to make a more general conclusion regarding the affect of normal contact force on friction.