I predict that the amount of friction will increase proportionally because the co-efficient is constant. Therefore, the ratio in which it increases with weight will be constant, so it will be proportional.
I intend to use 10 different masses ranging from 100g to 1000g. I will record the force of friction for 100g, 200g, 300g, 400g, 500g, 600g, 700g, 800g, 900g and 1000g.
I needed to select a surface that would produce a fair amount of friction. Not so much that all the results would be near each other at the top but not so small that there was little difference between how low they were. I also needed to find a range of masses to use that would produce fairly consistent but varied results.
I decided to use MDF as the surface for my experiment because the others were either too high or low. I also decided to use masses ranging from 200g – 1000g, as my preliminary test from-300 – 900gappeared to be successful.
Equipment needed:
- (10) 100g weights
- MDF block
- Drawing pin
- Length of string
- Balance
- Scales
- Newton meter
- I will begin by setting the equipment up as shown in the above diagram with a 100g mass on the block. I am going to use an MDF block because my preliminary work showed that this would probably be the best option.
- I will pull the block along at a constant speed with the Newton meter. I will keep the same piece of string for every result so the length is constant to keep it a fair test. I will also use the same Newton meter so there is the same degree of accuracy for all results. We must use the same strength of pulling for each test to keep it fair. I will repeatedly measure the length of the string to check that it does not move and become different from measurement to measurement. I am going to use a spring balance of 1-5 N because my preliminary work told me none of the results will go above 5N. This will also ensure that results can be read to a greater degree of accuracy.
- I will read the force off of the Newton meter and record it in a table. I will do this three times and take an average in order to get rid of any errors.
- I will repeat steps 1-3 for the mass of 200g, 300g, 400g, 500g, 600g, 700g, 800g, 900g and 1000g.
Obtaining Evidence
We repeated each observation three times and took an average in order to eliminate inaccurate results. Our method did not change at all from our plan. In addition to measuring all of the variables I was attempting to control I tried to check that the speed and power of the pull on the block was kept constant. I could not measure this so I had to rely upon my senses. I measured the length of the string each time to ensure that that mass was the only variable being changed. This creates more reliable results.
Analysis
My graph shows that as the mass increases so does the pulling force. As the graph of mass against friction is a straight line passing through the origin, the mass of an object is equivalent to the force needed to pull it.
Friction is a result of the microscopic ridges on objects locking together. Increasing the mass of the load will increase the weight force pulling the load downwards. This means the ridges will be pushed tighter together. When you pull it, it will require more force to pull the objects over each other because the teeth will lock together harder. Therefore the greater the weight of an object the more force is needed to pull it.
The graph of mass against pulling force is so close to a straight line passing through the origin and most points that it supports my prediction that the mass of an object is proportional to the force needed to pull it. Mass is 10 x weight so the mass axis will be proportionate to weight. As the mass increases, the friction increases by a certain amount each time, creating a straight line. This illustrates the point that the co-efficient of friction is proportional. This is because the co-efficient of friction is the constant ratio between friction and a normal contact force. Increasing mass leads to an increase in weight force. Weight = normal contact force, so an increase in mass will have a relationship with an increase in friction.
I measured the gradient of my line of best fit.
The gradient is equal to force ÷ mass.
The co-efficient of friction is friction ÷ weight so we need to convert mass into weight.
Weight = mass
10
Weight =40
Co-efficient of friction = friction
Weight
= 0.93
40
= 0.023 (3sf)
Evaluation
The result for 800g is slightly different to what might have been expected. It is slightly out of the ratio of increase in the other results, but not much. I expected it to be a bit higher.
As I repeated each result three times it is unlikely that the error was a result of poor measurement. The fact that I was unable to measure how hard the load was pulled and had to rely upon my sense of power or speed may have lead to an inaccuracy in how hard the load was pulled that I did not notice or sense. Also there were sudden jerks whilst pulling the load. This meant that the amount of friction was slightly inconsistent because it went from little to quite a bit because of the jerk. Hopefully taking the results several times eliminated this.
Because the repeated results agree with the original values it is reasonable to suggest that they are reliable. The fact that the bulk of the results were on or around the best fit line I can say that my prediction that mass is proportional to the pulling force on an object was correct.
The main difficulty with my investigation was that I could not measure the power with which the load was being pulled and therefore was not able to keep the variable entirely constant. The best way to tackle this complication would be to use a contraption or machine that would create the same power of pull each time. You could possibly use a car, which can measure its speed and can be used to keep it constant or a small electric motor to pull it at a constant force. To eliminate most measuring error you could use a digital Newton meter.
This investigation indicated to me that there is a vivid relationship between the mass of a load and the force needed to pull it. It showed that friction is roughly 0.025 times the weight force or normal contact force. As you increase the weight force friction increases in the same ratio. To extend this investigation I could look at the influence of different surfaces on friction. It may also be interesting to investigate the friction on surfaces with different sized slopes. I could maybe investigate the grip of different car tyres on the road. Tyres that grip better would provide better acceleration and braking. This is because the tyres would interlock more with the microscopic ridges on the road so that it could push off more to accelerate or lock in better to brake more effectively. I could test which brand tyres take the longest to accelerate to a certain speed and stop from a certain speed whilst going at the starting from the same speed each time. The tyre that took the longest would be the least grippy and, therefore, worst tyre.
