2. Attach the string onto the neutron-metre and fix the hook on the end to the end of the shoe;
3. Give a controlled tug on the neutron metre. The measurement scale will move. Carefully observe the measurement scale. Stop pulling when the shoe starts to move and record the pulling force displayed on the measured scale.
4. Add the required masses to the shoe and repeat.
Results Of the Preliminary Experiment
1. I will use the range of weights from 100g – 2.5kg. The masses under 200g made the experiment difficult because only a small force was needed to pull the shoe and consequently it was extremely difficult to read the displayed force on the neutron metre. I found that 200g was the first measurement that could be read accurately as it required a considerable amount of force. Two point five kilograms was the maximum weight I could fit inside the shoe, without causing damage to my friend’s trainer, and therefore was the finishing mass.
2. To ensure the shoe was pulled safely I tied a piece of string to the neutron metre and pulled the string over a metal bar attached to two Clamp stands
3. The forcemetre I used was a 20N-neutron metre as a 20.8 neutons was my maximum reading.
4. I attached the forcemetre to the shoe by placing the hook of the neutron metre to the end of the shoelaces.
5. The ranges of masses used were as follows :-
400g - 2.9N
500g - 3.3N
1000g - 7.0N
1500g - 14.7N
2000g - 18.1N
2500g – 20.8
6. Safety Procedures
(i) Make sure the weights are firmly in the shoe so it is not possible for the weights to fall out and cause damage to the surroundings.
(ii) Place the shoe in the middle of the table to prevent it from falling off.
(iii) Do not force too many weights in the shoe as they may damage the shoe or fall out.
Variables
Constant Variables
To make this experiment a fair test the following variables must be kept constant throughout the experiment
Ø Area of shoe in contact with surface
Ø The slope of surface
Ø The length of string
Ø Type of surface under shoe
Ø Thickness of tread of sole of shoe
Ø Nature of shoe surface in contact with surface under shoe
Changing Variables -
The weight of the overall shoe and the force applied to the shoe.
Dependent Variable -
Pulling force.
Independent Variable -
Mass of shoe.
Prediction
Statement: ‘The greater the mass of the shoe, the greater the force required to move the shoe.’ This is because increasing the mass will increase the friction between the shoe and the bench (this is because the surfaces are being pushed together).
If smooth looking surfaces are examined under a high power microscope, their actual roughness can be seen. They only touch where their high spots meet.
The high spots that are touching tend to stick together. The limiting friction is the force needed to separate these high spots. Once the high spots have been separated a lower force is needed to keep the two surfaces moving.
If the normal force is increased the surfaces are squashed together more. The high spots, where the surfaces are in contact, are larger:
A greater limiting friction is needed to separate the high spots and a greater dynamic friction is also needed to keep the surfaces sliding
Results
Analysing Evidence
The pattern that is shown on the graph is that the more weight that has been added, the more friction is produced. My results support my prediction 100% because I stated in my prediction that the friction that is being produced will increase when more weight is added.
Evaluation
I tried to get my results as accurate as possible but I would never be able to get them exactly accurate because of the following circumstances:
- I could not get an even force when I was pulling the shoe along the desk.
- The Newton measurers were very old and had been used by many pupils in the past so I could not receive their full potential.
- I could not get a continuous balanced pull on the trainer, so the trainer kept moving at different angles and suddenly jerking.
- The weights inside the trainer kept moving as the trainer was in motion.
On my graph I have some anomalous results I have marked these down as points A and B these results did not really fit in with the rest and my line of best fit, although the others more or less did.
I should have definitely repeated my results to try and get a similarity between the two and to back both results up. However, I did not achieve this through lack of time. This related to a minimal amount of results. To perform a reliable experiment I should have obtained more results and spent more time examining them.
There is a small resemblance between all of my results as the larger the weight put into the shoe the higher the force of Newton’s required.
The main thing I would comment on is that I wish that I could have had more time to perform my experiment and obtain more results.
