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.
A greater limiting friction is needed to separate the high spots and a greater dynamic friction is also needed to keep the surfaces sliding.
Apparatus
1. Newton metre
2. 1 kg weight
3. 4 kg weight
4. 3 kg weight
5. 4 kg weight
6. 1 piece of wood
7. 1 nail or hook
All these things are shown in the diagram below;
Method
1. Place the masses on top of the piece of wood and place the wood on a flat surface;
2. Attach the string onto the Newton metre and fix the hook on the end of the wood;
3. Give a controlled tug on the Newton metre. The measurement scale will move. Carefully observe the measurement scale. Record the pulling force displayed on the scale when the wood starts to move steadily and stop pulling.
4. Add the required masses to the shoe and repeat.
5. carefully note down the forces given.
Results Of The Preliminary Experiment
the experiment went quite well but I could have improved it in some ways. I have noted this down in the evaluation. To ensure the wood was pulled safely I tied a piece of string to the neutron metre and pulled the string.
The range of masses and the results found were as follows: -
200g - 2.5N
400g - 2.9N
500g - 3.7N
1000g - 6.1N
1500g - 8.9N
2000g - 13.7N
Evaluation
Overall, this experiment went quite well but I have listed below some constant variables, which may make the experiment fairer;
Constant Variables;
To make this experiment a fair test the following variables must be kept constant throughout the experiment.
Ø Area of wood in contact with surface
Ø The slope of surface
Ø The length of string
Ø Type of surface under wood
Ø Thickness of wood
Ø Size of piece of wood
Safety Procedures
(i) Make sure the weights are firmly on the wood so it is not possible for the weights to fall out and cause damage to the surroundings.
(ii) Place the wood in the middle of the table to prevent it from falling off.
(iii) Do not force too many weights on the wood as they may damage the wood or fall out.
By Muhammad Geloo
