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# Investigation into Friction.

Extracts from this document...

Introduction

Investigation into Friction

Aim

Our aim is to investigate the relationship between the frictional force between two surfaces, and the force needed to make the surfaces slide over one another. The investigation will also be into the type of surface and the amount of frictional force, and the force needed to make the surfaces slide. We will also be investigating the coefficients of friction on different surfaces.

Theory

Friction is the name given to the force, which opposes the relative sliding motion of two surfaces in contact with one another, as Ordinary Level Physics[1] by AF Abbott tells us. This means that when two objects slide over one another, or touch one another, there is a frictional force present that tries to stop any sliding movement. The attractive forces between the molecules on the two surfaces cause friction, the Ordinary Level Physics says. Fig 1[2] shows two surfaces, and the cause of friction between them. The surfaces are not totally smooth, and so the particles oppose the forward movement of the others, and are also attracted to each other, causing friction.

Fig 1.

The more force there is acting on the surfaces, the more friction there is. This means that if there were a force of 10 N acting on the top surface, the friction would be greater than if no force other than gravity were acting on the top surface. There is also less friction between two surfaces if there are less ‘rough’ places for friction to occur. This means that if the two surfaces were smooth, there would be less frictional force acting than if they were both rough surfaces. Therefore, the smoother the surfaces, the less resistance there is to movement due to friction. Also, the more force there is acting on surfaces, the greater the frictional forces between them.

Middle

Force meter calibrated in NewtonsString

Method

1. The apparatus was set up as shown in Fig 2.
2. The smooth hardboard was used first as the bottom surface.
3. Weights were added to the wooden block 10 N at a time.
4. The force meter was used to make the block move along the smooth hardboard.
5. The reading on the force meter was taken at the moment when the block moved.
6. This was repeated immediately for each weight.
7. The experiment was repeated using the rough hardboard as the bottom surface.
8. The results were recorded in Tables 2 and 3.

Results

 Smooth hardboard Added mass / kg Total mass / kg Total weight / N Reading on force meter (F) / N 1st exp 2nd exp Average 1.0 1.328 13.28 5.5 6.0 5.75 2.0 2.328 23.28 9.5 10.0 9.75 3.0 3.328 33.28 13.5 13.0 13.25 4.0 4.328 43.28 15.0 17.0 16.00 5.0 5.328 53.28 21.0 17.0 19.00 6.0 6.328 63.28 23.0 24.5 23.75 7.0 7.328 73.28 25.0 30.0 27.50

Table 3.

 Rough hardboard Added mass / kg Total mass / kg Total weight / N Reading on force meter (F) / N 1st exp 2nd exp Average 1.0 1.328 13.28 5.0 4.5 4.75 2.0 2.328 23.28 8.0 8.5 8.25 3.0 3.328 33.28 13.0 13.5 13.25 4.0 4.328 43.28 18.0 18.0 18.00 5.0 5.328 53.28 22.0 22.0 22.00 6.0 6.328 63.28 23.0 24.5 23.75 7.0 7.328 73.28 25.0 25.0 25.00

Table 4.

Observations

When we did the experiment, we discovered that the force taken to make the masses move on the second reading on the smooth hardboard was greater than the preliminary experiment predicted. When the weight added was 70 N, the force needed to make the burden move was 30 N for the second reading. This meant that the following masses would be too heavy for it to only take 30 N to move them. The experiment for the smooth hardboard was then only continued to an added weight of 70 N.

Analysis of Results

The masses added in the smooth surface experiment did not reach 93.28 N, as predicted. This could have been because the surfaces were not exactly the same throughout the experiment, and so when it came to adding 90 N to the block, the surface had been changed significantly enough for the frictional force to have increased. Also, the place where the preliminary experiment was done could have been different to the place where the actual experiment took place.

Conclusion

## Improvements to provide additional evidence

If the pressure sensor was used, the coefficient of dynamic as well as static friction could be investigated. Using the method described above, when the block touched the pressure sensor when it just started to move, the data logger would record the force needed. But because the pressure sensor is moveable, after the block has touched it, if the block kept being pulled, the pressure sensor would come with the block. This would mean that the force needed to keep the block and weights moving could be recorded. This could be used to give us the coefficient of dynamic friction, and we could then compare the two coefficients of static and dynamic friction, on different surfaces.

We could also investigate how the speed at which the force at F is applied affects the coefficient of friction for different surfaces. This would be done by pulling the burden at different speeds. To make sure that the speed was regular, markings could be made on the board, indicating the position that the burden needed to be in after a certain time. For instance:

speed = distance ÷ time

If the speed was to be 0.001 m/s, then the block will have to have moved 1 cm in 1 second, 2 cm in 2 seconds etc. The centimetres could be marked on the hardboard, and the ‘puller’ would have to pace himself so that the block was passing the centimetre mark on the second. This method could be used for any speed. The affect that the speed that force is applied at could then be investigated, extending the original experiment.

Graphs

Graph 1.

Graph 2.

Graph 3.

Graph 4.

[1]Ordinary Level Physics; AF Abbott; page 17

[2] Microsoft Encarta 96 Encyclopaedia

[3]Letts GCSE Physics Classbook; Graham Booth; page 58

[4]Ordinary Level Physics; AF Abbott; page 18

[5]Ordinary Level Physics; AF Abbott; page 18

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