Lab 5

Static and Kinetic Friction

Objective:
In this experiment you will determine the coefficient of static friction and the coefficient of kinetic friction between two wooden surfaces.


Materials:

Flat wooden ramp
Wooden block
Meter stick


Setup:

Discussion:
The force of friction is a contact force between two surfaces. A small scale view of friction involves molecular bonding between the two surfaces as well as any meshing together of the uneven surfaces. This meshing together would involve the hills and valleys of one surface being pushed into and filling the hills and valleys in the other surface. Thus you could infer that the force of friction would be directly proportional to how strongly the two surfaces are pushed together. In fact the constant of proportionality is µs the coefficient of static friction. If the two bodies in contact are on a flat surface, then the force that pushes them together is the weight of the object.

Here it is easy to see that the Normal force must be equal to but opposite to the weight of the object. Thus the equation that governs static friction is:

When the block is just on the verge of slipping:

For our setup the vector diagram would appear as follows:


From Newtons 1st Law and the free body diagram drawn above it can be shown relative to the coordinate system illustrated that:

Horizontally:
Fs + Wx = 0
and Vertically:

FN + Wy = 0


where Wx and Wy are the horizontal and vertical components of the weight vector W, Fs is the frictional force that opposes the motion, and FN is the normal force . In order to obtain a value for µs it is necessary to obtain values for Wx and Wy since they are equal to, but opposite to, Fs and FN.

From the diagram:
therefore:

Procedure:

1.Set up the apparatus as shown in the diagrams above.

2.Measure the mass of the block and record it's weight in the chart provided.

3.Place the mass on the ramp and increase the angle of the ramp until the block just begins to slip.

4.Record the angle several times for the same block and record the average and the average deviation for the angle in the chart provided.

5.Using the average value of , compute Wx, Wy, and finally µs.

6.Repeat the procedure with at least 5 different masses on the block.


Coefficient of Static Friction


Coefficient of Kinetic Friction (µk)

Procedure:
1.Set up the apparatus as shown in the diagrams above.

2.Measure the mass of the block and record it's weight in the chart provided.

3.Place the mass on the ramp and, while tapping on the ramp with your finger, increase the angle of the ramp until the block just begins to slip.

4.Record the angle several times for the same block and record the average and the average deviation for the angle in the chart provided.

5.Using the average value of , compute Wx, Wy, and finally µk.

6.Repeat the procedure with at least 5 different masses on the block.

Coefficient of Kinetic Friction

Questions:

1.How precisely did you determine your values for µs and µk ?

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2.By what percentage does the coefficient of static friction differ from the coefficient of kinetic friction?

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