8. Plot the graph of the limiting static friction (fL) against the normal force (R).
Since fL =μs R, find the value of coefficient of static friction (μs) from the graph.
The slope of the straight line () = 0.42
9. Plot the graph of the kinetic friction (fk) against the normal force (R). Since fk = μk R, find the value of coefficient of kinetic friction (μk) from the graph.
The slope of the straight line () = 0.26
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Repeat steps 5 to 9 by replacing the wooden block with the bricks. Find the values ofμs andμk for the bricks.
Mass (1 wooden block) = 1.52 kg
The slope of the straight line () = 0.63
The slope of the straight line () = 0.39
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Repeat steps 5 to 9 by replacing the bricks with the steel blocks. Find the values ofμs andμk for the steel blocks.
Mass (1 wooden block) = 2.02 kg
The slope of the straight line () = 0.87.
The slope of the straight line () = 0.65
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Connect the spring balance to a wooden block. Then pile another wooden block on the top of the original wooden block. Pull the spring balance attached to the two blocks in pile at a constant speed. Record the applied force (F = fk ) from the spring balance in the table below.
Results and Discussion
- Describe the shapes of the graphs plotted in step 4.
The graph shows the magnitude of the force of friction versus the applied force. The force
of friction at the contact surfaces between the block and the bench is opposite to the
applied force. The force of static friction equals the applied force as the block remains
static. When the applied force exceeds the force of kinetic friction, the block starts to
move. In general, the force of kinetic friction stays constant as the block continues to
move along on the bench. It is note that the maximum force of static friction is greater
than the force of kinetic friction.
- Comment the case that when the applied force is smaller than the static friction.
When the applied force is smaller than the static friction, the block will remain stationary.
- Explain why the coefficient of kinetic friction is smaller than the coefficient of static friction.
When the block is set in motion, the block actually slips over the bench. This slipping
motion indicates the retarding frictional force (i.e. the kinetic friction) is less than the
maximum force of static friction.
4. Compare the values of coefficients of friction for wooden blocks, steel blocks and bricks.
The coefficient of kinetic friction is less than the coefficient of the static friction in all
three examined cases. The heavier the block of same materials produces greater force of
friction. In general, the coefficient of friction depends on the nature of the surfaces. Some
values of and for different surfaces are given in Table A1.
Table A1 Coefficients of friction (approximate values)
- Referring to the table in step 12, compare the results of the blocks in pile and placed side by side. State your findings on the effect of contact surface area
The results show that the force of friction between the blocks and the bench does not
depend on the area of the contacted surface.
- State the sources of error and suggest improvements for this experiment.
(a) The value of fk (shown on the spring balance) is taken when the block is just set in
motion. The applied force recorded at this short instant of time is assumed to be the
same as the maximum force of static friction. Reading from the spring balance may
cause error especially viewing in a short time.
(b) The block was not moving in a straight line.
(c) The surfaces are not fully in contact with each other due to the regularities of the
surfaces.
7. Give a conclusion to this experiment.
The coefficient of kinetic friction is less than the coefficient of static friction. They vary
for the nature of the contacted surface.
New Way Physics for Advanced Level