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B8 Inquiry Activity
REQUIRED SKILLS
■ Designing an experimental procedure
■ Reporting results
Relating Static Friction, Kinetic Friction, and the Normal Force
Question
What is the relationship between the force of static friction, the force of kinetic friction, and the
magnitude of the normal force acting on an object?
Materials
 balance
 wooden block with different face areas and a
hook
 horizontal board
 spring scale, calibrated in newtons
 set of standard masses
Part 1 — Relating Static Friction and the Normal Force
Procedure
1. Read the steps of the procedure and design a chart to record your results.
2. State a hypothesis relating the magnitude of static friction and the magnitude of the normal
force.
3. Measure the mass of the block using the balance.
4. Place the largest face of the block on the horizontal board. Attach the spring scale to the
block. Pull with an ever-increasing horizontal force until the block just starts to move.
Record this force, which is the maximum magnitude of the force of static friction.
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5. Increase the mass of the block system by placing a standard mass on the upper surface.
Record the total mass of the block with the standard mass. Use the spring scale to
determine the maximum magnitude of the force of static friction for this system (see
diagram below).
6. Repeat step 4 three more times, increasing the added mass each time until you have five
different masses and five corresponding maximum magnitudes of static friction.
7. Calculate the magnitude of the weight corresponding to each mass system. Record the
magnitude of the normal force.
8. (a) Graph the maximum magnitude of the force of static friction as a function of the
magnitude of the normal force.
(b) Draw the line of best fit and calculate the slope of the graph.
Analysis
1.
Describe the graph you drew in step 8.
2.
As the magnitude of the normal force acting on the mass system increased, what happened
to the maximum magnitude of the force of static friction?
What is the relationship between the maximum magnitude of the force of static friction and
the magnitude of the normal force? Write this as a proportionality statement. Does this
relationship agree with your hypothesis?
3.
4.
On a level surface, how does the magnitude of the weight of an object affect the magnitude
of the normal force and the maximum magnitude of the force of static friction?
5.
Explain why adding a bag of sand to the trunk of a rear-wheel-drive car increases its
traction.
6.
Design and conduct an experiment to verify that contact surface area does not affect the
maximum magnitude of the force of static friction for a sliding object. Identify the
controlled, manipulated, and responding variables. Analyze your data and form
conclusions.
Part 2 — Relating Kinetic Friction and the Normal Force
Procedure
1. State a hypothesis relating the magnitudes of kinetic friction and the normal force.
2. Use the set-up shown in the diagram in part 1 to design an experiment. List the materials
you will use as well as a detailed procedure. You will need to place objects of different
mass on the block for each trial.
3. For each trial, measure the force that must be applied to keep the block system moving at
constant velocity. Then calculate the magnitude of the normal force.
4. Plot a graph of kinetic friction as a function of the normal force.
Continued on next page…
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Analysis
1. Analyze your data and form conclusions.
2. How well did your results agree with your hypothesis?
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