Physics Lab: Friction
Equipment list: friction blocks spring scales large weight set centigram balance
Purpose: To explore the properties of friction and calculate a kinetic and static coefficient of
friction between a wooden block and the surface below it.
This lab will test the three claims about friction:
Claim 1. The coefficient of friction is independent of the weight of the block and the force of
friction is linearly proportional to the weight of the block. µ should not change for different
normal forces. The frictional force will change, but µ should be constant.
Claim 2. The force of friction depends on the type of surfaces interacting but is independent of
the area of contact between them. If this statement is correct you should measure the same force
of friction regardless of which of the block's six possible surfaces is being dragged across the
table.
Claim 3. The coefficient of static friction is typically greater than the coefficient of kinetic
friction (µs > µk). Measure µs and compare to your value of µk as measured earlier. If you have
uncertainties, you can make a scientific claim as to which coefficient is greater.
Preparation: This lab investigates all three of the above statements to varying degrees of
thoroughness. But you will need to know how to take measurements with the spring scale and
then how to relate those values to the force of friction.
1. Starting with Newton's second law, derive a formula that relates the coefficient of kinetic
friction, µk, to the horizontal tension force, FT (measured by the spring scale) when the block
moves at constant velocity. This allows you to know the friction force by taking a
measurement from the spring scale.
2. Taking measurements with the spring scale. The spring scale has two sets of graduations on
it. One set reads mass in grams and the other set reads force in newtons. Make sure the spring
scale measurements for the tension force are taken in newtons, not grams.
3. Determining the uncertainty of the spring scale. Using a "rule of thumb" method for the
absolute uncertainty of a scale reading (i.e., the absolute uncertainty is equal to half the
smallest increment on the scale), record in your lab book the absolute uncertainty associated
with a measurement from the scale.
Procedure:
Testing claim 1. This is the main part of the lab and requires a full page graph and an explanation
of what your results show. The goal is to calculate the kinetic coefficient of friction between
your block and the table by measuring the slope of a graph plotted from experimental data. Be
sure you can calculate the value of µ from the weight and the force required to pull the mass.
Pull the block at constant velocity with the spring scale. From the spring scale reading, find the
force of friction on the block. By adding weights to the top of the block and thereby increasing
the normal force on the block, a new force of friction can be measured with the spring scale by
again pulling the newly weighted block at constant velocity. Take at least five data pairs of the
friction force (as ultimately measured by the spring scale) and the normal force on the block
(include the mass of the block and all weights added to the top of the block. From these data
construct a graph whose slope is equal to the coefficient of kinetic friction. Measure the slope
and find the coefficient.
Testing claim 2. This part can be done casually without a formal experimental process. Pull the
block at constant velocity with as many different sides in contact with the surface as is possible
(at least two different sides). Does the force of friction change (within the uncertainty of your
measurements) when the block is on different sides? Answer this question by reference to a data
table you collect for different surfaces.
Testing claim 3. After the coefficient of kinetic friction has been found, from claim 1 above,
calculate the coefficient of static friction. This will be done with a few measurements and then a
direct calculation. With the block at rest, gently pull the block with the spring scale. The spring
scale's reading (and so the force of friction) will increase to its maximum value where the block
will break free and start to slip. The reading on the spring scale when the block just breaks free
and slips is the maximum force of static friction on the block. From this and knowing the mass of
the block, the coefficient of static friction may be directly calculated without using the slope of a
graph.
CONCLUSION: On the basis of your results, refute or confirm the above three claims. Defend
your results.
TIPS:
While pulling the block, the spring scale should be held in a horizontal position so that the scale
neither pulls up or down on the block.
Make sure the spring scale reads zero when all the slack is taken up in the spring scale
mechanism as you first start to pull on it.
Make sure no dust, grime, or errant matter is between the block and the table.