Experiment: Encountering Friction
• Purpose •
In this activity, you will explore the nature of the friction force. You will measure the force of friction in
different situations and calculate a useful quantity called the coefficient of friction. You will investigate
the factors that determine the magnitude of the friction force.
• Materials •
___ friction block
___ set of masses
___ spring scale
___ meterstick
___ surface board
___ 2 “C” or table clamps
• Discussion •
Friction is not a fundamental force. Rather it is the result of surface irregularities and electromagnetic
interactions that occur at a microscopic scale. When two objects are pressed together, the microscopic
“hills” and “valleys” of one surface fit (to some extent) into the “hills” and “valleys” of the other surface.
Where the objects make close contact, the molecules of one object “stick” to the molecules of the other
through electromagnetic force. This is called molecular adhesion. The force of friction arises when an
attempt is made to slide one surface across another. The molecular adhesion can be broken if enough
driving force is applied, but even while the surfaces are in sliding contact, molecules adhere and break
away over and over. We do not see this sequence. We see friction on a macroscopic scale. By measuring
friction and the factors that influence it, we hope to better understand the causes on the molecular
level.
• Procedure •
1. STATIC (STATIONARY) FRICTION
a. Set-Up.
i. Place the friction block on the surface board as shown above.
ii. Place a 200g mass on top of the block as if the block were a sled on which the mass were to
ride.
iii. Attach the spring scale to the eye hook as shown above. (If the scale can be calibrated, be
sure to do so before using it in any lab.)
b. Procedure. Pull the spring scale (holding the end opposite to that attached to the friction block). Make
sure the scale is parallel to the surface board, not at some angle to it. Pull the scale until the friction
block starts to move.
c. Measure. Determine and record the following.
i. Normal Force (N). Think of this as the total load. It is the combined weight (not mass) of the
friction block and the additional mass—everything that you were pulling.
• Determine the weight of the friction block experimentally (use the spring scale):
W = __________________
• Determine the weight of the 200g mass experimentally or by using the weight equation (W=mg). (You
cannot attach a slotted mass directly to a spring scale):
W = __________________
• Normal force:
N = ____________________
ii. Static Friction Force (fs). This is equal to the maximum force you exerted—as indicated by the
spring scale—before the friction block moved. (Before motion occurred there was no
acceleration, so the friction force and your pulling force were equal in magnitude and opposite
in direction.) For greatest accuracy, each member of the group should perform the procedure
and an average should be determined. Record the raw data and the average.
Trial 1 Value: ______________
Trial 2 Value: ______________
Trial 3 Value: ______________
Average: _________________
iii. Contact Area (A). This is the area of the bottom of the friction block. Make and record
appropriate measurements and calculate the area of contact.
L = ___________
A = L x W = _________________________
W = __________
A = ____________
d. Determination of the coefficient of static friction (μs). The coefficient of static friction is the ratio of
the static friction force to the normal force. Use your data to determine the coefficient of static friction.
Show the calculation (letters, then numbers, then solution) and box your answer.
2. KINETIC (SLIDING) FRICTION
a. Set-Up. Same as in procedure 1 above.
b. Procedure. Pull the spring scale (holding the end opposite to that attached to the friction block). Pull it
so that the friction block moves with a constant speed of about 5cm/s. Practice first by moving your
finger along the meterstick at 5 cm/s. When you pull the friction block, make sure the scale is parallel to
the surface board.
c. Measure. Determine and record the following.
i. Normal Force (N). (same)
ii. Kinetic Friction Force (fk). This is equal to the force you exert—as indicated by the spring
scale—while the friction block moved. (While the friction block is in uniform motion there is no
acceleration, so the friction force and your pulling force are equal in magnitude and opposite in
direction.) For greatest accuracy, each member of the group should perform the procedure and an
average should be determined.
Record the raw data and the average.
Trial 1 Value: ______________
Trial 2 Value: ______________
Trial 3 Value: ______________
Average: _________________
iii. Contact Area (A).
L = ___________
A = L x W = _________________________
W = __________
A = ____________
d. Determination of the coefficient of kinetic friction (μk). The coefficient of kinetic friction is the ratio of
the kinetic friction force to the normal force. Use your data to determine the coefficient of kinetic
friction. Show the calculation (letters, then numbers, then solution) and box your answer.
3. QUESTIONS
a. Given the same circumstances of load, surface area, and surface roughness, which is greater: static
friction force or kinetic friction force? (The difference may or may not be significant, but one will be
greater than the other.)
b. Given the same circumstances of load, surface area, and surface roughness, which is greater: the
coefficient of static friction or the coefficient of kinetic friction? (The difference may or may not be
significant, but one will be greater than the other.)
4. RESEARCH
a. Question. Consider the following quantities: static friction force, coefficient of static friction, kinetic
friction force, coefficient of kinetic friction. Determine how each is affected—if at all—by changes in
the following quantities: normal force, contact area, and sliding speed (kinetic friction force and
coefficient of kinetic friction only).
b. Predictions.
i. If the normal force (load) were increased while all other factors remained unchanged,
• the static friction force would
___increase ___decrease ___not change.
• the coefficient of static friction would
___increase ___decrease ___not change.
• the kinetic friction force would
___increase ___decrease ___not change.
• the coefficient of kinetic friction would
___increase ___decrease ___not change.
ii. If the contact area were decreased while all other factors remained unchanged,
• the static friction force would
___increase ___decrease ___not change.
• the coefficient of static friction would
___increase ___decrease ___not change.
• the kinetic friction force would
___increase ___decrease ___not change.
• the coefficient of kinetic friction would
___increase ___decrease ___not change.
iii. If the speed of the block were increased while all other factors remained unchanged,
• the kinetic friction force would
___increase ___decrease ___not change.
• the coefficient of kinetic friction would
___increase ___decrease ___not change.
c. Plan and implement a procedure for determining an answer to the question above.
i. You must determine a method to vary the normal force and measure static friction force,
coefficient of static friction, kinetic friction force, and coefficient of kinetic friction. You will then
compare values to determine which of the quantities you measured—if any—depend on the
quantity you changed.
ii. You must determine a method to vary the contact area and measure static friction force,
coefficient of static friction, kinetic friction force, and coefficient of kinetic friction. You will then
compare values to determine which of the quantities you measured—if any—depend on the
quantity you changed.
iii. You must determine a method to vary the sliding speed and measure kinetic friction force
and coefficient of kinetic friction. You will then compare values to determine which of the
quantities you measured—if any—depend on the quantity you changed.
d. At the end of the class period, turn in one copy of this write-up and one copy of the raw data on
which you will base the conclusions in your Experiment Report. Your report may not include any data
not turned in at this time.
e. Record the procedure, data, calculations, and conclusions.
i. Describe your procedure so clearly that a classmate could repeat it without having to ask you
any questions.
ii. Construct and complete well-organized data tables. Pay attention to units of measure.
iii. Show calculations of appropriate percent differences.
iv. State the conclusion of experiment in two or three statements as follows
• There is or is not a dependence of one variable on the other (i.e., the static friction
force is dependent on the normal force).
• By changing one variable by a significant amount, the other variable also changed by a
significant amount (i.e., a significant—120%—increase in the normal force resulted in a
significant—70%—increase in the static friction force). Be sure to include percent
difference values here to justify either a dependence or a lack thereof.
• The nature of the dependence (i.e., increasing the normal force results in an increase
in the static friction force). This statement is not required if no dependence exists.
An example of a lab report for an experiment similar to this one can be found on the next page. The
purpose and other details are different, but the report requirements are the same. You should find it
instructive as you prepare your report.