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WHAT A DRAG
PHYSICS LAB: FORCE OF FRICTION
Introduction: We don’t often think about the force of friction until the moment we slip down a muddy
hill during a hike or hit a patch of ice while driving. In both the mud and ice examples, the force of
friction is reduced and it’s an obvious inconvenience to us. On the other hand, machines are lubricated
(like a car’s engine) in order to reduce friction as friction is not an advantage.
Concepts: Friction, Force
Objectives: For this lab, you will investigate the concepts of friction. A simple determination of the
coefficient of friction between surfaces is made through graphical analysis of force of friction vs. normal
force.
Materials:
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Smooth Tabletop or Counter
Force of Friction Set (Ziplock bag)
o (5) 100-gram masses
o (1) Spring Scale
o (2) Wood Blocks w/Eyebolts (one smooth and one w/sandpaper)
Equations and Keywords:
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Normal Force: On a level surface, the normal force (Fn) is always equal and opposite to the
weight (only on a level surface).
Force of Static Friction: The force of static friction (fs) cancels out the applied force right up to
and including when static friction reaches its maximum (fsmax).
Force of Kinetic Friction: For applied forces greater than the maximum force of static friction, the
block starts to slip and then the value for friction becomes kinetic friction (fk) and the wooden
block is then under a net force, so it accelerates to the left or right (depending on the direction
of the applied force).
Coefficient of Friction (μ): The ratio of the force that maintains contact between an object and a
surface and the frictional force that resists the motion of the object. Each surface combination
has a unique COF. Low COF smooth surfaces  less friction. High COF  rough surfaces 
high friction.
Equations
o 1) Normal Force: Fn = m*g (recall that the vector arrow for normal force is positive, so we
use 9.81m/s2 for gravity constant)
o 2) Coefficient of kinetic friction: μk = fk
3) Coefficient of static friction: μs = fsmax_
Fn
Fn
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Procedure:
Part A: Frictional forces versus surface area (Practice reading static and kinetic forces by sliding the block
several times BEFORE recording any actual trial data.)
1. Place wood block flat on a tabletop.
2. Attach a spring scale to the eyebolt.
3. Measure the static friction between the block and tabletop (max force just before the block
slides). You should practice first before recording any data. Record the results in Data Table A on
the Friction Blocks Worksheet.
4. Measure the sliding friction between the block and the tabletop (the force as the block slides
across the counter at a constant speed). You should practice first before recording any data.
Record the results in Data Table A.
5. Now, place the same wood block on its edge (thin side) and again attach a spring scale to the
eyebolt.
6. Measure the static friction between the block on its edge and the tabletop. Record the results in
Data Table A.
7. Measure the sliding friction between the block on its edge and the tabletop. Record the results
in Data Table A.
8. Repeat the steps in Part A using the rough wooden block. RECORD ONLY THE MOST RELIABLE
TRIALS (IN NEWTONS).
Part B: Frictional forces versus Normal force
9. Obtain 5 masses at 100-grams each.
10. Place the smooth wood block on the tabletop and attach a spring scale to the eyebolt.
11. Measure the static and sliding friction between the wood block and tabletop. Record the results
in Data Table B.
12. Place one 100-gram mass on the wood block. Enter the weight of the mass in Newtons in Data
Table B. (Multiply the mass in grams by 0.00981 to obtain the weight in Newtons.)
13. Again, measure the static and sliding friction between the wood block and tabletop, with the
additional mass. Record the results in Data Table B.
14. Repeat steps 12-13, increasing by 100-grams for each trial until you reach 500 total grams.
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FRICTION BLOCKS WORKSHEET
Data Table A*
Object
Tested Block
Surface
Tested Surface
Static Frictional Force (N)
Sliding Frictional Force (N)
EXAMPLE (if spring scale reads in grams)
Flat
Flat
Edge
Flat
Edge
Tabletop
Tabletop
Tabletop
Tabletop
Tabletop
Ff = gramsspringscale * 0.00981 = _____N
Ff = gramsspringscale * 0.00981 = _____N
Smooth Block
Smooth Block
Rough Block
Rough Block
*Some spring scales read only in grams. If that is the case, see the example above in order to convert to the appropriate frictional force (Ff).
Data Table B (Smooth Block)
Normal Force (weight added to block): See Equation 1
Static Frictional Force (N)
Sliding Frictional Force (N)
Static Frictional Force (N)
Sliding Frictional Force (N)
0 Newtons (initial)
Fn = 100g * 0.00981 = 0.981N
200g
300g
400g
500g
Data Table B (Rough Block)
Normal Force (weight added to block): Fn = m*g
0 Newtons (initial)
Fn = 100g * 0.00981 = 0.981N
200g
300g
400g
500g
Post Lab Questions (answer on a separate sheet of paper)
Part A: Frictional forces versus surface area
1. Does it take more force to start an object sliding over a surface or to keep it sliding at a constant speed?
2. How do the frictional forces between the flat and edge surfaces compare? What influence does the surface area have
on the frictional force? Which two items determine the frictional force acting against motion?
Part B: Frictional forces versus Normal Force
3. Using graph paper, draw a graph of static frictional force versus Normal force using the information in Data Table B.
Draw a “best fit” line through the data points. (Include a color coded key to distinguish between smooth and rough
blocks.)
4. Does the data produce a straight “best fit” line? If yes, what does the slope of each line represent?
5. On the same sheet of graph paper, draw a graph of the sliding frictional force versus Normal force using the
information found in Data Table B. Draw another “best fit” line through this data. (Include a color coded key to
distinguish between smooth and rough blocks.)
6. Does this data produce a straight “best fit” line? If yes, what does the slope of each line represent?
7. Determine the coefficients of static and sliding friction (i.e. slope*) between the wood blocks and the tabletop from
the corresponding graphs.
Wrap-up:
8. Organize lab equipment in the correct ziplock bag (check that your lab set numbers all match). Return lab equipment.
9. Turn-in completed lab, completed post lab questions, and graph to the basket.
*Refer to equations 2 and 3 listed in the lab.
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