HPP Activity A22.v1
Slipping and Sliding with Shoes
This semester you have studied various motions such as walking, jumping, and falling through
the air, and Newton's laws of motion. You have discussed the effects of friction on moving carts
and projectiles flying through the air. Today you are going to start a more detailed study of
friction. What is it? Can it be incorporated into Newton's laws of motion? Does it have a
"simple" quantitative mathematical model?
Exploration - Take a look at that shoe…
There should be a shoe located at each of the lab stations around the room. Using your group's
shoe and any necessary additional equipment, record the following basic information describing
this shoe.
GE 1.
1. Write a brief description describing this shoe including how it looks and
the materials from which it is made.
2. Using a provided balance, measure the mass of this shoe in kilograms.
3. Using a provided spring scale, measure the weight of this shoe (in
Newtons).
4. Compare the two values you just measured. Are they the same? Should
they be? What is the relationship between the two?
Be ready to share your results with the class and record any helpful notes from
the class discussion in space below.
Exploration - Pulling a shoe sitting on a level surface
GE 2.
1. What do you think will influence the amount of force required to cause
your shoe to slip on a level surface? Make a list of what you think are the
important properties of a shoe/level surface system.
Activity Guide
 2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
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You will now conduct a series of experiments to test your ideas.
Experiment #1 - Shoe on the floor
Set up your equipment as pictured. Pull on the shoe and measure the force required to just keep
the shoe moving. Have each person in your group try this and record the values. Be sure to pull
parallel to the floor without lifting up on the shoe. Make sure you zero (tare) the force probe
before each measurement. Start the data acquisition and the slowly begin to pull on the shoe as in
the figure below until the shoe begins to slide at a constant velocity across the floor. Begin by
having each group member pull on the shoe with the force probe.
Force
prob e
GE 3.
1. How reproducible is your data? Are each group member's graphs the
same? Similar? Very different?
2. Describe the graphs from the moment you just started to pull until just
before the shoe began to slide?
3. Describe the graphs from the moment the shoe began to slide and while
the shoe was moving at a constant velocity.
We define two kinds of friction forces based on these observations: 1) static
(not moving) friction force which can be anywhere between zero and some
maximum value and 2) the kinetic (moving) friction force that is constant and
independent of the speed of the sliding object. The friction forces always
oppose any motion or potential motion.
4. What can you conclude about the force required to keep the shoe moving
at a constant velocity?
5. On the picture below, draw all the force vectors acting on the shoe while
the shoe is sliding at a constant velocity. Try to draw appropriate lengths for
the vectors. I.e. make a free-body diagram of the shoe while sliding. Does the
surface exert a force on the shoe? In which direction(s)?
Activity Guide
 2010 The Humanized Physics Project
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Force
probe
As we have seen, forces are vectors and so to add them we must define a
coordinate system and find the sum of the x-components and the ycomponents separately.
6. Using your labels for the forces in the free-body diagram above, write
down the two sums of the x and y forces (or their x and y components).
According to Newton’s laws of motion, what should these two sums equal?
7. Now slide the shoe across the floor with differing, but constant, velocities.
Does the sliding friction change with velocity?
8. Is a constant force required to move the shoe across the floor? Is this
consistent with Newton's laws of motion? Explain.
9. Now collect data for your shoe sliding across different surfaces. Ask your
instructor for ideas if you need some. Test at least 5 different surfaces. Put
your data in the table below.
Surface
Floor
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 2010 The Humanized Physics Project
Force [N]
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10. What conclusions can you draw from your data? Does the smoothness of
the surfaces play a role in friction? Does the “stickiness” of one of the surfaces
play a role? What has the highest friction force? The lowest?
Experiment #2 - Weight a minute!
11. In question #1 above, many of you may have predicted that the heavier a
person is, the more force it will take to move his/her shoe across the floor.
Make a detailed study to test this prediction. What variable will you change?
What variable will you measure to test the prediction? What will you hold
constant?
Collect your data and fill in the table below. You will need at least 5 data
points. Based on your data, can you predict what the friction force will be if
the weight of the shoe could be made zero?
Weight of shoe [N]
Friction force [N]
From this data, create a graphical model of the force required to keep the shoe
sliding at a constant velocity vs. the total weight of the moving shoe
(including the additional weights). Be sure to include all necessary
information on your graph. Remember to also save your Excel file often and
record the name of any saved files in the space below.
Once you have created a graphical model, then create the corresponding
mathematical function model representing this data. Paste a copy of this
graphical and mathematical function model in the space below. Be sure to
rewrite your mathematical function model using appropriate variables and
units. (paste special -> picture -> don’t float over text)
12. In words describe the relationship between the force required to keep the
shoe moving at a constant velocity as a function of the total weight of the
shoe.
Activity Guide
 2010 The Humanized Physics Project
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Experiment #3 - Wider is better!
Conduct an experiment to measure the friction force as a function of surface
area of the object. Because of the difficulty with shrinking or enlarging your
shoes, we will instead model our shoes with a block of wood with three
different flat faces. Measure the friction force for each of the three faces and
record the data in the table below.
Contact area [cm2]
Friction force [N]
13. What conclusion(s) can you draw from your data? Does this make sense?
Should wider tires on your car give you better traction?
Invention
Be prepared to share your results to these three experiments with the rest of the class. Record a
summary of the results of the class and any notes you have from this discussion.
GE 4.
1. Go back and calculate the coefficient of kinetic friction for each of your
experiments above.
Description of experiment
Shoe on floor
Shoe on
Shoe on
Shoe on
Shoe on
Shoe on ? weight =?
Shoe on ? weight =?
Shoe on ? weight =?
Shoe on ? weight =?
Activity Guide
 2010 The Humanized Physics Project
Coefficient of Kinetic friction
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Shoe on ? weight =?
Block on table area = ?
Block on table area = ?
Block on table area = ?
Application - Walking
Try the following activity to help think about the role that friction plays when
we are walking.
GE 5. The Pencil Experiment
Incline a pencil to the table and make it slide by pushing on it, along the
direction of the incline. Starting from straight up and down, determine the
angle when the pencil just starts to slide.
1. How does that angle change (or not change) as you increase the force you
exert on the pencil? Try a very gentle push, a mid-size push, and a very strong
push. Try each condition at least three separate times and record the resulting
angles. Describe your results.
2. Draw a free body diagram of the forces acting on the pencil's eraser at the
table's surface. Double-click on the empty picture below and then carefully
sketch all the forces.
Activity Guide
 2010 The Humanized Physics Project
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3. Based on the forces in your diagram, write down expressions for the sum
of the x-components and the sum of the y-components.
4. Solve the two equations in 3 for the coefficient of friction k in terms of
the angle of incline . You can consider the weight of the eraser to be very
small (= 0) compared to the other forces.
5. Discuss how your solution in 4 relates to your experimental results in 1.
GE 6. Getting Your Shoe (with you in it) to Slide
1. Make yourself rigid, like a pencil, and see what angle is required between
your body and the vertical to just get your feet to slide on the floor. Compute
the coefficient of kinetic friction between your shoe and the floor. Show your
calculations.
2. How does your result compare to the coefficients measured earlier
between a shoe and the floor by the class?
3. Predict the angle that you could make with the rug before slipping.
Explain your prediction. (Hint, think about the results with the carpet earlier.)
4. Make yourself rigid, like a pencil, and see what angle is required to just get
your feet to slide on the provided rug. Compute k between your shoe and the
rug.
Activity Guide
 2010 The Humanized Physics Project
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5. How well do your prediction and experimental results agree?
Application
GE 7. Walking on floors and carpets:
1. What is the maximum stride you can take without your shoe slipping on
the floor? on the rug? Why? Explain any differences.
Walking on ice:
2. Suppose you are walking on ice and the coefficient of friction between
your shoe and the sidewalk is now reduced to one-tenth of your value for the
floor. What is the maximum stride you can take? Explain your calculation.
3. Compare your stride on ice to your stride on the floor. Does this make
sense to you intuitively? Explain.
Sliding on ice:
4. Based on your results to Experiment #3, is the frictional force exerted on
your shoe by the ice more when you walk on ice or slide on ice? Explain.
Spinning the wheels of your car to get out of soft snow:
15. How good of a strategy is it to spin the wheels of your car to get out of soft
snow? Based on your study of friction, what would be a better strategy?
Explain.
Activity Guide
 2010 The Humanized Physics Project
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