HPP A15.v2
Slipping and Sliding with Shoes
Slipping and Sliding with Shoes

You should be working with new lab partners starting with this lab.
So far this semester you have been studying various motions such as walking, jumping,
standing, and falling through the air. Today you are going to start looking at what happens
when different forces are brought into the mix.
The provided shoes will be used by many people. Please do not write on them!! If
you want to write on a shoe, then please use your own shoe for the experiments.
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.
1.
(a)
Write a brief description describing this shoe including how it looks and the
materials from which it is made.
(b)
Using a provided balance (triple beam and/or electronic), measure and record the
mass of this shoe.
(c)
Using a provided spring scale, measure and record the weight of this shoe.
(d)
Compare the two values you just measured. Are they the same? Should they be?
How can you tell if the values are in agreement?
Invention Discussion #1
Be ready to share your results with the class and record any helpful notes from the class
discussion in your logbook.
Pulling on a shoe sitting on a level surface
2.
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.
You will now conduct a series of experiments to test your ideas.
 Humanized Physics Project
2005 - UNL
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Slipping and Sliding with Shoes
 Humanized Physics Project
2005 - UNL
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Slipping and Sliding with Shoes
Experiment #1 - Shoe just starting to move on the floor
Set up your equipment as pictured. Pull on the shoe and measure what force is required
to just get the shoe to start moving. Be sure to pull straight out without lifting up on the shoe.
3.
Have each person in your group try this and record the values. How well do the force
values agree?
Your answer to question #2 may have included the idea that the heavier a person is, the
more force it will take to start moving his/her shoe across the floor. To test this idea, put at
least five different values of additional mass into the shoe and measure the force required to
just get the shoe to start moving for each mass value. Record the additional mass and the
resulting force values in your logbook. Convert the mass values to weight values and
determine the total weight for each instance (weight of the shoe plus additional weight).
From this data, create a graphical model of the force required to get the shoe to start to slide
vs. the total weight of the moving shoe (including the additional weights). Hint! You should use
the provided Excel file for your analysis. 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
your logbook.
Once you have created a graphical model, then create the corresponding mathematical
function model representing this data. Print a copy of this graphical and mathematical function
model for each member of your group. Be sure to rewrite your mathematical function model
using appropriate variables and units.
4.
In words describe the relationship between the force required to just get the shoe to start
moving as a function of the total weight of the shoe.
Experiment #2 - Shoe just starting to move on carpet
Repeat experiment #1, only this time place the shoe on the provided carpet square. Use
a c-clamp to keep the carpet from moving around. Again use five different total weight values
and record the force required to just get the shoe to start moving. After you have collected your
data, create a graphical and a mathematical function model for this data.
 Humanized Physics Project
2005 - UNL
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HPP A15.v2
5.
Slipping and Sliding with Shoes
How does the behavior of a shoe on the floor compare to that of a shoe on carpet? How
do your models reflect any differences?
 Humanized Physics Project
2005 - UNL
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Slipping and Sliding with Shoes
Experiment #3 - Shoe just starting to move compared to moving at a constant speed
Conduct an experiment to compare the following two forces: (1) the force required to
just get the shoe to start moving and (2) the force required to keep it moving with a constant
speed once it starts to slide. Try this both on the floor and on the carpet. The spring scales are
not always very accurate when used to measure something that changes quickly like these
forces. Instead of reading actual force values, try to qualitatively observe the stretch of the
spring inside the spring scale to judge what is happening.
6.
7.
Which of these forces seems to need to be larger? Describe your results.
Draw a free body force diagram for the shoe as it just starts moving identifying all
relevant forces
Invention Discussion #2 - Static and Kinetic Frictional Forces
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.
8.
(a)
From your data, what is the coefficient of static friction between the shoe and floor?
Between the shoe and carpet? Do these values seem reasonable to you? Explain.
(b)
How do the coefficients of friction compare for different shoes on the same surface?
Which shoe(s) have the highest value? Which the least? Does this make sense to
you? Explain.
Apply the Concepts to Walking
Try the following activity to think about the role that friction plays when you walk.
The Pencil Experiment
Incline a pencil to the table and make it just start to slide by pushing on it, along the
direction of the incline. Starting from straight up and down, determine the angle when the
pencil's eraser just starts to slide.
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2005 - UNL
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HPP A15.v2
9.
Slipping and Sliding with Shoes
(a)
(b)
(c)
(d)
How does that angle change (or not change) as you increase the force you exert on
the pencil? Try a gentle push, a mid-size push, and a strong push. Try each
condition at least 3 times and record the resulting angles. Describe your results.
Draw a free body diagram of the forces on the pencil's eraser. Assume that the
weight of the eraser is very small compared to the other forces and can be ignored.
Based on the forces in your diagram, write an equation relating all of the forces in
the y direction. Write another equation relating all of the forces in the x direction
From your equations, derive an expression for the coefficient of static friction s (i.e.,
s = ?). How does the expression for s depend on the angle of incline, ? Discuss
how this relates to your experimental results in (a).
Getting Your Shoe (with you in it) to Slide
10.
(a)
Make yourself rigid, like a pencil, and lean against
the wall (do not lean against a white board). See
what angle of incline, , is required between your
body and vertical to just get your foot to slide on
the floor. Compute the coefficient of static friction
between your shoe and the floor.
(b)
How does your result compare to the coefficients
measured earlier between a shoe and the floor by
the class? Are they in the same "ballpark"?
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2005 - UNL
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11.
Slipping and Sliding with Shoes
(a)
Predict the angle that you can make with the rug before slipping. Explain your
prediction. (Hint, think about the results with the carpet earlier.)
(b)
Make yourself rigid, like a pencil, and experimentally see what angle is required to
just get your foot to slide on the rug. Compute s between your shoe and the rug.
(c)
How well do your prediction and experimental results agree for the angle? How
well do the two values of s agree?
 Humanized Physics Project
2005 - UNL
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Slipping and Sliding with Shoes
Applying Your Results
Walking on floors and carpets:
12.
Using your angle values and the length of your leg, estimate the maximum stride
you can take without your shoe slipping on the floor and on the rug. Explain any
differences between the two stride lengths.
Walking on ice:
13. (a)
If you are walking on an icy sidewalk and the coefficient of static 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.
(b)
Compare your stride on ice to your stride on the floor. Does this make sense to
you intuitively? Explain.
Sliding on ice:
14.
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.
End of Lab Procedures

Return all equipment (shoe, weights, etc.) to the table of your lab station.

Quit the Excel software. If you are the last lab of the day, then shut the computer
down.
Want More Information?
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
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Using triple-beam and electronic balances (see Reference F)
Mass and weight (see Walker, Sections 1-2 & 5-1)
Forces (see Walker, Chapter 5)
Static and kinetic frictional forces (see Walker, Section 6-1)
 Humanized Physics Project
2005 - UNL
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