Download LAB 07: What does Friction Depend on and Mμ

NAME:
DATE:
LAB 07: What does Friction Depend on and Mμ
PURPOSE
Investigate friction and measure friction coefficients of various shoes. In this experiment we will
determine the sliding coefficients of friction for various shoes and surfaces, and compare them to each
other.
TERMS and Equations:



Friction: a force that opposes the motion (or impending motion) between two surfaces
Normal Force: a force between two surfaces that is perpendicular to each surface
Coefficient of friction (μ): a quantity used to determine the amount of friction between two surfaces
Ffriction =  * Fnormal
where  is the coefficient of friction
(Eqn 1)
MATERIALS





Spring scale(s) (with capacity to measure weight of shoes)
Various shoes (i.e., sneakers, slippers, dress shoes, boots) and wood block
Two different surfaces (here we will use each side of the piece of wood)
250g, 500g, 1 kg or other masses
Connecting Material: String (1-2 m) / Tape
PRELIMINARY QUESTIONS
1.
In pushing a heavy box across the floor, is the force you need to apply to start the box moving greater than, less
than, or the same as the force needed to keep the box moving? On what are you basing your choice?
2.
How do you think the force of friction is related to the weight of the box? Explain.
3.
Do you think different types of shoes have different coefficients of friction? If so how do you think they differ?
4.
Select two different types of shoes to be tested in this experiment. Make a prediction about how the
coefficient of frictions will compare to each other. Discuss what made you choose your answer.
5.
Draw a Free-Body Diagram of
 a shoe AT REST
 a shoe that is being dragged along a horizontal surface at a CONSTANT VELOCITY.
PROCEDURE
1.
Measure the Weight of each shoe and block and record the measurement in your data table.
2.
Drag the first shoe horizontally with the spring scale at a constant slow velocity. Record the reading on the
scale and repeat two more times.
3.
Add weight to the shoe in increments to get at least 4 different data points and repeat the experiment three
times for each different weight.
4.
Repeat the whole process with two other types of shoes and the block.
5.
Repeat the process using the second surface for the block, and if you have time the shoes
Lab 07: What Does Friction Depend on and Mμ
CCC Physics I
1/2
NAME:
DATE:
EXTENSION: Try to determine the coefficient of friction a second way.
 Place the block on the board.
 Incline the board until the block just starts to move and continues at a constant velocity and measure the
angle at which this happens.
 Repeat the process for the surface on the other side.
 Using your knowledge of Forces and Trigonometry, use the measured angle to determine the coefficient of
friction between the block and the surfaces.
ANALYSIS
1.
Using your Free Body Diagram, create an equation for the coefficient of friction  of your shoe in terms
of its weight Fg and the force of friction Ff.
2.
Make a graph the force measured on the spring scale vs. the weight for each shoe. This graph may
be linear. Take the slope to determine the coefficient of friction (μ).
3.
How is the weight of the shoes related to the frictional force?
4.
How is the weight of the shoe related to the coefficient of friction?
5.
You measured the sliding (kinetic) which is at play when surfaces are in motion. Static friction is at
play before surfaces start to move but are being forced toward motion. How do you think they
compare to each other, is one greater? Why might this be?
6.
Compare the calculated coefficients of friction with the ranking in your hypothesis. What shoes have a
high coefficient of friction? Which ones have a lower value? What about the surfaces do you think
causes these differences?
7.
What are the units of the coefficient of friction? Why is this the case?
8.
Would it make any difference if you used the empty shoe or the loaded shoe to calculate the
coefficient of friction for a particular surface? Why or why not?
9.
Compare the calculated coefficients of friction with the values for the friction force. Does a bigger friction
force mean a bigger coefficient of friction? Why or why not might this be the case?
10. Athletic shoes tend to have a greater coefficient of friction than dress shoes. Why might they be
designed that way? What part of their design may attribute to their higher coefficient of friction?
11.
Rubber is often placed on stairs and sand is often placed on ice to make them safer. What would
this do to the coefficient of friction? Why does this make them safer?
12. EXTENSION: How did your coefficients of friction compare using the different methods? Which
one do you think is more accurate?
Lab 07: What Does Friction Depend on and Mμ
CCC Physics I
2/2