Inv. 6. Friction
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Investigation 6. Friction
Background
We have noted that Newton (1642-1727) stated that
objects in motion stay in motion and objects at rest stay at rest
unless they are acted upon by unbalanced forces.
He further said that this phenomenon is due to a property of matter he called
inertia. Challenging Newton’s statement, we observed that a rolling sphere on a
smooth surface does not roll forever. Why is this so?
Before Newton, Galileo (1564-1642) had described a force called friction that
he claimed resisted movement. We can get a feeling for Galileo’s friction by
pushing our hand over cloth. It takes force to push our hand across the cloth.
Therefore, there must be a force pushing against us or it would take no force to
push across the cloth. This raises the question of whether we can find and
measure such a force in other things that are rubbing against each other. Does
Galileo’s friction explain the slowing of a moving sphere on the floor?
Problem
What is the nature of friction? How does it affect motion?
Procedure
1. Carry out Activities 6–1 and 6–2. Repeat each measurement enough times
so that the results are consistent and reproducible. Answer the questions at
the end of each activity.
2. After completing the activities, answer the Investigation 6 Summary
Questions.
Activity 6–1. Frictional Forces
Questions
1. Can we measure a frictional force acting on a backpack when we pull it at a
constant speed across a table?
2. How might frictional forces opposing the pulling of an object be increased or
decreased?
Materials
• spring scale
• backpack
• materials as requested
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Fig. 6–1. Pulling a backpack with a spring scale on a table.
Directions
1. Place a backpack on a table. Slowly pull the backpack by hand across the table.
Keep your pull parallel to the table. Note what, if any, evidence you have of
friction. Record your observations.
2. Measure the force needed to pull the backpack across the table.
a. Attach a spring scale to the backpack as shown in Fig. 6–1.
b. Record force readings on the scale before the backpack starts to move, and
as it moves.
c. Pull the backpack at a constant speed and record the readings on the scale.
d. Note what, if any, evidence you have of friction. Record your observations.
3. Explore ways you can increase or decrease the force needed to start and
maintain movement of the backpack at a constant speed.
Discussion Questions
1.
Galileo’s frictional force was described in the Background. Was there such a
force produced in moving the bag? Explain and give evidence to support
your answer.
2.
Describe the force that acts on the bag when it is stationary and when it is
moving at a relatively constant speed. Provide evidence to support your
answer.
3.
Draw a graph that shows the amount of force applied to the backpack over
time from rest until it is moving at a constant speed. Start from zero force
and continue until the backpack has moved over the entire length of the
table.
Practices in Physics and Technology
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4.
Were you able to increase the frictional forces produced by an object pulled
across a surface? If so, how?
5.
Were you able to decrease the frictional forces produced by an object pulled
across a surface? If so, how?
6.
What variables affect the friction on an object?. Develop hypotheses about
how each variable affects friction.
Activity 6–2. Resistance to a Force
Questions
1. What can we learn about friction through the use of digital technology?
2. What does the graph of forces necessary to move a stationary object tell us
about friction?
3. How do the graphs of increased or decreased frictional forces on an object
compare?
Materials
• 1 backpack with weights
• 1 computer with interface
• 1 dual-range force sensor
• computer software (LabQuest, LoggerPro, or other software)
• printer
Directions
1. Predict the shape of the time vs. force graph when you pull the backpack
across the table. Sketch your prediction.
2. Follow the instructions of your teacher and set up the computer with installed
software. Interface or connect the computer with a dual-range force sensor.
3. Test the predictions you made in Direction 1. Place a backpack on a table
and hook it to the force sensor as shown in Fig. 6–2.
4. Push the collect button on the computer and slowly pull the backpack at a
constant speed, with the force sensor parallel to the surface. Save the
electronic time vs. force graph shown on the screen by printing or sketching
it.
5. Repeat Direction 4 to validate results. You may be able to save each run and
then display all runs on the same plot. Runs may appear in different colors.
6. Use the dual range force sensor to test each of your hypotheses on
Practices in Physics and Technology
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increasing and decreasing friction on the backpack (Activity 6–1 Question
6). Save the graphs.
Fig. 6–2. Hooking a force sensor to a backpack
Discussion Questions
1. Look at the shape of the electronic time vs. force graphs showing the force on
the backpack.
a. Describe the common features of the graphs.
b. Identify the events that correspond to the common features on the graphs.
c. How do the graphs compare with the shape you predicted? Explain why they
are the same or different.
2. What are the dependent and independent variables in this investigation? What is
your evidence?
3. What does the shape of the graph tell us about the friction on objects sliding
over surfaces?
4. What seems to be occurring at the moment an object starts to move?
5. As you tested your hypotheses about increasing or decreasing friction, how did
you determine whether friction increased or decreased?
6. What effect did changes in the following have on friction?
a. materials.
Practices in Physics and Technology
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b. shape of the backpack
c. surface area of the backpack
d. weight of the backpack
e. other
Investigation 6. Summary Questions
1. When we electronically measured the force on the backpack, what did the graph
reveal about the force necessary to get the stationary bag to start moving?
2. How can we increase or decrease the maximum value of the force of static
friction or the force of kinetic friction necessary to move an object horizontally
with no acceleration? Provide several examples of different ways.
3. Under what conditions do water and air create friction? Give several examples of
each kind of friction.
4. What are three or four examples of ways that friction helps or hinders us in our
daily lives?
5. What would walking though your home be like if there were no friction? Write a
brief tale about going from your bed to the kitchen area in a frictionless world.
6. How does friction help explain why a sphere stops rolling on the floor? What is
your evidence supporting your answer?
7. Newton stated that an object in motion will stay in motion unless acted upon by
an unbalanced force. What evidence from these activities supports or refutes his
statement?
8. How is inertia different from friction? How does each affect motion?
9. Fig. 6–3. (below) shows graphs of a backpack being pulled horizontally under
varying conditions. Time is on the x axis; force (N) on the y axis. Examine the
graphs. In Table 6–1, record
a. your explanation for each color-coded graph, and
b. what conditions could have been changed to get each of the different results.
Practices in Physics and Technology
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Fig. 6–3. Graphs showing forces of friction when a backpack is pulled across a horizontal surface
Table TG 6-1. Interpretations of graphs shown in Fig. 6–3
Graph Color
Your explanation
(What conditions may have been changed to get each graph?)
Green
Blue
Red
Orange
Pink
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