Download Static Friction

Physics 2 Lab 4: Static and Kinetic
Friction (this will go in your lab notebook)
Introduction: If you try to slide a heavy box resting on the floor, you may find it difficult to get
the box moving. Static friction is the force that is counters your force on the box. If you apply a
light horizontal push that does not move the box, the static friction force is also small and directly
opposite to your push. If you push harder, the friction force increases to match the magnitude of
your push. There is a limit to the magnitude of static friction, so eventually you may be able to
apply a force larger than the maximum static force, and the box will move. The maximum static
friction force is sometimes referred to as starting friction. We model static friction, Fstatic, with
the inequality Fstatic  s Fn where s is the coefficient of static friction and Fn the normal force
exerted by a surface on the object. The normal force is defined as the perpendicular component of
the force exerted by the surface. In this case, the normal force is equal to the weight of the object.
Once the box starts to slide, you must continue to exert a force to keep the object moving, or
friction will slow it to a stop. The friction acting on the box while it is moving is called kinetic
friction. In order to slide the box with a constant velocity, a force equivalent to the force of
kinetic friction must be applied. Kinetic friction is sometimes referred to as sliding friction. Both
static and kinetic friction depend on the surfaces of the box and the floor, and on how hard the
box and floor are pressed together. We model kinetic friction with Fkinetic = k Fn, where k is the
coefficient of kinetic friction.
In this experiment, you will use a Force Sensor and an Accelerometer to study static friction and
kinetic friction on a wooden block.
OBJECTIVES

Use a Dual Force Sensor to measure the force of static and kinetic friction.
 Measure the coefficients of static and kinetic friction for a particular block in contact with
several surfaces.
MATERIALS
computer
Vernier computer interface
Logger Pro
Wax paper
Vernier Force Sensor and accelerometer
Physics with Vernier
string
block of wood with hook
balance or scale
Sand paper
12 - 1
Computer 12
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. Predict which surface pair will show the lowest coefficients of friction: the table surface, the
sandpaper, or the wax paper.
PROCEDURE
Part I Starting Friction
1. Measure the mass of the block and record it in the data table.
2. Connect the Accelerometer to channel 1 of the interface, and the Force Sensor to channel 2.
You should see a dual screen on the laptop screen showing both force and acceleration.
3.. Hold the Force Sensor in position, ready to pull the block, but with no tension in the string.
Click
to set the Force Sensor and the accelerometer to zero.
4. Click
to begin collecting data. Pull the block as shown below in Figure 1, taking care
to increase the force gradually. Click
to begin collecting data and pull as to gather
force vs. time data and acceleration vs. time data.
5. With your group, decide what the data is telling you. Think about these questions and decide
on a procedure to gather the needed data to find the coefficients of both static and kinetic
friction.
-Do you want the block to move with constant velocity? If so, how will you know that the
velocity is constant?
-Where on the graph is the force due to static friction? How can you measure it?
-Where on the graph is the force due to kinetic friction? Again, how can you measure it?
-How many trials do you need for each surface pair to accurately characterize the force for
both static and kinetic?
- How will you find the coefficients of friction from that data?
12 - 2
Physics with Vernier
Static and Kinetic Friction
Mass
Wooden block
Dual-Range
Force Sensor
Pull
Figure 1
Outline your procedure below:
Data Tables:
Mass of block
kg
Data Table 1: Peak Static Friction and Kinetic Friction on the Bare Table
Fill in your data table below
Physics with Vernier
12 - 3
Computer 12
Data Table 2: Peak Static Friction and Kinetic Friction on the Sand Paper
Data Table 3: Peak Static Friction and Kinetic Friction on the Wax Paper
Data Table 4: Peak Static Friction and Kinetic Friction on the 4th surface. Record the
surface here:__________________________________
12 - 4
Physics with Vernier
Static and Kinetic Friction
ANALYSIS QUESTIONS:
You need to explain here how you are using the date you took to calculate the coefficients of
friction, and give one example for static and one for kinetic friction, stating the formulas
used. Then summarize your results in a table.
Conclusion: Write a paragraph conclusion summarizing your data. Compare the results to
what you might have expected when you started the lab. State any sources of error that you felt
could not be eliminated entirely.
Lab Notebook Requirements- This should be around 4-6 pages.
Title
Objectives
Preliminary Questions
Materials
Equipment set-up. Use a diagram or photo.
Procedure- enough detail for another physics student who has not done this lab. Be very specific
about your data collection you should have a sketch or printout of the graphs and tell me exactly
how you took data for this lab. Be specific about the number of trials you did. If you used the
analysis tools in Logger Pro, be sure to tell me what you did with them.
Data Tables- Use a ruler, title and number each one
Calculations and Results- define how calculations were done with just one example for kinetic
and one for static. Summarize your results in a results table.
Conclusion- State your results here, what are the coefficients of static and kinetic friction for each
one of the four surface pairs.
Physics with Vernier
12 - 5