Download Abstract: What could the force of kinetic friction depend on? Theory

Kinetic Friction Lab
Abstract: What could the force of kinetic friction depend on?
Theory:
Of all the possibilities above, let’s see how the force of kinetic friction depends on the normal force.
Use the same set-up as last time. Set-up: see page 128. This way, you don’t need to re-sketch it.
Do an FBD for mass/block system moving to right with constant v:
Data:
Zero the force probe when there is slack in the string. Start taking force data, very gradually increasing the
pull on the force probe until the block starts to slide. Then continue pulling with constant velocity. Your
graph should look something like this:
Kinetic Friction Region
f (N)
0
t (s)
Click and drag to get the region of interest and use the STAT button to get the mean.
You don’t need to print out any LoggerPro graphs. Record your kinetic friction data in the following table in
Excel! Do at least 7 trials, with 7 different added masses. Choose a good range of added masses! Go up by at least
200 g (0.200 kg) each time!
How do you calculate FN in your table? Hint: look at your FBD!
Data Table:
mBlock (kg)
etc.
madded (kg)
mtotal (kg)
FN (N)
fk (N)
Print off your Excel data table and paste it into your PJ at the bottom of this page.
Graph fk vs FN in Excel (remember, that means putting fk on the “y” axis.) If the graph is linear, use Excel to plot
the trend-line and determine the slope. Display the equation on your chart. Print off your graph and paste it into
your your APJ on the top of the right hand page.
Questions:
1. What is the relationship between fk and FN?
2. Express this relationship as an equation… fk = ?
3. What else do you think friction could depend on?
Conclusion: If your graph of fk vs FN is linear, you can conclude that
fk α F N
fk = slope FN
fk = constant FN
fk = µ k F N
where µk, “pronounced mew sub k”, is the coefficient
of kinetic friction. Though the name is kind of strange,
just remember µk is simply the slope of your fk vs FN
graph.
Isn’t it surprising how good your data is? If you kept a = 0, it should be!