Intro to Static and Kinetic Friction Lab
Seed Question: Static friction occurs when there is no relative motion between an object
and the surface it is resting on. Kinetic friction occurs when you have sliding, slipping
skidding etc. Which kind of friction can be bigger?
Exploration: Put together the following set-up: Sketch it!
Block-mass
combination
Force probe set to 10 N
1000 g
Newspaper
Do a free body diagram of all the forces acting on the block-mass combination in each of the following cases
using hash marks where appropriate:
Case 1: Block remains at rest but has a pull to the right
Case 2: Block slides to the right with constant velocity.
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:
f (N)
0
t (s)
Once you get a good graph, use “Print Graph” to get a nice copy for your PJ. Tape it in on this page.
Questions:
1. Why does the force probe read the force of friction? (Explain, using your free body diagram in the procedure.)
2. Indicate the region of your graph where the block is stationary: the static region: trace over it in blue.
3. Indicate the region of your graph where the block is moving: the kinetic region: trace over it in red.
4. What is the range of static friction? Use numbers!
Label the maximum static friction on your graph.
5. What is the kinetic friction? (Use the flat part of your force graph where the block is moving, and a = 0.)
Big Result: How did the maximum force of static friction compare with kinetic friction?
fs max > fk usually (except for Teflon, where fs max = fk )
fk = µk FN
Where µk is the coefficient of kinetic friction.
(From the Kinetic Friction Lab, p. 131)
fs max = µs FN
Where µs is the coefficient of static friction.
(By analogy with the expression above)
For Further Contemplation: ?