Download Lab 7: Friction Multi-blocks

Lab 7: Friction Multi-blocks
Instructor: Professor Dr. K. H. Chu
Pre-Lab Questions:
Due before lab begins.
1. Explain briefly the different types of frictional forces.
2. How does the coefficient of friction depend upon the area of contact? Explain.
3. Give some examples in which the force of friction causes the object to accelerate.
4. If you push on a heavy box that is at rest, you must exert some force to start its motion.
5. However, once the box is in motion you need a smaller force to maintain that motion. Why?
6. Find the force needed to pull a mass of 30 kg at constant velocity on a rough incline making an angle
of 400 with horizontal and with µk = 0.3
Friction is everywhere. In introductory physics, professors often present problems by saying “assume
there is no friction,” but we can’t just ignore it in the real world. In general, friction is the force that
slows down the motion of an object. The force of friction is directed along the surface of contact
between the object and surface and directed opposite to the direction of motion of object. We deal
with:
a. Static friction (fs)
This exists when the object is at rest relative to the surface. This force must be overcome in order to
make the object start moving. It is given by fs = µsn.
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b. Kinetic friction (fk)
This exists when the object is in motion and is given by fk = µkn.
where µs and µk are the coefficient of static and kinetic friction and n
is the normal force which presses the two surfaces together. In
general, µs > µk because it takes a larger force to start an object
sliding (static friction) than to keep it sliding (kinetic friction). They
depend only on the materials in contact and independent of the area
of contact.
Another way to find µs is to set up the board as an incline plane.
The coefficient of friction µs is related to the maximum angle θ to
which the board can be elevated before the block starts its motion is:

µs = tan 
(1)
In this experiment, using the board in a horizontal position, we measure the frictional force fk and fs
as they vary with respect to the normal force n, at a variety of scenarios and confirm that this
coefficient does not change as the normal force varies. Using the second law of motion, we can
calculate the coefficients of friction between block and board.
1. Friction on Level Surface - The simplest example of friction involves static friction on a flat surface.
Here we will use a hanging weight to pull a block and measure the coefficient of static friction,
.
1.1. Measure the mass of each block and record them.
1.2. Add about 200 g of mass (side-by-side) into the block to start. Tie a piece of string to the block
on one end and a 50 g hanger on the other. Hang them over the pulley at the edge of the table.
Make a mental note of where the block is located and start from that spot every time.
1.3. Add weights to the hanger until the block starts to slide. Record in a table that has Mass of
Block in one column and Mass of Hanger in another.
1.4. Add 100 g into the block and repeat the previous experiment.
1.5. Continue adding 100 g and repeat until you have 5 measurements.
1.6. Put your results into Excel. We can then make a graph with Block mass as x-axis and Hanger
mass as y-axis. The slope of this graph is our value for
.
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1.7. Repeat for the other block.
2. Static Friction on a Slope - Let’s try to do another experiment to get an independent measure of
that we can compare to our values in 1.6.
2.1. Untie the hanger and remove the string. Place your block with 200 g added onto the slope at
the same position as before.
2.2. Slowly increase the angle of the slope until the block starts to slide. Practice a couple times
before doing the actual trials.
2.3. Record the angle of the first movement for 4 trials all using the same weight (block + 200 g).
2.4. Copy your numbers into excel. In the next column perform the correct calculation to find the
value of
for each case. Take the average and standard deviation of
.
2.5. Compare your values from parts 1 & 2 by doing a percent difference.
2.6. Repeat for the other block.
3. Final Exercise:
a. Sometimes the coefficient of static friction can be equal to or greater than 1. What angle
would be necessary in part 2 if
=1?
b. Derive equation 1, µs = tan  (Apply 2nd law of motion for constant velocity)
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