Download Investigating Friction

Investigating Friction
In this experiment you will investigate the effects of changing normal force, surface area, and type of
surface on the static and kinetic friction between the surfaces.
The premise of this investigation is that we can measure both the frictional resistance and the normal force
on an object if we tow it at a constant velocity on a level surface. An object will only move with a constant
velocity if the net force acting on it is zero. Consequently, the following force diagram would be appropriate
for an object towed along a level surface by a horizontally applied force.
FNBT
Block
FfBT
FTBR
LoggerPro Buttons:
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Since the block does not accelerate horizontally, the tension force on the block by the rubber band is
balanced by the frictional force on the block by the table. Similarly, since the block does not accelerate
vertically, the gravitational force on the block by the earth is balanced by the normal force on the block by
the table. If we can measure the tension force, we will know the magnitude of the frictional force. If we can
measure the gravitational force we will know the magnitude of the normal force.
Procedure
Begin by checking to make sure that the Lab Pro is plugged in to a power supply and the USB cable is
connected from it to the computer. Connect the force sensor to the Analog 1 input. Open the LoggerPro file
called Friction Experiment. (If you are using a WDSS, open the WDSS friction file, and pair the WDSS
with your computer.)
Begin by recording the mass of your block. You will need to calculate the gravitational force on this mass,
and use the reasoning described above to determine the normal force acting on the block.
Attach a long rubber band between the block and the force sensor Before any data run, be sure that the force
probe is zeroed. To do this, make sure that the rubber band is slack, and then click on the zero button at the
top of the LoggerPro window. Click the collect button, and drag the block with the force sensor,
horizontally, at a slow constant speed. A force vs. time graph, with 10 seconds of data will appear. You will
need to record two values from this graph. Drag across the graph in the section that includes the highest
static friction force. Select the STAT tools button from the top of the LoggerPro window. Record the MAX
value. This is your maximum static friction force value. Then drag across the graph in the section where you
were towing the block at constant velocity. This should be a fairly constant level section of the graph. Select
the STAT tools button again. This time record the MEAN value of the force. This is your kinetic friction
force value.
Add 100 grams to the block. Calculate the new normal force. Repeat the procedure above, recording the
maximum static friction force and the kinetic friction force. Continue this process until you have added
1000 grams to the block.
Repeat the experiment for the other surface area block.
Plot the following graphs on one set of axes (separate data sets for each)
1. Ff vs. FN, for the maximum static friction, small surface area
2. Ff vs. FN, for kinetic friction, small surface area
3. Ff vs. FN, for the maximum static friction, large surface area
4. Ff vs. FN, for kinetic friction, large surface area.