Lab 6.3A – Calculating Coefficients of Kinetic Friction
Name ____________
Discussion When the surfaces of two objects are in contact, molecules on the surface of one object are attracted to the molecules of the
other. Ridges and valleys of one surface settle into the valleys and ridges of the other surface. When one surface slides over another; a
slip-and-stick sequence occurs as molecules cling and break away from each other. For a
frictionforce( F f )
block sliding on a horizontal surface, the force that presses the surfaces together is the
 =
normalforce( Fn )
normal force. From the formula, Ff = ..Fn. If you solve for the coefficient of friction, ,
you get the ratio of friction force to weight.
What helps us in our calculations is that the normal force = the weight of the object, so you can find the weight of the object or the mass
of the object (you’ll have to do both!) and substitute in accordingly. It must be emphasized that this relationship holds true only on a flat
surface when the force that presses the surface together is only the weight.
Computing the Coefficients of Static and Sliding Friction
Station 1: Weigh the bowling ball, using the bathroom scale. Conversions: 1 kg = 2.2 lbs, 1 kg = 9.8 N. Record the weight, in Newtons,
in Data Table A. Attach a 20 N scale to the hook and drag the ball across the inverted lab tray. Try to measure the maximum
force before the ball will move, and also measure the force to drag it at constant speed. Record these in Data Table A. Be sure to hold
the scale horizontal. The static friction force, Ff,s , is the force required to get it moving. The sliding friction force, F f,k , is the force
required to keep it moving at constant speed. The scale might jump around. Make your best judgement for the scale readings.
Station 2: Weigh the lead mass by suspending it from a 50 N spring scale. Record the weight in Data Table A. Use a 20 N scale to
drag the mass across an inverted lab tray. Measure the static friction force and the kinetic friction force. Record in Data Table A.
Station 3: Weigh the shoe with the two 1000 g masses inside, by suspending it from a 50 N scale. Record in Data Table A. Use a 20 N
scale to drag the mass across the lab counter top, and find the static and kinetic friction forces. Record in Data Table A.
Station 4: Mass the friction block, using a triple beam balance. Convert the mass to weight units before recording in Data Table A.
Use a 5N scale to drag it across the smooth board on the counter top. Find the static and kinetic friction forces. Record in Table A.
Station 5: Mass the friction block, using the triple beam balance. Convert the mass to weight units before recording in Data Table A.
Use a 5 N scale to drag it across the sand paper on the board, and find the static and kinetic friction forces. Record in Data Table A.
Station 6: Mass the friction block, using the triple beam balance. Convert the mass to weight units before recording in Data Table A, line 1.
Use a 20 N scale to drag the block across the board on the counter top, and find the static and kinetic friction forces.
Record in Data Table A, line 1. Repeat the procedure with a 1 kg mass on top of the block, and place the data on line 2.
Station 7: Mass the friction block, using the triple beam balance. Convert the mass to weight units before recording in Data Table A, line 1.
Use a 20 N scale to drag the block across the board on the counter top, and find the static and kinetic friction forces.
Record in Data Table A, line 1. Repeat the procedure with the block on its side, and place the data on line 2.
Station 8: Drag the block at different speeds. Record the drag forces at different speeds. No other calculations for this station.
Data Table A
Fg
Ff,s
Ff,k
F f ,s
F f ,k
Weight of
Force to Just
Drag force at
Data Table A
static =
kinetic =
Fg
Object
Get Going
Constant Velocity
Fg
Station 1
Station 2
Station 3
Station 4
Station 5
Station 6 - line 1
- line 2
Station 7 - line 1
- line 2
Station 8
No measurement No measurement
|
No calculation
No calculation
Questions
1. Does the dragging speed have any effect on the coefficient of sliding friction, kinetic?
(Did you get the same force for the different speeds at station 8?) (Even if ‘yes’, put ‘no’!)
____________
2. At each station, how does static compare to kinetic?
_______________________________________________________
3. Does kinetic depend on the weight of the friction block? Explain.
4.
If your press down upon a sliding block the force of friction increases but  does not. Explain.
5.
Why are there no units for ?
6.
Does the surface area make a difference in the coefficient of friction?