Slip-Stick Lab
Name: _________________________
Coefficients of Friction
Procedure
To investigate three types of friction and to measure the coefficient of friction for each type.
Discussion
When the surfaces of two objects are in contact with each other the ridges and valleys of
one surface settle into the ridges and valleys of the 2nd surface. This is what we call friction. The
smoother a surface is the less ridges and valleys and the less friction. Air acts like a lubricant, so
the more tightly the surfaces are pressed together, the more air is squeezed out and the greater
the friction between the objects. Therefore the force of friction depends on (1) the type of
surfaces and (2) how hard the surfaces are pressed together. The force that presses the
surfaces together is simply the weight of the object.
The ratio of frictional force to weight is called the coefficient of friction and is denoted by
the Greek letter μ (mu).
The coefficient of friction depends on whether sliding is taking place. When sliding occurs, μ is
less. This is because you are not giving the ridges of one surface enough time to fully settle into
the ridges of the 2nd surface. If two surfaces are sliding past one another we refer to the
coefficient of sliding friction. When two objects are at rest we refer to the coefficient of
static friction.
The force of friction depends on the coefficient of friction μ and the normal force N:
Ff = μN
(N = mg)
Ff=μ(m)(g)
SURFACES
µs (static)
µk (kinetic)
Steel on steel, dry
0.60
0.30
Steel on wood, dry
0.40
0.20
Steel on ice
0.10
0.06
Wood on wood, dry
0.35
0.15
Metal on metal, greased
0.15
0.08
Part A: The Coefficient of Static and Sliding Friction
1.) Weight the block by suspending it from a spring scale
2.) Record the weight in Table I
3.) Determine the coefficient of static and sliding friction by dragging the block with a spring
scale. Be sure to hold the spring scale horizontally. The static force is the force it takes to get
the block moving. The sliding force is the force it takes to keep the block moving.
4.) Increase the force pressing the surfaces together by adding weights to the block.
Table I
F(initial)
F(constant)
W
μ (static)
μ (sliding)
Force to get the
Force at
Weight of block
F(initial)/W
F(constant)/W
block going
constant
velocity
Analysis
1.) How does the μ static compare with μ sliding?
2.) Does μ sliding depend of the weight of the block?
3.) As you added more weight, the force of friction increased but μ did not. Why?
Part B: The Effect of Surface Area on Friction
1.) Drag the block with a spring scale. Record the friction force and the weight of the block in
table II.
2.) Repeat step one use a different side of the block (with different area).
3.) Compute μ sliding for both areas.
Table II
Force of Friction
Weight
Area of Contact
(normal force)
1
2
Analysis
1.) Does surface area make a difference in the coefficient of friction?
Coefficient of
Friction (μ)
FRICTION PRACTICE PROBLEMS
Name: __________________________
Homework
Period:________ Date: ____________
1. A 267 N dog runs and slides across the tiled kitchen floor.
a. If the force of friction that opposes its motion is 175 N, what is the coefficient of friction between
the dog and the kitchen floor?
b. If Tommy, a small 178 N toddler decides to hitch a ride for the next trip and this time they slide
down the hardwood living room floor. If a 250 N force of friction opposes their motion, what is
the coefficient of friction between the dog and the living room floor?
2. A sled is pulled at a constant speed with a force equal to 30 N. The mass of the sled and the rider is 40 kg.
a. Determine the coefficient of friction.
b. What would the acceleration of the sled and rider be if it were pulled with a force of 50 N? (use
the same coefficient of friction you found in part a).
c. What would happen if another person jumped on the sled? Explain!
d. What if the snow turned into ice after many sledders went over it and it became much more
slippery? Explain!