FRICTION LAB
Starter
Consider a box sliding across a floor. What are
two factors that determine how strong the
frictional force is?
Practice
The two factors are:
1. How rough the surfaces are.
A measure of the roughness is given by a dimensionless number called
the coefficient of friction.
The symbol is the Greek letter m, or m.
2. How hard the objects are pressed together.
This is given by the normal force, N.
f = mN
FRICTION LAB
Starter
From the F vs. t graph for a box on a horizontal surface
that weighs 20 Newtons, determine the
coefficient of static friction.
m = f/N = 4/20 = .200
Practice
Static Friction – at break point: f = msN
Kinetic Friction – sliding:
break point
sliding
static
f = mkN
Horizontal Surface N = mg
At break point,
Pull = msmg
After it starts sliding at a
constant speed,
Pull = mkmg
Therefore, to experimentally find the
coefficients,
ms = Pulling force at break point/mg
mk = Pulling force while sliding/mg
Application
Design a lab to determine the coefficients of
friction between wood and cork, Teflon, and
felt by recreating the friction vs. time graphs
and finding the pulling forces at break point and
while sliding.
Available Equipment:
Force sensor
Friction blocks with wool, Teflon, and felt surfaces
Masses
Logger pro / Data studio
Lab Report Items DATA
Felt
Trial
1
2
3
Average
Total Mass
Normal Force
Friction Force
at Break Point
Cork
Trial
1
2
3
Average
Total Mass
Normal Force
Friction Force
at Break Point
Plastic
Trial
1
2
3
Average
Total Mass
Normal Force
Friction Force
at Break Point
Friction Force
while sliding
ms
mk
Friction Force
while sliding
ms
mk
Friction Force
while sliding
ms
mk
LAB REPORT ITEMS
Cover Sheet
Data table
Sample Calculations
Sample Graph
Question: As the mass of the cart increases,
how does the coefficient of friction
change?
Summary
Connection
Name some real life applications where
friction is desirable, and not desirable.
Exit
If you change the mass in the friction cart,
what effect would that have on the coefficient?