FINDING THE COEFFICIENT OF
FRICTION
Also the well known “piece of cake”
It really is easy.
Trust me. Of course you will need a
CALCULATOR!!!!!!

An engine provides 5000N of force on a
1600 kg car moving at uniform speed.
What is the coefficient of friction between
the road and the tires?

Time to work an equation

Put it on your sheet.
HERE IT IS
First
uniform tells us there is
no acceleration.
So we have:
F= 5000 N
m = 1600kg
g = 9.8
Coefficient = ?
SO WE PUT THEM
TOGETHER
Ff =mg x cof
Using algebra isolate cof
Now we have cof = Ff/m x g
In numbers cof = 5000 / (1600)(9.8)
Do the math in order!!!!!
Answer will be .3188…..
Round up .32
NOW WE GET COMPLICATED
Static Friction-(Mus) We know what
this is.
Kinetic Friction(Muk) We know this.
So how do we work a problem?
SIMPLE!!!!
Like so:
• We have a box that has a mass of 50kg.
• If it takes 156.8 N of force to move it what
is the static coefficient ?
• First convert the mass ( 50 x 9.8) (490N)
• Then we divide force needed (156.8) by
force mass (490) 156.8/490 = .326…..
Round to
• 0.33
EASY NO? NOW FOR KINETIC FRICTION
Kinetic means moving.
You push a 100 kg rock down the road.
( You are STRONG!!!)
If the coefficient of kinetic friction (Muk) is
.25 between the road and rock how much
force is needed to keep the rock moving?
First we find Fn for the rock
(100kg) ( 9.8 m/s^2) = 980N
NOW we plug in our formula
 (Muk)
(Fn)
=
?
 (.25)
(980N) = 245N
 So it takes 245 N of force to keep
the rock moving.
 Easy!!!!!!!!!!!!
NOW YOU DO IT.
• A car with a mass of 1500 kg needs a
force of 2000 N to get it moving. What
is the Mus?
• .1360…………. (.14)
• Andrea wants to move a 70 kg John. If
the Mus is .20 how much force does
she need?
• 137.2 N
• Simple?????
Last Ones
• 60 kg Joe is moving down the hall, but
looks like he wants to stop. If the Muk is .50
how much force must Coach Ahrens apply
to keep him moving?
• (60N) (9.8) x .50 = 294 N
• 110 kg Colton is headed for the end zone. If
Tristain has to apply 460 N of force to help
him score what is the Muk?
• 460/ (110 x 9.8) = .4267….. Round
.43
OH NO
 Review tomorrow
 TEST
THURSDAY!!!!!!!!!!!!!!!!