Do Now:
A 40 N chair is pushed across a room with an
acceleration of 2 m/s2. Steven pushes with a
force of 15N. What is the force of friction
acting on the chair?
Static – not moving
Kinetic - moving
Maximum Force of Static Friction
 F fs
 When the max force is applied to an object
before the object begins to move, force of
static friction is at a maximum
Force of Kinetic Friction
 F fk
 friction when object is moving
Friction is dependent upon:
 static or kinetic
 normal force (which is equivalent to weight
on a flat surface)
 texture of surface
Which is Greater: Max Force of
Static Friction or Kinetic Friction?
 Do an experiment to find out which is greater. Use
spring scales and pull a cork tray. Increase the mass of
the tray and repeat to verify.
Which is Greater: Static or Kinetic
Friction?
F fs > Ff
k
When a force is applied to an object at rest it
does not move initially because the force of
static friction balances with it. As the applied
force increases so does the static friction…
UNTIL it reaches the maximum force of static
friction. If the applied force continues to
increase the object moves and kinetic friction
begins to act on the object.
Relationship Between Friction and
Normal Force
 Ratio of friction to normal force = a constant called
the coefficient of friction
Ff
FN

Rearrange:
Ff   FN
Coefficient of friction (μ)
 Number that describes the amount of friction between 2
surfaces
 has no units, cannot be negative, usually (but not
always) less than 1
 2 surfaces have a max coefficient of static friction (
and coefficient of kinetic friction

( k)
 s > k
m s)
Example:
 ice on ice,  s = 0.1,  k = 0.03
 rubber on concrete,
 s = 1.0,  k = 0.8
Example Problem 1
A 4 kg box is pushed with a force of 15 N to the right.
Determine the coefficient of friction between the box
and floor if the acceleration of the box is 2 m/s2.
Example Problem 2
A 29.43 N box is pushed with a force of 25 N to the right.
What is the acceleration of the box if the coefficient of
friction between the box and floor is 0.4?