Download Friction and Normal Force

I. Normal Force
 The Force that opposes gravity.
 A 2 kg book is resting on a table.
FN
The Net Force is zero.
Fg
What is the
Normal Force?
Fg = FN
B. According to Newton’s
rd
3 Law
 For every action, there is an equal and
opposite reaction.
 The action is Gravity.
 The reaction is called the Normal Force.
 Lucky for us, The two are equal on a flat
surface.
C. When are they not equal
Normal Force
F of gravity
II. Friction
 Force that opposes motion.
 Whenever two surfaces are in contact.
 Always opposite direction of motion
 Depends on surface type and Normal
force ONLY.

NOT SURFACE AREA!
B. Kinetic Friction
 Friction on a moving object.
 Ff = k FN
 k = coefficient of kinetic friction.
It is a constant that depends on surface
type
 You look it up on a chart.

C. Static Friction
Friction on a stationary object.
Ff = s FN
s is the coefficient of static
friction.
It is always higher than kinetic
friction.
EXAMPLE
 You push a 25 kg box across the floor. It is
travelling at a constant speed of 1.0 m/s. If the
coefficient of kinetic friction is .20, what force are
you pushing with?
F net = Fpush - Ff
F push
Ff