1. Friction and Normal Force

2. 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

3. B. According to Newton’s 3rd 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.

4. C. When are they not equal
Normal Force
F of gravity

5. 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!

6. 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.

7. 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.

8. 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
F f