1. The Normal Force and Friction

2. The Normal Force
The normal force (FN) is the reaction force of a surface.
It is directed perpendicular to the surface…
So, when I am standing on a level surface it is straight up
If I am standing on an inclined surface, the normal force will be at right angles to the surface

3. Calculating the Normal Force
The normal force is equal to the sum of the forces acting perpendicular to the surface.
Most often this is just the weight of the object, but it can be different
For example…

4. Example 1
A 5 kg book rests on a table surface. What is the normal force on the book?

5. Example 2
You push on the book in example 1 with a force of 10 N. Now what is the normal force?
If you use 10 N to lift the book instead, what is the normal force?

6. Example 3
Suppose the forces you used in example 2 was directed at a 35⁰ angle. What will the normal forces be then?

7. Friction Friction is a force that resists motion.
Friction is caused by:
Inter-locking irregular surfaces
Weak molecular interactions
There are two types of friction
Static – resists initial movement
Kinetic – resists continued motion
Static is usually greater than kinetic

8. Coefficient of Friction
All pairs of surfaces have a rating for the amount of friction between them
Called the coefficient of friction (µ)
See table in your book, page 138

9. Calculating Friction Ff = µFN Example 1…
A 5 kg book is resting on a table. The coefficient of friction between the book at the table is What force is needed to move the book with constant speed?

10. Example 2
You apply a force of 50 N to the book in Example 1. What is the book’s acceleration?

11. Example 3
You apply a force of 50 N directed at a 30⁰ angle WRTH to the book in example 1. What is the book’s acceleration?

12. Example 4
A ball rolls to a stop over a distance of 3 m. If its initial velocity was 1.3 m/s, what is the coefficient of friction between the ball and the ground?