1. Physics: Forces and Newton’s Laws
1/1/2019
Objective: I can determine the effect that friction has on a moving object and solve problems involving friction

2. Normal Force
The normal force is a contact force exerted by one object on another
It always acts perpendicular to the plane of contact

3. Normal force
The normal force is a general term for forces from several different sources
For example, gravity pulls me toward the floor; the floor provides a normal force that holds me in place.

4. Tension
If I pull on one end of a rope with a 5 N force, the other end of the rope pulls on whatever it’s attached to with an equal force
Tension is another force we often encounter in physics problems – It’s the force exerted by a string or rope
From Newton’s Laws, we know that tension must be equal at both ends of a rope

5. Tension
Even if the ends of the rope are in different directions, they still exert equal forces
Tension always pulls an object toward the rope or string.

6. Friction
Friction is always a contact force that opposes motion between two objects.
We have two types of friction: Static Friction and Kinetic Friction

7. Friction Static Friction Kinetic Friction
Force exerted by one object on another when there is no motion between them
Example: You try to push a heavy couch across a carpeted floor. Because of the static friction, you are unable to get the couch to move.
Force exerted by one object on another when they rub against each other because one or both is moving.
Example: You push your book across your desk, and feel the force of kinetic friction resisting your movement

8. Friction and the Normal Force
The force of friction, either static or kinetic, is always related to the normal force
The greater the normal force, the greater the force of friction.

9. Coefficient of Friction
The coefficient of friction is a property of materials. It tells you how much friction there will be when two objects are in contact.
Notice that the coefficient is different for static and kinetic friction

10. Coefficient and Forces
We represent the coefficient of friction with the Greek letter μ.
We use subscripts to define which type of friction we’re dealing with: μk = kinetic, μs = static
We find the force of friction by multiplying the coefficient by the normal force: Ff = μFN

11. Force of Friction
So, if we want to find the force of static friction on a 2 kg box with μs = .75:

12. Homework Problems
P. 128, #
These will be due Friday. That means you’ll have a couple days to work on them and some time tomorrow to ask questions about them.
P. 130, # 22-26
Some of them are tough, so I recommend starting them tonight so you can get help tomorrow.