1. Friction
Newton’s 2nd Law with Friction
Examples
Outline

2. Friction
Friction is the force which resists sliding of two surfaces across each other.
We distinguish between static and kinetic friction:
Static Friction :
- there is NO relative motion
- fs prevents sliding
Kinetic Friction :
- the block is sliding
- fk is opposite to v FA v

3. The friction force response to an increasing applied force
fs,max=μsN fk fs
Fa = push
a=0
a>0

4. Demo
Example: Two blocks are pulled with a force of 20N over a surface with a coefficient of kinetic friction of 0.2. Block 1 has a mass of 1kg and block 2 a mass of 3kg. What is the acceleration of the system ?

5. QQ38: Inequality
Example: A 2kg block is sliding down a 30o ramp that has a coefficient of kinetic friction of 0.2. What is the acceleration of the block ?

6. QQ38: Inequality
Example: A car is moving at a constant speed of 24m/s. The driver hits the brakes which causes the car to slide. Given that μk=0.8 for rubber on concrete:
a) what is the acceleration of the car ? b) how far will it move before stopping ?

7. Do for next class: Read: Sections 5.6
Suggested problems: 5.50, 5.51, 5.53
Do as many more of the problems from the end of the chapter as it takes you to realise that they are all the same. If this takes you more than two full problems, make sure that you are following the procedure that I followed for the examples in class.