1. Friction and Traction
Unit 8

2. Friction A force that works against motion.
Static Friction – friction between 2 objects not in motion.
Kinetic (sliding) Friction – friction between 2 objects that are in motion.
Once an object has overcome that initial push (or static friction), it has kinetic friction.

3. Static vs Kinetic Friction

4. Friction
There are 2 factors that determine the amount of frictional force that can occur between 2 surfaces:
The coefficient of friction
The normal force
Ff = uFN

5. Coefficient of Friction

6. Coefficient of Friction
Describes the grippyness of 2 surfaces sliding against one another.
Slippery objects have a low coefficient of friction while sticky objects have a high coefficient.
The coefficient can range from 0 – 1.

7. Coefficients of Friction for Common Materials
Materials in Contact
Coefficient of Static Friction
Coefficient of Sliding Friction
Steel on Steel
0.78
0.42
Tires on Dry Concrete
1.0
0.65
Tires on Wet Concrete
0.8
0.5
Tires on Snow
0.75
0.1

8. Normal Force

9. Normal Force Force that presses 2 sliding surfaces together.
Is equal and opposite to gravity or the weight of an object.

10. Traction The friction between the wheel and the surface it moves on.
It is the amount of force a wheel can apply to a surface before it slips.
To increase traction you must either
Increase the coefficient of friction (between the wheel and the surface).
Increase the normal force (the weight of the robot).

11. VEX Traction

12. Friction in VEX Different wheels have different traction
Plastic on plastic parts reduces friction. Metal on metal is not desirable for moving systems.

13. Tasks THINK Phase – keyterms and notes BUILD Phase – pp. 27-34
AMAZE Phase – Challenge on p. 36. Take note of modification ideas.
4. Engineering Notebook p. 37
5. STEM – all
6. Skills Logbook – new skills acquired and new skills to work on.
7. Group and Self Assessments