1. Frictional Force
A force that opposes
motion
Acts parallel to the
surfaces in contact.

2. Friction Force vs. Weight
Cause of Friction
Friction Force vs. Weight
Cause of Friction: the microscopic roughness between surfaces…like two gears locking together. Ff W

3. This is called a Free Body Diagram (FBD)
If it is not moving, then there is no friction until an applied force acts on the object! What if it is moving? Ff F
This is called a Free Body Diagram (FBD)

4. Sliding Friction – occurs when two solid objects slide over each other.

5. 2 Kinds of Sliding Friction
1. Static Friction – not moving
Static > Kinetic
2. Kinetic Friction - moving
So… it takes more force to start moving an object

7. Force vs. Time
Maximum Static Friction
Constant Kinetic Friction F t

8. Factors that affect Sliding Friction:
The normal Force acting on an object. FN
The coefficient of Friction ( µk < µs, unitless) The texture of the surfaces. The stickiness between surfaces.

9. Ff = µFN General Formula for Friction: or Static - µs Kinetic - µk
friction force = coefficient of friction x normal force or
Ff = µFN
Static - µs
Kinetic - µk

10. Now for the problems…
…draw your FBD
Consider x-axis and y-axis separately:
ΣFx = ma
ΣFy = ma
Remember:
Ff = µFN

11. 1. A 30 N block has a coefficient of friction of 0. 26
1. A 30 N block has a coefficient of friction of What force is required to slide it along a level surface at constant velocity?
2. A 35 kg crate requires a force of 110 N to slide at an acceleration of 2 m/s2. What is the coefficient of friction?

12. A crate having a mass of 60 kg falls horizontally off the back of a truck which is traveling at 30 m/s. Compute the coefficient of kinetic friction between the road and the crate if the crate slides 45 m on the ground with no tumbling along the road before coming to rest. Assume that the initial speed of the crate along the road is 30m/s.
30 m/s

13. Radio Control Car
A 35 kg radio control car runs out of gas and coasts to a stop in 3 seconds. Knowing the coefficient of friction between the ground and the car is 0.4, find the initial velocity and the distance the car rolled.

14. Worksheets