A force that opposes motion Acts parallel to

-A force that opposes motion -Acts parallel to the surfaces in contact.

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

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)

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

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

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

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

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

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

1. A 30 N block has a coefficient of friction of 0. 26. 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/s 2. What is the coefficient of friction?

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 30 m/s. 30 m/s

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.

Worksheets