FRICTION!FRICTION!

FrictionFriction – force that opposes – force that opposes (slows) motion(slows) motion– Vector QuantityVector Quantity

Types of FrictionTypes of Friction – Rolling, – Rolling, Sliding and FluidSliding and Fluid

Frictional Force Depends on – Frictional Force Depends on – – Normal Force of object Normal Force of object (Weight)(Weight)

– Type of surfaceType of surface

Static FrictionStatic Friction – Opposes the START – Opposes the START of motion (when object is at rest)of motion (when object is at rest)

Kinetic FrictionKinetic Friction – Objects in contact – Objects in contact that are moving past each other.that are moving past each other.

Kinetic Friction < Static FrictionKinetic Friction < Static Friction

Frictional forces are PARALLEL to Frictional forces are PARALLEL to surfacessurfaces

In order to get an object moving In order to get an object moving or keep it moving, it must or keep it moving, it must overcome the frictional force. overcome the frictional force.

20 N Steel Box

Ffs = 14.8 N Fapplied = 14.8 N

**If Frictional Force = Applied Force**If Frictional Force = Applied Force

then the object is at rest or moving then the object is at rest or moving at constant velocity**at constant velocity**

**If Applied Force > Frictional Force**If Applied Force > Frictional Force

then the object is accelerating in then the object is accelerating in the direction of the applied force.** the direction of the applied force.**

Force of FrictionForce of Friction

Nf FF

ProblemProblem

A force of 10 Newtons towards the right is A force of 10 Newtons towards the right is exerted on a wooden crate initially moving exerted on a wooden crate initially moving to the right on a horizontal wooden floor. to the right on a horizontal wooden floor. The crate weighs 25 Newton’s. The crate weighs 25 Newton’s.

Calculate the magnitude of the force of Calculate the magnitude of the force of friction between the crate and the floor.friction between the crate and the floor.

What is the net force acting on the crate?What is the net force acting on the crate? Is the crate accelerating?Is the crate accelerating?