do now: a 40 n chair is pushed across a room with an acceleration of 2 m/s 2. steven pushes with a...

Do Now:A 40 N chair is pushed across a room with an

acceleration of 2 m/s2. Steven pushes with a force of 15N. What is the force of friction acting on the chair?

Static and Kinetic Friction

Static – not movingKinetic - moving

Maximum Force of Static Friction When the max force is applied to an

object before the object begins to move, force of static friction is at a maximum

sfF

Force of Kinetic Friction

friction when object is movingkf

F

Friction is dependent upon:static or kineticnormal force (which is equivalent to

weight on a flat surface)texture of surface

Which is Greater: Max Force of Static Friction or Kinetic Friction?Do an experiment to find out which is

greater. Use spring scales and pull a cork tray. Increase the mass of the tray and repeat to verify.

Which is Greater: Static or Kinetic Friction?

> sfF

kfF

When a force is applied to an object at rest it does not move initially because the force of static friction balances with it. As the applied force increases so does the static friction… UNTIL it reaches the maximum force of static friction. If the applied force continues to increase the object moves and kinetic friction begins to act on the object.

Relationship Between Friction and Normal ForceRatio of friction to normal force = a constant

called the coefficient of friction

Rearrange:

f

N

F

F

f NF F

Coefficient of friction (μ)Number that describes the amount of friction

between 2 surfaceshas no units, cannot be negative, usually (but

not always) less than 12 surfaces have a max coefficient of static

friction ( ) and coefficient of kinetic friction ( )

>

k

s k

Example:ice on ice, = 0.1, = 0.03rubber on concrete, = 1.0, =

0.8

ss

k

k

Example Problem 1A 4 kg box is pushed with a force of 15 N to the right. Determine the coefficient of friction between the box and floor if the acceleration of the box is 2 m/s2.

Example Problem 2A 29.43 N box is pushed with a force of 25 N to the right. What is the acceleration of the box if the coefficient of friction between the box and floor is 0.4?