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