Friction

Friction• Friction is the force that opposes a sliding

motion.• Friction is due to microscopic irregularities in

even the smoothest of surfaces.• Friction is highly useful. It enables us to walk

and drive a car, among other things.• Friction is also dissipative. That means it

causes mechanical energy to be converted to heat. We’ll learn more about that later.

Microscopic View

W

N

Friction may or may not exist between two surfaces. The direction of friction, if it exists, is opposite to the direction object will slide when subjected to a horizontal force. It is always parallel to the surface.

Fpushf

(friction)

Small view:Microscopic irregularities resist movement.

Big view:Surfaces look perfectly smooth.

Friction depends on the normal force.

• The friction that exists between two surfaces is directly proportional to the normal force.

• Increasing the normal force increases friction; decreasing the normal force decreases friction.

• This has several implications, such as…– Friction on a sloping surface is less than friction on a flat

surface (since the normal force is less on a slope).– Increasing weight of an object increases the friction between

the object and the surface it is resting on.– Weighting down a car over the drive wheels increases the

friction between the drive wheels and the road (which increases the car’s ability to accelerate).

Static Friction

• This type of friction occurs between two surfaces that are not slipping relative to each other.

• fs sN

– fs : static frictional force (N)

– s: coefficient of static friction

– N: normal force (N)

fs < sN is an inequality!

• The fact that the static friction equation is an inequality has important implications.

• Static friction between two surfaces is zero unless there is a force trying to make the surfaces slide on one another.

• Static friction can increase as the force trying to push an object increases until it reaches its maximum allowed value as defined by s.

• Once the maximum value of static friction has been exceeded by an applied force, the surfaces begin to slide and the friction is no longer static friction.

Static friction and applied horizontal force

Physics

N

W

Force Diagram

surface

fs = 0There is no static friction since there is no applied horizontal force trying to slide the book on the surface.

Static friction and applied horizontal force

Physics

N

W

Force Diagram

surfaceFfs

0 < fs < sN and fs = FStatic friction is equal to the applied horizontal force, and there is no movement of the book since F = 0.

Static friction and applied horizontal force

Physics

N

W

Force Diagram

surfaceFfs

fs = sN and fs = FStatic friction is at its maximum value! It is still equal to F, but if F increases any more, the book will slide.

Static friction and applied horizontal force

Physics

N

W

Force Diagram

surfaceFfk

fs = sN and fs < FStatic friction cannot increase any more! The book accelerates to the right. Friction becomes kinetic friction, which is usually a smaller force.

Static friction on a ramp

Physics

N

surface

f s

Wx = mgsin and N = mgcosAt maximum angle before the book slides, we can prove that s = tan

W = mg

Without friction, the book will slide down the ramp. If it stays in place, there is sufficient static friction holding it there.

Static friction on a ramp

Physics

N

surface

f s

fs = mgsin and N = mgcosAt maximum angle before the book slides, we can prove that s = tan

W = mg

F = 0Wx = fs

mgsin =smgcoss = sincos = tan

Assume is maximum angle for which book stays in place.

x

W x

Kinetic Friction

• This type of friction occurs between surfaces that are slipping past each other.

• fk = kN– fk : kinetic frictional force (N)– k: coefficient of kinetic friction– N: normal force (N)

• Kinetic friction (sliding friction) is generally less than static friction (motionless friction) for most surfaces.

Friction Summary• Friction is the force that opposes a sliding motion (or

impending sliding).• Friction is due to microscopic irregularities in even the

smoothest of surfaces.• Friction is calculated based on 3 things: 1) materials involved (), 2) FN, & 3) state of motion.• Friction is not a function of surface area

Static Friction: occurs between two surfaces that are not slipping relative to each other.

fs sFN

Kinetic Friction: occurs between surfaces that are slipping past each other.

fk = kFN

Sample Friction Problem #1

A 10-kg box rests on a flat/level table top. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30.

a) What is the maximum horizontal force that can be applied to the box before it begins to slide?

b) What force is necessary to keep the box sliding at constant velocity?

A box with mass 35.0 kg is pushed across a horizontal surface by a horizontal force of 82.0 N @ 0°. The box slides at constant velocity of 7.00 m/s @ 0°. (a)Find the coefficient of sliding (kinetic) friction. k = ?(b) What would be the acceleration of the crate should the horizontal force suddenly cease? a = ?

Sample Friction Problem #2

A 10-kg wooden box rests on a flat/level table top. Between the surfaces of the box and the table the coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30.

What is the friction force on the box and the acceleration of the box if…

a) no force horizontal force is applied to the box?

b) a 20 N horizontal force is applied to the box?

c) a 60 N horizontal force is applied to the box?

Sample Friction Problem #3

A 10-kg wooden box rests on a wooden ramp. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30. What is the friction force between the box and ramp if

a) the ramp is at a 25o angle?

b) the ramp is at a 45o angle?

c) what is the acceleration of the box when the ramp is at 45o?

Sample Friction Problem #4

Friction LabDetermine the coefficients of static and kinetic friction between the wooden block (felt side) and the cart track. The only additional equipment you may use is a balance, a spring scale, and some weights.

Write a mini-lab report that includes only the analysis section. Include diagrams (free-body), calculations, and results for each kind of friction.

(a)Find the horizontal force necessary to slide a 250 N crate across a floor (where k = 0.175) at constant velocity @ 180°.

(b)What would be the acceleration of the crate should the horizontal force suddenly cease?

(a)Lawn Mower Problem

(b)Block pulled by angled string

(c)Books Stacked

(d) Box pushed across rough surface.