FrictionPhysics 11

What’s Stopping You?

Humour again..

What is friction?

The force that opposes the relative motion of two surfaces and acts along the surfaces.

Frictional forces inhibit relative motion between two objects in contact with each other

CONTACT FORCE

Expanded…

When two surfaces are at rest and in contact, the surface atoms interact to from relatively strong attractive forces. When you push one object, static friction “pushes back” with exactly the same magnitude as an applied force until the applied force is great enough to break the attractive forces between the surface atoms.

Once the object is in motion, new “bonds” continually form and brake over and over.

Two types of friction:

1. Static Friction: This value represents the relative force

necessary to make an object move

2. Kinetic Friction: This value represents the relative force

necessary to keep an object moving at a constant rate

Fri

ctio

nal

For

ce

Res

isti

ng

Mot

ion

Force Causing the Object to Move

Kinetic RegionStatic Region

sF

kFMax

kFF s

kFF s

What affects the force of friction? Type of surfaces in contact

Ex: Rubber on rubber verse rubber on ice Coefficient of friction (µ)

Magnitude of the normal force “the surface pushes back against the weight of

the object” Ex: Which is easier to move a box full of bricks

or a box full of t-shirts?

Coefficients of friction:

“stickiness value”

Known values for certain surfaces, determined experimentally

Units = none!

Normal Force

The normal force is sometimes—but certainly not always—equal in magnitude and opposite in direction to weight (Fg)

When an object lies on a level, non-accelerating surface (rest or constant motion):

Fg =FN

Frictional Forces Occur When Materials are in Contact:

W

fsF

N

Surfaces in Contact

M1

F = Force Causing Motion (Pull on Scale)Fs = Force of Static Friction (Resists Motion)N = Force Normal Holds Surfaces in ContactW = Weight of Object ( Mass x Gravity)

Calculating force of friction:

Direction of Ff is always opposite to the direction of motion

Ff = µ FN

For both static and kinetic friction

Ex 1:A glass of water is being pushed along a glass table. If the glass has a mass of 0.675 kg, calculate the force of friction.

Fg = mg

= (0.675 kg)(9.81) = 6.62 N

Fg = FN = 6.62 N

FF = µFN

Look up µ in table on pg 140

= (0.40)(6.62) = 2.65N

Ex 2: A group of people are pushing a large wooden trunk across a wood floor at a constant velocity and you are exerting a force of 625.0 N, what is the mass of the block?

Constant velocity Ff = Fapp

FF = µFN

Look up µ in table on pg 140

(625) = (0.20)FN

625 = FN

0.20 3125 N = FN

FN = Fg = 3125 N

Continued…

Fg = mg

(3125) = m (9.81) 3125 = m

9.81 319 kg = m

Practice:

Pg 144 # 5, 6 Pg 151 # 30 - 33 Friction and Forces Sheet