Day 51 Friction

Aim: What are the different types of Friction?LO: Relate friction to the normal forceLO: Calculate friction for different surface combinationsLO:

AGENDADo Now - WorksheetNotesWorksheetHW# Due

Friction

Friction

Friction is a special force that is caused by the surface roughness of an object.

It always acts in the opposite direction of the motion of the object.

There are two types of friction– Static, and kinetic

Coefficient of Friction

All surfaces exhibit friction, some more than others.

It depends on the roughness of the surface of the object.

It is represented by the symbol – For static friction: s

– For kinetic friction: k

Sliding Friction – Microscopic model Depends on microscopic (electrostatic) bonding

forces Depends on roughness of the surface

Kinetic Friction

Kinetic friction is the force of friction on an object when it is moving

The formula is:

Ff = kFN

Static Friction

Static Friction is the force of friciton on an object when it stands still.

We find that it is harder to start an object moving than it is to keep it moving.

The formula is:

Fs sFN

Graph of the behavior of sliding friction

s sf Nk kf N

A Table of coefficients of sliding friction

ExampleExample

A boy exerts a 36N horizontal force as he pulls a 52N sled across a cement sidewalk at a constant speed. What is the coefficient of friction between the sidewalk and the sled (ignoring air resistance)?

52N

36N

Solution

Known:

FN = Fg = 52 N

Fpull = Ffriction = 36N because the sled is moving at constant velocity

Ffriction = FNk Therefore k = Ff/FN

k= 36N/52N = ?

Example 2

Suppose the sled runs on packed snow. The coefficient of friction is now only 0.12. If a person weighing 650N sits on the sled what is the force needed to pull the sled across the snow at a constant speed?

= 0.12Fw = mg= 650NWhat force to pull sled?

Inclined PlaneInclined Plane A common free body

diagram used is often the inclined plane.

Another name for an inclined plane is a ramp.

Look at the diagram to the right showing the usual forces on an inclined plane

FN

Ff

W

Vector Vector DiagramDiagram

If we look at just the vector diagram we see some interesting things

We usually know the weight of the object, so we can find the normal force.

The normal force is perpendicular to the friction force and the force of the inclined plane

FN

Ff

W

Example 3

A skier (Ki) has just begun to descend a 30o slope. Assuming the coefficient of kinetic friction is 0.10 calculate:

(i) his acceleration and (ii) his speed after 4 s

Example 3

A skier (Ki) m = 7 kg has just begun to descend a 30o slope. Assuming the coefficient of kinetic friction is 0.10 calculate:

(i) his acceleration and (ii) his speed after 4 s

Approach:

(i) Resolve forces | | and to slope

(ii) Calculate frictional force

(iii) Find net force down the slope => acceleration

(iv) Use vf = vi + at => vf

Solution

Force of gravity down the slope is:

Fgpara = FgSin()

Fgpara = 7kg*10 m/s/s*0.5 = 35 N

Calculate Normal = Fgperp

Fgperp = FgCos()

Fgperp = 7kg * 10m/s/s *0.866 = 60.62 N

Solution Continued

Calculate Frictional Force:

Ff = kFnormal

Ff = 0.1 * 60.62 N = 0.6062 N

Caluculate Net force down the slope

Fnet = Fgperp – Ff

Fnet = 35 N – 0.6062 N = 34.4 N

Solution last page

Calculate Acceleration down the slope:

Fnet = ma

a = 34.4 N/7kg = 4.9 m/s/s Calculate velocity at t = 4 seconds

Vf = Vi + at

Vf = 0 m/s + 4.9 m/s/s * 4 s = 19.7 m/s

Inclined PlaneInclined Plane A common free body

diagram used is often the inclined plane.

Another name for an inclined plane is a ramp.

Look at the diagram to the right showing the usual forces on an inclined plane

FN

Ff

Fp

W Fp = the force caused by the ramp

Vector Vector DiagramDiagram

If we look at just the vector diagram we see some interesting things

We usually know the weight of the object, so we can find the normal force.

The normal force is perpendicular to the friction force and the force of the inclined plane

FN

Ff

Fp

W