day 51 friction aim: what are the different types of friction? lo: relate friction to the normal...
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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
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Friction
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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
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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
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Sliding Friction – Microscopic model Depends on microscopic (electrostatic) bonding
forces Depends on roughness of the surface
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Kinetic Friction
Kinetic friction is the force of friction on an object when it is moving
The formula is:
Ff = kFN
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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
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Graph of the behavior of sliding friction
s sf Nk kf N
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A Table of coefficients of sliding friction
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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
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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 = ?
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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?
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= 0.12Fw = mg= 650NWhat force to pull sled?
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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
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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
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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
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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
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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
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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
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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
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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
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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