Introduction of Mechanics Lecture Notes 3 for Students
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Transcript of Introduction of Mechanics Lecture Notes 3 for Students
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Introduction of mechanics
Lecture notes 3 Force
Inertia
Newtons laws of motion
Tension, Normal Reaction, Frictionand Weight
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1 Force
Unit: newton, N
It can be measured byspring balance.
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2 Is a force needed to keep a body moving?
Will the cart move even if the man stopspushing it? NO!
it seems thata
moving.
force isneeded to
keep things
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e.g.
flick off
sharply
3 Inertia
or ofconstant speed along astraight line.
Inertia is the tendency of a body
tomaintain its state ofrest
Galileos law of Inertia
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Every object
remains in a stateofrest
4 Newtons first law of motion
Isaac Newton (1642-1727)
...similar to Galileo's law,but I give a meaning of
force
or uniformspeed along a straight line
unless acted on by a netforce.
* net force = sum of
forces
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Why?
5 Newtons idea of force
Force changes the state of rest oruniform motion of an object.
But in everyday life, it doesnt seemright.
frictionopposes motion
need to apply a force to oppose
don't need force to move at constantspeed
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6 Newtons second law of motion
Fma
F= constant ma
Newtons 2nd law
a of an object is
directly proportional to&in same direction asF
inversely proportional tom
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Example 1
A driver braked a car moving at 72 kmh1 to stop. The braking distance,measured by the brake marks on theroad, was 40 m. The mass of the carwas 1000 kg.
What was theave. brakingforce?
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Example 1
Finding the acceleration of car:
By v2 = u 2 + 2 x ___ x ___,
___ = _____________+ _________
a = __________
By F= ma,Ave. braking force = ______ x ______
= _________
a s
0 2(a)(40)5 m s2
1000 (5)
5000 N
72 x 1000
3600( )
2
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7 Tension in a string
Tension
either left/right
force in a string
(depending on what it acts
on)
tension
Tension can act in oppositedirections.
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8 Normal reaction
The force acting on a boy by thesurface in contact with him is normalreaction.
An upward force provided by the surfac
acting perpendicular (normal) to the bo
weight
Any force tobalance
the weight?
Net force actin on the bo = 0
normalreaction
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8 Normal reaction
No matter he stands atA (a horizontal surface)or
B(an inclined surface)
the normal reaction acting on boy is
perpendicular (normal) to the road
normalreaction
normalreactio
n
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9 FrictionWhere does friction come from?
bumps ofsurfaces
When the blockmoves,
the tiny bumps catcheach other friction (opposing motion)
vfriction
v friction
Friction arises when object slides
(or tends to slide) over another
Likewise
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Friction opposes or prevent themotion of an object.
People find ways to reduce it, forexample:
9b Reducing friction and using friction
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Weight is the gravitational forceacting on an object by the Earth.
if an object falls freely, its weight Wgives it an acceleration ofg.
ApplyingF=ma , W=
mg
Measured in newton (N)
10 Weight the pull of gravity
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Taking g to be 10 m s2,
W=mg
the weight of a 1-kg mass is
10 Weight the pull of gravity
W= 1 kg 10 m s2
= 10 N
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Newtons third law of motion statesthat...
It applies to two objectsinteracting
with one another. It does not apply tooneobject by itself.
to every action there is an equal &
opposite reaction.
11 Newtons third law of motion
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A fan cart is switched on, blowing airin the forward direction.
Example 2
The motion of a fan cart is stolen by a sail!
(a) When the cart is placed on thebench, it moves backwards.Explain why.
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By Newton's 3rd law,
This pushes the cart backwards.
Example 2
The motion of a fan cart is stolen by a sail!
force on air bthe fan Fair exerts anequal and
opposite force Fon the fan
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(b) When a sail ismounted on the cartas shown, the cartdoes not move.Explain why.
sail
Example 2
The motion of a fan cart is stolen by a sail!
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sail
Example 2
The motion of a fan cart is stolen by a sail!
net force on cart = 0 it does not move
force on thecard by airFforce on air
by the cardF
