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7/31/2019 Chapter 7: Dynamics (f2)
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7/31/2019 Chapter 7: Dynamics (f2)
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7.1 FORCE
FORCE = a push or a pull.
Cannot see a force, but can see and feelits effect.
What a forcecan do to an
object?
Change itsspeed
Change itsposition ordirection
Change itsshape
Move it
Stop it
Increase ordecrease its speed
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Use a force to change the shape of
things
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Use the force to
move, stop and
increase or
decrease the speed
of things
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Use a force to change the direction or position of the things
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Various Types OfForces
Gravitational force
- The force that makesthings fall to the ground
Electrostatic force
- The force due to theelectrical charges
Magnetic force
- The force which actsonly on iron, steeland a few othermetals
Frictional force- The force acting on thesurface of an object when
it moves against thesurface of another object
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Example of gravitational force
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Example of
frictional
force
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Example of magnetic force
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Examples of electrostatic
force
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7.2 MEASUREMENT OF FORCE
The strength of a force is measured innewtons (N).
Spring balance or newton balance to
measure the size of a force exerted on abody.
Sir Isaac Newton
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7.3 APPLICATION OF FRICTIONALFORCE
Surfaces in contact have friction.
This force helps us walk, run, move orstop.
Different types of surfaces affect themagnitude of frictional force.
(the rougher the two surfaces are, thegreater the frictional force between
them).
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Direction of the frictional force is parallelto the surface & in opposite direction of
motion.
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Advantages
of friction
Allows us todrive our cars
or ride ourbicycles
Makes shoesgrip the
ground- can
walk withoutslipping
Enables us to
hold things
Produce thesound likethe guitar
Brake systemsallow us to slow
down and stopvehicles
Enables thingsto rest on thetable or on the
ground andlean against
the wall
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Disadvantages
of friction
The engines of a shiphave to produce anextra force to keep
the ship movingforward against the
frictional force
Produces heat thatcan damage sensitive
parts in a machine
Shoes and tyres canwear out dangerous,can cause accidents
Produce noise
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Application ofincreased friction
How???
Increasing theroughness of
contactsurfaces
Why???
(a)To slow downmovingobjects morequickly
(b) to hold things
more firmly(c) to grip the
surfacesbetter
Application
i) Rubber pads or knobs fixed to the bases ofmany householdappliances to preventfrom slipping while in
use.ii) Tyres & shoes given
deep treads or groovesfor a good grip onslippery surfaces.
iii) Brake pads used onwheels to slow down orstop vehicles
iv) Rough surfaces onstaircases & floors prevent us from slippingwhile walking or running
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Applicationof decreased
frictionMethod
Smoothingthe contact
surfaces
Aim
a) To move
things moreeasily
b) To reduce theheat producedby rubbingparts
c) To slidethings overeach othermore easily
Application
i) Round & smooth sphericalobjects(rollers or ballbearings) placed aroundthe axles in machines toallow the axles to turn
smoothly.ii) Layer of air between 2
contact surfaces layer of airbetween hull of hydrofoil &the water allows the hydrofoilto move at high speeds.
iii) Lubricants (oil or grease) applied on moving partsinside car engines.
iv) conveyor belt moves onwheels or rollers used inairports to transport luggage
etc.
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7.4 APPLICATION OF WORK
When a force moves something, work issaid to be done on the thing.
When work is done, energy is transformed
into a different form.
WORK = FORCE X DISTANCE MOVED
(in the direction of the
force)
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Unit for WORK = joule (J).
Exampe 1:
Q: A horizontal force of 50 N is applied topush an object a distance of 2.0 m ona table. What is the work done by the
force?A: Work done (WORK)
= Force X Distance moved
= 50 N X 2.0 m
= 100 Nm= 100 J
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Example 2:
Q: A man does 1500 J of work to lift a
box to a height of 2 m. What is theweight of the box?
A : Work done = Force X Distcance moved
1500 = F X 2F = 1500
2
= 750 N
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Example 3:
Q : A student of mass 50 kg carries a load
of 5 kg. He walks up a staircase of heightof 4 m. The work done by student is?
[1 kg = 10 N]
A : Work done = Force X Displacement= (50X10 + 5X10) X 4
= 550 X 4
= 2200 J
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7.5 APPLICATION OF POWER Power = rate at which work is done.
POWER = WORK DONE
TIME TAKEN= FORCE X DISTCANCE MOVED
TIME TAKEN
Power is measured in J s
-1
or watt (W). 1 W = 1 J s-1
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Example 1:
Q : A motor can lift an object weighing
200 N to a height of 5 m in 10 s. Whatis the power delivered by the motor?
A: POWER = WORK DONE
TIME TAKEN= Force X Distance moved
Time taken
= 200 N X 5 m
5 s= 100 Nm s-1 = 100 J s-1
= 100 W
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Example 2:
Q : A lift can carry 4 people to the top of
a building 50 m in 2 minutes. What is thepower delivered by the motor of the lift?The average mass of one person is 72 kg.[Assume 1 kg = 10 N]
A: Weight of one person = 72 X 10 = 720 N
Total force = 4 X 720 N = 2880 N
Total work done = Force X Distance
= 2880 N X 50 m= 144 000 J
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POWER = WORK DONE
TIME TAKEN
= 144 000 J
2 X 60 s
= 1200 W
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7.6 THE IMPORTANCE OF FORCEIN LIFE
Gravitational force hold and keep all theobjects on Earth.
Frictional force help us to carry out
activities. Electrostatic, magnetic and mechanical
forces to run the various types ofmachines for industries, transportation &
communications.
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Created by : Teacher Faasilla