Chapter 7: Dynamics (f2)

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

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Chapter 7: Dynamics (f2)