chapter 7 force and motion form 2
TRANSCRIPT
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CHAPTER 7
FORCE ANDMOTION
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Definition ofForceA force is defined as a push or
a pull. We use a pushing forceto push a trolley. We use a
pulling force to open door.
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What aforce can do
to object?
Change itsspeed
Move it Stop it
Increase or
decrease itsspeed
Change itsposition
Change itsshape
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Pushing Force
Pushing a pram
Hitting a shuttle-cockwith a racket
Cutting a loaf ofbread
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Pulling Force
Stretching aspring
Climbing a tree
Lifting a pail of water
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Types Of Force
Magnetic force
A magnet possesses magnetic force.
Magnetic force attracts iron, nickel and cobalt.A magnet has two poles a north pole and asouth pole.
Like poles repel each other. Unlike poles attract each other.
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The magnetic force is strongest at the poles.
A freely suspended magnet always comes to restin a north-south direction, with its north polepointing towards the Earths north pole.
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Gravitational Force
An object falls because it is pulled by the Earth.
The force of the Earth pulling an object towardsits centre is called the force of gravityorgravitational force of the Earth.
Theweight of an object is the gravitationalforce pulling it.
An object weight slightly heavier at the polesthan at the equator because the poles are nearer
to the centre of the Earth.the Earths gravitational force is about six timesgreater than that of the Moons.
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Electrostatic Force
When a plastic ruler is strongly rubbed with apiece of wool, both the ruler and the piece ofwool become charged with an electrostaticforce.
One object becomes positivelycharged whilethe others becomes negativelycharged.
A charged object (positively or negativelycharged) attracts an uncharged object.
Like charges repel each other.Unlike charges attract each other.
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Frictional Force
Africtional force is produced when 2 surfacesrub against each other.
A frictional force produces heat and can start afire, e.g. striking a match to get a fire.
A frictional force acts in the direction oppositeto that of motion. Hence, a frictional force slowsdown motion.
A frictional force wears out the surface is
contact.
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Measuring Force
The unit of force
The SI unit for measuring force is the newton(N).
A force has magnitude, i. e. size.
Weight of 1 kg 10.0 N
Weight of 100 g 1.0 NWeight of 1.0 g 0.01 N
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Force is measured in Newton (N) by using aspring balance or a compression balance.
A spring balance is works on the principle thatthe extension of the spring is directlyproportional to the force pulling it.
Spring Balance
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A graph showing the extension of a springagainst the force pulling it is a straight line
originating from zero. This is true only if the force is not too large.
Force(N)
0 1 2 3 4 5 6
Extension
(cm)
0 2 4 6 8 10 12
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Graph of extension of spring against force