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2.8 ::UNDERSTANDING

GRAVITY

CHAPTER 2 :FORCES AND MOTION

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History ab0ut gravity.

Newton was the 1 st person who studied the force of gravity.He got an inspiration whenan apple fell from a tree,hitting his head.Hehheehhe Newton cute laaa

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Hist0ry about gravity

Newtons opinion

The falling apple was acted on by a force of attraction,[the force of attraction due to

gravity from the centre of earth]

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Objects fall because they are pulled towards theEarth by the force of gravity.

This force is known asthe pull of gravity or theearths gravitationalforce. The earths gravitationalforce tends to pulleverything towards itscentre.

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FREE FALL

Any object which falls only under theinfluence of the force of attraction due to

gravity and without any influence of other forces is said to experience

FREE FALL.

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FREE FALL

An object is falling freely when it is falling under

the force of gravity only.

A piece of paper does not

fall freely because its fallis affected by air resistance.

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FREE FALL

An object falls freely only in vacuum .

The absence of air means there is no air

resistance to oppose the motion of the object.

In vacuum, both light and heavy objects fallfreely. They fall with the same acceleration that is the acceleration due to gravity, g .

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FREE FALL

(a) With air resistance(b) Without air resistance

(in vacuum)

Practically, free falling can only take place in vacuum.

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1. Explain Acceleration due to gravity

All objects which experience free fall will fall with thesame acceleration, that is ACCELERATION DUE TOGRAVITY.This constant acceleration is known as thegravitational acceleration, g.The standard value of the gravitational acceleration,g is 9.81 m s -2 .The value of g is often taken to be 10 m s -2 forsimplicity.The magnitude of the acceleration due to gravity depends on the strength of the gravitational field.

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2. State what a gravitational field is

The gravitational field is the region around the earthin which an object experiences a force towards thecentre of the earth.

This force is the gravitational attraction between theobject and the earth.

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3. Define gravitational field strength

The gravitational field strength is defined as thegravitational force which acts on a mass of 1kilogram.

Its unit is N kg -1.

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4. Determine the value of acceleration due to gravity

Gravitational field strength, g = 10 N kg -1

Acceleration due to gravity, g = 10 m s -2

The approximate value of g can therefore be written either as 10 m s -2 or as 10 N kg -1.

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Gravitational Field Strength vs. Gravitational Acceleration

When considering a body falling freely , the g is thegravitational acceleration. g (= 9.8 m s-2)

When considering objects at rest , g is the Earthsgravitational field strength acting on it.

g (= 9.8 N kg-1)

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5. Define weight ( W) as the product of mass ( m) and acceleration due to gravity ( g) i.e . W = mg.

The weight of an object is defined as thegravitational force acting on the object.

Weight = mass x gravitational acceleration

W = mg

SI unit : Newton, N and it is a vector quantity

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6. Solve problems involvingacceleration due to gravity

Example 1Can you estimate the gravitational force act to your body?

mass = 60 kg, g = 9.8 N kg-1, F = ?

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Example 1Can you estimate the gravitational force act to your body?

mass = 60 kg, g = 9.8 N kg-1, F = ?

F = mg

= (60) (9.8)= 588.0 N

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Example 2 A satellite of mass 600 kg in orbit experiences agravitational force of 4800 N. Calculate the

gravitational field strength.

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Example 2 A satellite of mass 600 kg in orbit experiences agravitational force of 4800 N. Calculate the

gravitational field strength.

Given : m = 600 kg. F = 4800 N, g = ? g = F = 4800 . = 8 N kg-1m 600

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Example 3 A stone is released from rest and falls into a well. After 1.2 s, it hits the bottom of the well.

(a) What is the velocity of the stone when it hits the bottom?(b) Calculate the depth of the well.