Gravity

1. Gravity

2. Introduction

4. • If the year 1666, sir Isaac Newton first described force
of gravity mathematically.
• It is said that he observed an apple falling from a tree
towards the ground.
• He wondered which force made the apple fell down
towards earth and not moved toward the sky.
• Another scientist Albert Einstein, further added to
Newton’s ideas about gravity with his theory of
relativity.
• The earth rotates around itself and around the sun
with a speed, though we do not realise it.
• At this speed the earth should ideally throw us away
from its surface towards empty space, but this does
not happen.

13. • This is because the earth exerts a force on all the
objects on its surface.
• This force exerted by earth called earths
gravitational force.
• Every time you jump you, you experience gravity.
• It pulls you back down to the ground.
• Without gravity you would float off into the space
along with all other matter on the earth.
• Gravity is a force of attraction.
• Gravity physically acts like a magnet which pulls
two objects together.

14. • The sun has a very large gravitational force
which ensure that all the planets keep
revolving around it.
• Similarly, all the planets have a gravitational
force of their own.
• The earth’s gravitational force keeps the
moon revolving around it, and ensure that all
animals and things on earth stay bound to it.
• The gravity of sun is much more than that of
a earth, still we are not pulled by the sun, this
is because we are much closer to earth.

15. • The earth’s atmosphere stays around the earth
because of the gravitational force exerted by the
earth.
• Gravity is also responsible for the flowing air and
wind.
• The earth’s gravity is six times stronger than that of
the moon.
• Weight is actually the measurement of the force of
gravity pulling on an object.
• Our weight on Earth is how hard gravity is pulling
on us toward Earth’s surface.
• Our weight on other planets whether weigh more
or less depends on the gravity of that planet.

16. • Gravitational force exerted by any object is not the
same.
• The gravitational force exerted by each body or things
is different.
• Gravity is largely influenced by the mass, size and
closeness of the objects.
• If you are standing on a top of a tower and how you
throw an iron ball and a plastic ball simultaneously
towards the earth, which of the se would reach the
surface first?
• The iron ball will reach the surface first because it has
more mass than the plastic ball.
• Now if two objects are placed on the ground , one a
large piece of paper and other a small piece of
cardboard that has the same mass as that of the large
piece of paper, on what would gravity have more
effect?

17. • Here the gravitational force acting on the cardboard will be
less because the area of the paper is larger than that of the
cardboard and hence more gravitational force is acting on it.
• If you place a screw at a larger distance lets say 5 cm from
the magnet, it will take longer to attract a screw, however if
you place it at 2 cm from the magnet the screw will stick to
the magnet sooner.
• The same is with gravity.
• The closer the object, the greater is the force of attraction.
• However, it is very important to remember that mass, size
and distance are not individual properties that can be
considered independently.
• If you were to determine which object would reach the earth
first if released from the same height all of these factors
would have to be considered simultaneously for the objects
being dropped.

18. • In short, gravitational force is greater, if the masses
are bigger and objects are closer.
• In this way the amount of gravity that somebody
possesses is proportional to its mass and distance
between it and other object.
• Also there is no gravity in space that is why we feel
weightless in space.
• We have weight because of gravity.
• Mass of an object never changes, but the weight of
the object changes depending on the gravity.
• Gravitational force is always directed vertically
downwards i.e. towards the centre of the earth.

19. • The earth is not uniformly round and is flatter
towards the poles and bulges at the equator.
• Hence the distance between the centre of the
earth and the equator is more than the distance
between centre of the earth and the poles.
• Thus, gravitational force is not uniform around the
earth as well it is more at the poles and gradually
reduces as you move towards the equator.
• For all practical purposes the average gravity on
earths surface is calculated at 9.8 m/s2.
• In physics, weight is described as a force and can
also be measured in Newtons.

20. • Important facts:
• Tides are caused by the rotation of the earth and the
gravitational effects of the moon and sun.
• Because mars and the moon have a lower gravity than
earth, a person weighing 100 kg on earth would only
weigh 38 kg on mars and 16.6 kg on the moon.
• The moon’s gravity is 1/6 of earth’s gravity so objects
on the moon will weigh only 1/6 of their weight on
earth.
• There is zero gravity in outer space so you would be
weightless if you were floating out in space!
• The force of gravity at a distance of 100 kilometers
from the earth’s surface is about 3% less than on the
earth’s surface.

21. • Objects weigh a little bit more at sea level than they do
on the top of a mountain.
• This is because the more distance between you and
earth surface the less gravitational force earth exerts
on you.
• So the higher you go, the less gravity pulls on you, and
the less you weigh.
• However, the difference is very small and hardly
noticeable.
• To escape the earth’s gravitational pull the object
should move at a speed of 11 km/s.
• Thus gravity is a major force in the universe which
keeps us from floating away, controls the oceans tides,
guides plant growth, keeps earth and other planets in
orbit.

30. Multiple
Choice
Questions

31. 1) Moon revolves round the earth because of the
gravitational force of
a) Sun
b) Earth
c) Mars
d) Venus
2) ________ influences gravitational force.
a) Mass
b) Surface area
c) Proximity
d) All of these
b) Earth
d) All of these

32. 3) If a man drops two balls of the same size, an iron ball of mass 10
kg. and a copper ball of mass 9 kg. from a distance of one kilometer
from the earths surface, which of these will reach the earth first?
a) Iron ball
b) Copper ball
c) Cant say
d) Both will reach at the same time
4) What is the strength of gravitational force in space?
a) 9.8 m/s
b) 4.9 m/s
c) 0 m/s
d) None
5) The earth is flatter towards _________.
a) The centre
b) Poles
c) Planets
d) In the oceans
a) Iron ball
c) 0 m/s
b) Poles

33. 6) On earth, the gravitational force is more ________
a) At the centre b) at the poles
c) In the planets d) in the oceans
7) What is the average acceleration due to gravity?
a) 9.8 mm/s b) 9.8 cm/s
c) 9.8 m/𝑠2
d) 9.8 km/s
8) An object will weigh ______ on the moon if it
weighs 16 Newton on earth.
a) 96 kg b) 96 newtons
c) 2.66 newtons d) 2.66 kg
a) At the centre
c) 9.8 m/𝑠2
C) 2.66 newtons

34. 9) What is the mass of an object if it weigh’s 109
Newton on the moon?
a) 11.12 kg b) 66.73 kg
c) 18.16 kg d) 654 kg
10) An object has mass 50 kg. on earth. What would
its mass on Mars?
a) 13 kg b) 16 kg
c) 19 kg d) 50 kg
11) Tides are caused by the rotation of the _______
and the gravitational effects of the _______
a) Moon, earth and sun b) sun, earth and moon
c) earth, moon and sun d) none
c) earth, moon and sun
b) 66.73 Kg
d) 50 Kg

35. 12) By how much does the earth gravitational force
reduce at a distance of 100 km, from the earth’s
surface?
a) 51% b) 23%
c) 11% d) 3%
13) Gravity is largely influenced by mass, size and
_______ of the object.
a) Temperature b) closeness
c) Height d) depth
14) ________ has maximum gravitational force.
a) Sun b) earth
c) Moon d) man
d) 3%
b) closeness
a) Sun

36. 15) Gravity physically acts like a _________.
a) Magnet b) wire
c) Bulb d) fuse
16) Space has no gravity.
a) True b) false
17) Objects would have no mass in the absence of
gravity
a) True b) false
a) Magnet
a) True
b) false

37. 18) Force can be numerically counted.
a) True b) false
19) The gravitational force on mars is more than that
on the moon
a) True b) false
20) We feel weightless in space because of the
space suit
a) True b) false
a) True
b) false
a) True