Download and gravitational acceleration

Why does an object fall downwards…………….?
Does the object also fall downwards on the
moon or on other places………….?
Wee see objects falling every day…………….
Do they fall with constant velocity or with an acceleration?
Does the boy have the same weight on the
earth and on the moon….?
The Moon
The Earth
Video
What do you see in the video………?
Where is it…………….?
What Will Happen If There Is No Gravitation On The Earth…?
Without gravitation, everything will floats in the air, including us, so
how can we walk on the ground? Also, have you even thought about that :
without gravitation, how can we go to the toilets? What will happen to the
substances in the toilets? Can you imagine that?
More important, if there is no gravitation, the Earth will not rotate around
the Sun at all. At that time, there will be no change in season. So
that some places will have 24 hours daytime and some places will have 24
hours nighttime. Can you imagine how our life will be?
UNDERSTANDING GRAVITY
Gravity
Gravity is a
force of
attraction
only between
bodies that
have mass.
Gravitational
Force
All objects fall to
the surface of the
earth because they
are pulled towards
the earth by a force.
Gravitational force
The force that pulls
everything towards the
center of the earth.
An apple fall from a
tree, it’s accelerated,
since its velocity
changes from zero as
it is hanging on the
tree and moves
toward the ground.
Gravitational
Field on the Earth
Gravitational Field is
The region around
the earth which an
object experiences a
gravitational force.
It is also said gravitational
acceleration (g).
At this point, the gravitational
field strength (g) is defined as
the gravitational force acting on
a 1 kg mass place at this point.
Gravitational
field strength at
the surface of
the earth is 9.8
N kg-1.
W F
g 
m m
An object of mass
1 kg will experience
gravitational force
(f) of 9.8 N.
g = gravitational field strength (N kg-1)
F = Gravitational force
m = mass of body
Acceleration
Due to Gravity
The gravitational
force cause object
to fall with
increasing velocity
Acceleration
F
g (m s-2)
Acceleration
Due to Gravity
Falling Objects
Situation 1:
Situation 2:
1. Acceleration
1. Acceleration
2. With air resistance
2. No air resistance
Acceleration due to gravity
g = 9.8 ms-2
FREE FALL
vacuum
FREE FALL
1. No air resistance
vacuum
2. Acceleration due to
gravity
g = 9.8 ms-2
g = 9.8 ms-2
video
Weight
The Weight is the gravitational force exerted
on the object by the earth or other Massive body
Weight (W) depends on
mass (m) and gravitational
acceleration (g).
W=mg
W = weight (N)
M = mass (kg)
g = gravitational acceleration (ms-2)
The bigger g
The bigger W
The smaller g
The smaller W
m = 15 kg
The Earth (g = 9.8 ms-2)
The weight of the baby on
the earth: 15 x 9.8 = 147 N
m = 15 kg
The Moon (g = 1.6 ms-2)
The weight of the baby on
the moon: 15 x 1.6 = 24 N
Differences between weight and mass
Weight
1. Measurement of the
gravitational force on body.
Mass
1. Measurement of the
quantity of matter in a body.
2. The weight on an object
varies with the gravitational
field strength of the
location. Ex: The weight of
an object on earth is
different from its weight
when on the moon.
2. The mass of an object is
constant. (everywhere).
3. A vector quantity (as it
involves forces).
4. A derived quantity : Weight,
W=mg.
5. SI unit : Newton (N)
3. A scalar quantity.
4. A base quantity : Mass = m
5. SI unit : kilogram (kg).
Problem Solving
1. A spacecraft of mass 800 kg is orbiting above the Earth’s
surface at a height where its gravitational field strength is
2.4 N kg-1. Calculate the gravitational force experienced by
the spacecraft.
Solution:
F
g
m
F = mg
F = 800 x 2.4
F = 1920 N
2. A rock has a mass of 20 kg and weight of 90 N on the surface of a planet.
a. What is the gravitational field strength on the surface of the planet?
b. What are the mass and the weight of the rock on the surface of the Earth
where its gravitational field strength is 9.8 N kg-1 ?.
Solution:
F
m
90
g
20
a. g 
g = 4.5 N kg-1
b. Mass remains unchanged, that is 20 kg
Weight = mg
Weight = 20 x 9.8
Weight = 196 N
Quiz
Problem
1. A cat has a mass of 7 kg. what is its weight if
a. It is on the surface of the earth where the
gravitational field strength is 9.8 N kg-1?.
b.It is on the surface of the moon where the
gravitational field strength is 1/6 of that on the
surface of the earth? Answer
Summary
1. The gravitational field strength at a point in the
gravitational field is the gravitational force acting on a
mass of 1 kg placed at that point.
2. The gravitational field strength on the surface of the
Earth is 9.8 N kg-1.
3. An object on the surface of the Earth experiences free
fall with an acceleration of 9.8 m s-2.