Section 5-4
Question: Why
is it easier for a
person to
bounce on the
moon than it is
to bounce on
Earth?
Answer:
GRAVITY!!

Gravity is a force of
attraction between objects
that is due to their masses
The force of gravity can
change the motion of an
object by changing its speed,
direction, or both
 Since all objects have mass,
and gravity is a result of
mass, all matter is affected
by gravity


All objects experience an attraction
towards other objects that pulls them
together

For example: you are applying a
gravitational force on your pen/pencil
and it is applying a gravitational force on
you
 Why doesn’t the pencil move towards you
when you drop it then?

You don’t’ notice objects moving
toward each other because their
gravitational is too weak compared to
Earth’s

Earth is HUGE

Because of its huge mass, Earth has an
extremely large gravitational force
 You must apply forces to overcome Earth’s
gravitational force to lift objects or to
move

Earth’s gravitational force pulls
everything towards the center of the
Earth



Sir Isaac Newton knew that unbalanced forces
are needed to change the motion of objects
He concluded that the same unbalanced force
that caused an apple to fall to the ground also
makes the moon orbit around the Earth –
gravity!!
He summarized his ideas in a law called The law
of Universal Gravitation


Describes the relationships between gravitational force,
mass and distance
It applies to all objects in the universe

All objects in the universe attract each
other through gravitational force
Size of the force depends on the masses of the
objects and the distance between the objects
 2 parts to the law
 Gravitational force increases as mass increases
 Gravitational force decreases as distance
increases



Gravitational force is small between objects
that have small masses
Gravitational forces is large when the mass of
one or both objects is large

Why is an elephant harder to pick up
than a cat?
 Because an elephant is much larger, which
increases the gravitational force between Earth and
the elephant

Why do astronauts bounce on the moon?

The moon has less mass then Earth so there is a
smaller gravitational pull between the two


Gravitational force is strong when the distance
between two objects is small
If the distance between them increases, the
gravitational force pulling them together
decreases rapidly


The sun is 300,000 times more massive than the
Earth
It is this large mass of the sun and the large
masses of the other planets that keep them all
in orbit


Why don’t you get pulled toward the sun instead of
back down to Earth?
Because the Sun is so far away from us, its
gravitational force on us is very small

Weight is the measure
of gravitational force
on an object

It can either be a
measure of Earth’s
gravitational force on
an object or the
gravitational force
exerted on objects by
the moon or any other
planet

Mass is related to weight, but they are
not the same thing


Weight changes when gravitational force
changes
Mass is the amount of matter in an object
 An object’s mass does not change when
gravitational force changes
 What is the only way to change an object’s
mass?

What would happen to an objects weight if it
was measured on a planet that had a stronger
gravitational force than here on Earth?


Its weight would increase
What if it were measured on a planet that had a
weaker force?

Its weight would decrease
 The moon’s gravitational force is about 1/6 of Earth’s
 That means, you would weigh 1/6 on the moon of what
you weigh on Earth, but does your mass change?

Gravitational force is
about the same
everywhere on Earth
Because of this, an object’s
weight is about the same
everywhere
 Because mass and weight
are constant on Earth,
they are often used to
mean the same thing


Weight is a measure of gravitational force



Since it is measuring a force, the unit for weight is N
(Newtons)
Spring scales are used to measure weight
Mass is measured in g (grams) or kg
(kilograms)

Triple beam balances are used to measure mass