The document explains gravity as a force of attraction between objects, initiated by Isaac Newton's contemplation on why an apple falls towards Earth. It details the law of universal gravitation, which states that gravitational force depends on the mass of objects and the distance between them, alongside discussions on weight and mass. It also covers historical perspectives on gravity and motion from philosophers like Aristotle and Galileo, emphasizing that all objects accelerate towards Earth at the same rate, regardless of their mass.

1. Gravity: A Force of Attraction
Gravity: A Force of Attraction

2. It all starts with an apple…
It all starts with an apple…
 One beautiful spring day in 1655, a man named
One beautiful spring day in 1655, a man named
Isaac Newton
Isaac Newton was sitting under an apple tree in
was sitting under an apple tree in
his garden, enjoying a glass of tea.
his garden, enjoying a glass of tea.
 Suddenly, one of the apples fell and crashed on
Suddenly, one of the apples fell and crashed on
his head.
his head.
Disclaimer: This last part has been fictionalized, most
Disclaimer: This last part has been fictionalized, most
believe that the apple did not actually hit Newton on the
believe that the apple did not actually hit Newton on the
head, but rather fell nearby and caught his attention.
head, but rather fell nearby and caught his attention.

3. A story about an apple
A story about an apple
 That got Newton thinking (once the
That got Newton thinking (once the
bump had gone down of course).
bump had gone down of course).
 “
“Why did the apple fall towards the
Why did the apple fall towards the
Earth?”
Earth?”
 “
“Why did it not “shoot upwards''
Why did it not “shoot upwards''
when it came away from the
when it came away from the
branch?”
branch?”
 And, “why did I plant that stupid
And, “why did I plant that stupid
apple tree anyway?!!''
apple tree anyway?!!''

4. A story about an apple
A story about an apple
 Newton knew that
Newton knew that
unbalanced forces are
unbalanced forces are
necessary to move or
necessary to move or
change the motion of
change the motion of
objects.
objects.
 So, he came up with the
So, he came up with the
idea that the Earth must
idea that the Earth must
attract the apple towards it
attract the apple towards it
with some “unseen force''.
with some “unseen force''.
 He named this force
He named this force
gravity
gravity.
.

5.  Gravity is a force of attraction between
objects.
 The atoms of one object pull on the atoms of
another object.
 It's like the Earth pulling on you and keeping
you on the ground.
 That pull is gravity at work.

6. All matter is affected by gravity
All matter is affected by gravity
 Definition of matter:
Definition of matter:
 Matter is anything that has mass and volume.
Matter is anything that has mass and volume.
 Since all matter has mass, all matter is
Since all matter has mass, all matter is
affected by gravity.
affected by gravity.
 Gravity (aka:
Gravity (aka: gravitational force
gravitational force)
) pulls
pulls
objects towards each other.
objects towards each other.
 It acts on anything with mass.
It acts on anything with mass.

7. Ok, then why don’t we see objects
Ok, then why don’t we see objects
being pulled towards one another?
being pulled towards one another?
 This is because the mass of most objects is
This is because the mass of most objects is
too small to cause an attraction large enough
too small to cause an attraction large enough
to cause the objects to move towards each
to cause the objects to move towards each
other.
other.
 Even though gravity is “pulling” the pencil
Even though gravity is “pulling” the pencil
you’re holding, its mass is so small that it’s
you’re holding, its mass is so small that it’s
not really moving.
not really moving.
 There is, however, one object that is big
There is, however, one object that is big
enough to cause a noticeable attraction…
enough to cause a noticeable attraction…

8. Gravity on Earth
Gravity on Earth
 The Earth!
The Earth!
 Earth has an enormous
Earth has an enormous
mass and thus an enormous
mass and thus an enormous
gravitational force.
gravitational force.
 When the Earth spins and
When the Earth spins and
gravity pulls on the clouds,
gravity pulls on the clouds,
weather can be affected.
weather can be affected.
 The Earth's gravity even
The Earth's gravity even
holds the atmosphere close
holds the atmosphere close
to our surface.
to our surface.

9. The Law of Universal Gravitation
The Law of Universal Gravitation
 Now that we know what gravity is, let’s
Now that we know what gravity is, let’s
go back to Newton.
go back to Newton.
 He generalized his observation in
He generalized his observation in
something called
something called the Law of Universal
the Law of Universal
Gravitation.
Gravitation.
 This law states:
This law states: All objects in the
All objects in the
universe attract each other through
universe attract each other through
gravitational force.
gravitational force.

10. The Law of Universal Gravitation
The Law of Universal Gravitation
The size of the gravitational force
The size of the gravitational force
depends on two things:
depends on two things:
1.
1. Mass
Mass of the objects
of the objects
2.
2. Distance
Distance between the objects
between the objects

11. Cavendish Experiment
Cavendish Experiment
Find the constant, G
Find the constant, G
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12. The law of universal gravitation
The law of universal gravitation
The formula for this law is:
The formula for this law is:
F = G x
F = G x m
m1
1 x m
x m2
2
r
r2
2
F = force
F = force
G = gravitational constant = 6.673
G = gravitational constant = 6.673
x 10
x 10-11
-11
Nm
Nm2
2
/kg
/kg2
2
- always an
- always an
uppercase G, do not confuse
uppercase G, do not confuse
with g, which is for gravity
with g, which is for gravity
M = objects’ mass
M = objects’ mass
R = distance between objects
R = distance between objects

14. The law of universal gravitation
The law of universal gravitation
Basically, there are two major parts to this law
Basically, there are two major parts to this law
that you need to know:
that you need to know:
1.
1. Gravitational force increases as mass
Gravitational force increases as mass
increases
increases
2.
2. Gravitational force decreases as
Gravitational force decreases as
distance increases
distance increases

15. What is weight?
What is weight?
 Weight is a
Weight is a measure of the gravitational
measure of the gravitational
force exerted on an object.
force exerted on an object.
 Most of the time, when we’re talking about
Most of the time, when we’re talking about
weight, we’re referring to the
weight, we’re referring to the Earth’s
Earth’s
gravitational force on an object.
gravitational force on an object.
 Since gravity is a force and weight is a
Since gravity is a force and weight is a
measure of gravity, weight is expressed in
measure of gravity, weight is expressed in
newtons (N).
newtons (N).
 On Earth, a 100 gram object would weigh 1 N.
On Earth, a 100 gram object would weigh 1 N.

16. What is mass?
What is mass?
 Mass is
Mass is the amount of
the amount of matter
matter in an object
in an object.
.
 This does not change… ever!
This does not change… ever!
 Whereas
Whereas weight
weight changes when gravity changes,
changes when gravity changes,
mass
mass always remains the same.
always remains the same.
 Remember, mass is measured with a balance,
Remember, mass is measured with a balance,
where the mass of one object is
where the mass of one object is compared
compared to
to
another object.
another object.
 On Earth, mass and weight are both constant
On Earth, mass and weight are both constant
since gravity is a constant force, which is why
since gravity is a constant force, which is why
they seem like the same thing to us.
they seem like the same thing to us.

18. Part 2
Part 2
Gravity & Motion
Gravity & Motion
Chapter 6.1
Chapter 6.1

19. Actually, it all started with a
Actually, it all started with a
cannonball…
cannonball…
 Well, and before the
Well, and before the
cannonball, it started with a
cannonball, it started with a
philosopher & scientist
philosopher & scientist
named
named Aristotle
Aristotle.
.
 In ancient Greece around
In ancient Greece around
400 BC, he proposed that
400 BC, he proposed that
the rate at which an object
the rate at which an object
falls depends on its mass.
falls depends on its mass.
 In other words, Aristotle
In other words, Aristotle
believed that the heavier
believed that the heavier
the object, the faster it falls.
the object, the faster it falls.

20. A story about a cannonball
A story about a cannonball
 In the late 1500s, an Italian
In the late 1500s, an Italian
scientist named
scientist named Galileo
Galileo
Galilei decided to prove
Galilei decided to prove
Aristotle wrong.
Aristotle wrong.
 Galileo theorized that all
Galileo theorized that all
objects will land at the same
objects will land at the same
time when they are dropped
time when they are dropped
from the same height.
from the same height.
 To prove this to his critics,
To prove this to his critics,
he set up a little experiment.
he set up a little experiment.

21. A story about a cannonball
A story about a cannonball
 Galileo carried a cannonball and a wooden ball up
Galileo carried a cannonball and a wooden ball up
the 300 steps of the Leaning Tower of Pisa.
the 300 steps of the Leaning Tower of Pisa.
 Dropping the two different balls at exactly the
Dropping the two different balls at exactly the
same time, the crowd was amazed with what they
same time, the crowd was amazed with what they
saw…
saw…
 The two balls, with extremely different masses,
The two balls, with extremely different masses,
landed at the exact same time!
landed at the exact same time!
Disclaimer: this story may or may not be true, but a similar
Disclaimer: this story may or may not be true, but a similar
and well-documented experiment was performed by
and well-documented experiment was performed by
Benedetti Giambattista in 1553.
Benedetti Giambattista in 1553.

22. A story about a cannonball
A story about a cannonball
 Can we demonstrate this?
Can we demonstrate this?

23. Elephants & feathers
Elephants & feathers
 What did Galileo prove?
What did Galileo prove?
 Objects fall to the ground at the
Objects fall to the ground at the
same rate because
same rate because acceleration
acceleration
due to gravity is the same for
due to gravity is the same for
all objects.
all objects.
 For example, an elephant and a
For example, an elephant and a
feather fall with the
feather fall with the same
same
acceleration
acceleration even though they
even though they
have
have different masses.
different masses.

24. Gravity &
Gravity &
Acceleration
Acceleration
 The rate at which
objects accelerate
towards Earth is 9.8
m/s/s.
 This acceleration is the
same for all objects,
regardless of their
mass.
 In other words, falling
objects accelerate at a
constant rate of 9.8
m/s2
.
 Gravity = g = 9.8 m/s2

25. Gravity &
Gravity &
Acceleration
Acceleration
 Notice, the ball isn’t
Notice, the ball isn’t
traveling at 9.8m/s
traveling at 9.8m/s2
2
, it’s
, it’s
accelerating
accelerating at 9.8 m/s
at 9.8 m/s2
2
.
.
 It’s velocity is continually
It’s velocity is continually
increasing.
increasing.
 Only the acceleration
Only the acceleration
remains constant.
remains constant.
 Each picture was taken 1
Each picture was taken 1
second apart, but notice
apart, but notice
how the ball travels a
how the ball travels a
greater distance between
greater distance between
each second.
each second.

26. Air Resistance
Air Resistance
 Watch me drop two pieces of paper, one
Watch me drop two pieces of paper, one
crumpled in a tight ball and the other kept flat.
crumpled in a tight ball and the other kept flat.
 As you know, the crumpled paper has more
As you know, the crumpled paper has more
velocity and hits the ground first, whereas the
velocity and hits the ground first, whereas the
flat paper sashays slowly until it lands on the
flat paper sashays slowly until it lands on the
ground.
ground.
 You also know that air has friction & causes the
You also know that air has friction & causes the
objects to move more slowly.
objects to move more slowly.

27. Air Resistance
Air Resistance
 The amount of air resistance depends on the size
The amount of air resistance depends on the size
and shape of an object.
and shape of an object.
 Air resistance increases as an object’s
Air resistance increases as an object’s speed
speed
increases and its
increases and its surface area
surface area (or cross-sectional)
(or cross-sectional)
increases.
increases.
 While gravity pulls an object down, towards the
While gravity pulls an object down, towards the
Earth, the air resistance pushes an object up, or
Earth, the air resistance pushes an object up, or
away from the Earth.
away from the Earth.

28. Free Fall
Free Fall
 When there is no air
resistance, an object is in
free fall.
 An object is in free fall only
if gravity is pulling it
down and no other forces
are acting on it.
 Free fall can only occur
where there is no air, such
as in a vacuum.

29. The role of gravity in orbit
The role of gravity in orbit
 In space, there is very little gravity.
In space, there is very little gravity.
 This is because the distances between
This is because the distances between
you and other large objects are so vast,
you and other large objects are so vast,
the force is minimal.
the force is minimal.
 You would still have mass, and because
You would still have mass, and because
there is still a little bit of gravity, you
there is still a little bit of gravity, you
would still have some weight.
would still have some weight.

30. The role of gravity in orbit
The role of gravity in orbit
 We know that an object in orbit is traveling in a
We know that an object in orbit is traveling in a
circular or near-circular path around another object.
circular or near-circular path around another object.
 There are two forces that act on a spaceship in
There are two forces that act on a spaceship in
orbit.
orbit.
 First,
First, gravity is pulling the spaceship back to
gravity is pulling the spaceship back to
earth
earth, so technically, the shuttle is in
, so technically, the shuttle is in free fall
free fall.
.
 Second, the shuttle is being propelled forward and
Second, the shuttle is being propelled forward and
is moving at a constant speed.
is moving at a constant speed.
 As a result, the path of the orbit is a curved
As a result, the path of the orbit is a curved
combination of the two forces.
combination of the two forces.

32. Some additional info:
Some additional info:
Terminal Velocity
Terminal Velocity
 Watch as this sky-diver falls.
 Once the parachute opens, the upwards force of the air
resistance begins to counter the downwards force of gravity.
 Eventually, his acceleration slows to 0 m/s2
.

33. Terminal Velocity
Terminal Velocity
 As an object falls, the air resistance is
As an object falls, the air resistance is
continually increasing.
continually increasing.
 Eventually, the force of the air resistance will
Eventually, the force of the air resistance will
match the force of gravity pulling the object
match the force of gravity pulling the object
down.
down.
 When this happens, the overall force is 0 N
When this happens, the overall force is 0 N
and the object stops accelerating.
and the object stops accelerating.
 The object is now falling a constant velocity,
The object is now falling a constant velocity,
which is called the
which is called the terminal velocity
terminal velocity.
.