Gravity is a Physics PowerPoint Presentation

For Physics students

1. Gravity: A Force of AttractionGravity: 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 NewtonIsaac 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 appleA 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 appleA 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
gravitygravity..

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 gravityAll 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 forcegravitational 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 EarthGravity 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 GravitationThe 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 GravitationThe 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.MassMass of the objects of the objects
2.2.DistanceDistance between the objects between the objects

11. Cavendish ExperimentCavendish Experiment
Find the constant, GFind the constant, G
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12. The law of universal gravitationThe law of universal gravitation
The formula for this law is:The formula for this law is:
F = G x F = G x mm
11 x m x m
22
rr
22
F = forceF = force
G = gravitational constant = 6.673 G = gravitational constant = 6.673
x 10x 10
-11-11
Nm Nm
22
/kg/kg
2 2
- always an - always an
uppercase G, do not confuse uppercase G, do not confuse
with g, which is for gravitywith g, which is for gravity
M = objects’ massM = objects’ mass
R = distance between objectsR = distance between objects

14. The law of universal gravitationThe 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
increasesincreases
2.2.Gravitational force decreases as Gravitational force decreases as
distance increasesdistance 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’sEarth’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 ofthe amount of matter matter in an objectin an object..
This does not change… ever!This does not change… ever!
Whereas Whereas weightweight changes when gravity changes, changes when gravity changes,
massmass 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 comparedcompared 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 2Part 2
Gravity & MotionGravity & Motion
Chapter 6.1Chapter 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 AristotleAristotle..
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 cannonballA story about a cannonball

In the late 1500s, an Italian In the late 1500s, an Italian
scientist named scientist named GalileoGalileo
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 cannonballA 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 cannonballA story about a cannonball
Can we demonstrate this?Can we demonstrate this?

23. Elephants & feathersElephants & 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
accelerationacceleration even though they even though they
have have different masses.different masses.

24. Gravity &Gravity &
AccelerationAcceleration
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/s
2
.
Gravity = g = 9.8 m/s
2

25. Gravity &Gravity &
AccelerationAcceleration
Notice, the ball isn’t Notice, the ball isn’t
traveling at 9.8m/straveling at 9.8m/s
22
, it’s , it’s
acceleratingaccelerating at 9.8 m/s at 9.8 m/s
22
..
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 ResistanceAir 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 ResistanceAir 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 speedspeed
increases and its increases and its surface areasurface 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 FallFree 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 orbitThe 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 orbitThe 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
earthearth, so technically, the shuttle is in , so technically, the shuttle is in free fallfree 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 VelocityTerminal 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/s
2
.

33. Terminal VelocityTerminal 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 velocityterminal velocity..