Gravity: A Force of AttractionGravity: A Force of Attraction

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.

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?!!''

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..

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.

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.

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…

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.

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.

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 objects2.2. DistanceDistance between the objects between the objects

Cavendish ExperimentCavendish Experiment

Find the constant, GFind the constant, G

https://video.search.yahoo.com/video/play;_ylt=A2KLqIEGEDFVzlsA1RosnIlQ;_ylu=X3oDMTByZWc0dGJtBHNlYwNzcgRzbGsDdmlkBHZ0aWQDBGdwb3MDMQ--?p=youtube+cavendish+experiment&vid=ceffb99e10a7de8282dd343f188c360e&l=1%3A59&turl=http%3A%2F%2Fts1.mm.bing.net%2Fth%3Fid%3DWN.UX5sc%252bcC6tcV5sLBvVqkFQ%26pid%3D15.1&rurl=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DdyLYbvZIYoU&tit=Cavendish+Experiment&c=0&sigr=11bulhojq&sigt=10k9bc3d1&sigi=121e0bcmj&age=1381239063&fr2=p%3As%2Cv%3Av&hsimp=yhs-001&hspart=mozilla&tt=b

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 mm11 x m x m22

rr22

F = forceF = force

G = gravitational constant = 6.673 G = gravitational constant = 6.673 x 10x 10-11-11 Nm Nm22/kg/kg2 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

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

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.

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.

Part 2Part 2Gravity & MotionGravity & Motion

Chapter 6.1Chapter 6.1

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.

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.

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.

A story about a cannonballA story about a cannonball

Can we demonstrate this?Can we demonstrate this?

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.

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/s2.

Gravity = g = 9.8 m/s2

Gravity &Gravity &AccelerationAcceleration

Notice, the ball isn’t Notice, the ball isn’t traveling at 9.8m/straveling at 9.8m/s22, it’s , it’s acceleratingaccelerating at 9.8 m/s at 9.8 m/s22..

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.

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.

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.

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.

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.

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 circular or near-circular path around another object.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.

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/s2.

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..