Gravity

and the

Laws of

Motion

MassMass is the amount of

“stuff” (matter) in an object.Measured in grams (kg,

mg, cg, etc.)Mass will not change

unless matter is added or taken away.

Weight Weight is the gravitational force

exerted on an object by Earth (or the moon, Jupiter, Sun, etc.)

It is a FORCE so it’s measured in

Newtons (N). Weight can change

depending on where

you are located.

Galileo Galilei 1564-1642

Italian astronomer and physicist

First true “Experimentalist”.

Set forth the basic ideas of Motion.

All objects fall at the same rate.

(heavy or light!)

Sir Issac Newton 1643-1727

English mathematician

and physicist

Laws of MOTION

Quick Review of Newton’s discoveries…

Laws 1st Law – The Law of Inertia 2nd Law – acceleration = Force mass 3rd Law – Forces occur in action-reaction pairs

Acceleration due to gravity, g All objects regardless of their mass are accelerated

equally by gravity Since this acceleration is so important to the motion

of all objects on the surface of the earth it is assigned g = 9.8 m/s2 (or 10 m/s2)

What is gravity? Gravity – the force that pulls objects

toward the earth.

Gravity Continued…

Freefall - An object accelerates as it falls.

Near the earth objects fall at the same rate regardless of their mass.

g = 9.8 m/s2

Gravity continued… Law of Universal

Gravitation

This law states that the force of

Gravity acts between all

objects in the universe.

Newton & Gravity Isaac Newton was the first

to deduce that the force that makes an apple fall is the same force that keeps the moon in its orbit.

But then he asked himself… Does the falling apple exert a

force on the earth? Why doesn’t the moon fall into

the earth?

Gravity . . .

The apple DOES exert a force on the Earth! Does the Earth notice? Why?

The moon IS falling! HUH?!? Satellite Motion!

Newtonian Mountain

Space Dangers

The Story of 9.8… From before we know that a 1 kg mass

exerts a force of 9.8 N on the Earth because…

Fweight = mass x gravity

Fw = (1kg)(9.8 m/s2)

Fw = 9.8 N

Where did this “9.8” come from in the first place?

The Story of 9.8…

F = 9.8 N

1 22 2

becomes... object Earth

Earth

m mmmF G F G

d r

Earth

1 kg mass

distance = radius

11 2 22

1 5.97 x 1024 kg6.67 10 /

6.38 x 106 m

kgF N m kg

Acceleration due to Gravity at Different Places on Earth…

g varies slightly due to differences in the distance from the center of the earth (altitude or latitude) North Pole g = 9.832 m/s2

Panama Canal g = 9.782 m/s2

New York City g = 9.803 m/s2

Denver g = 9.796 m/s2

Acceleration due to gravity on Different Planets…

Jupiter 2.54 x gNeptune 1.20 x gSaturn 1.06 x gUranus 0.92 x gEarth 1.00Venus 0.92 x gMercury 0.38 x gMars 0.38 x gPluto uncertainMoon 0.16 x g

These depend on the planet’s mass and radius.

Acceleration due to gravity depends on the planet’s mass and radius.

Acceleration due to gravity depends on the planet’s mass and radius.

Acceleration due to gravity depends on the planet’s mass and radius.

Guess your weight on Mars!

This website will tell you how much you would weigh if you were on any of the other planets.

http://www.exploratorium.edu/ronh/weight/index.html

Newton’s Universal Law of Gravitation

Depends on… Mass of the two attracting objects (m1 & m2)

Directly proportional

Distance between the two objects (d) Inversely proportional

Universal Gravitation Constant (G) G = 6.67 x 10-11 Nm2/kg2

Ya’ Gotta Obey the

Law!

Who are you attracted to??

Maybe not Romantically by

definitely Gravitationally!

Two bodies are attracted to each other due to their mass and the

distance between them.

Why don’t we always feel a gravitational attraction?

In order for a gravitational attraction to be felt the mass of one of the bodies must be very large. The earth is many times more massive than you are. You can sense the Force of Attraction.

Kepler’s First LawAll planets move about the sun in an

elliptical orbit. In an elliptical orbit the distance between the planet and the sun is continuously varying:

Why do we have seasons?

Are we closer to the sun in summer?

And farther away in the

winter?

What does the earth’s revolution around the sun

cause?The earth is

tilted on its axis 23.5 degrees. to the plane of the earth’s orbit around the sun

Seasons Mid-latitudes enjoy

a change of seasons.

Earth Revolution

APHELION / PERIHELION

“Helios” - Greek word for sun

Prefix “Ap” - means farthest

Prefix “Peri” - means closest

Which way did it go?

Better watch out for the

cops!