What is Gravity?

• Write down anything you may know about gravity.

• Any examples of gravity

• Where you may find it or not find it!

What is Gravity?Gravity is an attraction that acts between any two objects that have a mass.So why isn’t your notebook attracted to you now?1. Gravity is only significant if, one or both of the objects are very massive (like a planet, star, or moon).Scientist’s know the acceleration of gravity is…

= 9.8m/s2

Let’s review…What’s matter?Anything that has a mass and takes up

spaceSo…What’s mass?How much matter is in an object.Then what’s Weight?How much gravity is pulling on an object.

An example of GravityEvery object applies a gravitational force to every other object.

So why does an apple fall from a tree?

The apple falls because it is more attracted (gravitational force) to the Earth than it is to the tree.

Differences Between Gravity and Friction

1. The force of gravity acts even when objects are not touching.2. Friction must ALWAYS be touching!3. The force of friction can never make an object speed up!4. Friction always points in the opposite direction of motion, 5. But gravity sometimes points in the direction of motion.

Mass and Distance

Mass and Distance affect Gravitational Force.The force of gravity depends on the masses of two objects and the distance between them.2. The greater the mass of an object, the more gravity it has pulling on it and other objects.

More about Gravity3. The further away the object is from the center of the Earth, the less gravity it will have pulling on it.

The Acceleration Due to Gravity:How does g change with altitude?

Altitude (m)  

Distance From the Center of

g due to the Earth's

Gravity  What has This

Altitude?the Earth

(m) (m/s2)0 6378100 9.799823 Lying on the Earth 2 6378102 9.799817 Height of a person4 6378104 9.799811 On a small roof

137 6378237 9.799402 Highest point in Delaware381 6378481 9.798652 Empire State Building

8848 6386948 9.772690 Height of Mount Everest12000 6390100 9.763051 Airplane

340000 6718100 8.832994 International Space Station6378100 12756200 2.449956 Twice the Earth's Radius

378024900 384403000 0.002698 The Moon

Does the gravitational pull near the surface change much?

No! The gravitational pull remains nearly constant! You would have to be hundred’s of miles away for gravity to really affect you!

Summarizing Gravity1. Gravity is only significant if, one or

both of the objects are very massive (like a planet, star, or moon).

2. As object get larger, the more gravity is pulling on them.

3. As objects get further away from each other, the less gravity is pulling on them.

Earth vs. Terrestrial Planets (Rocks)

Earth vs. Jovial Planets (Gas)

Sun vs. The Planets

Sun vs. Other Stars

Other Stars

More about Gravity Gravity is the reason why weight and

mass are two different measurements!!!

Mass is related to the amount of matter in an object.

Weight = the force of gravity pulling on any object. (w=mg)

So…which one would change if you were on the moon? Why?

Weight, because the force of gravity pulling on an object on the moon is only 1/6 of that on the Earth!

Gravity DropIf I drop a golf ball and a plastic fuzzy ball at the same time, which one hits the table first?The both hit at the same time!

BUT, what if I drop a volley ball and a bowling ball at the same time? Which one will hit the ground first?Let’s try it!So what happened?

Gravitational Force Formula My name is

Sir Isaac Newton and I figured this

out!

d is the distance between the centers ofthe two objects.

G is called the “gravitation constant” it equals (6.67x10-11 N x m2/kg2)and makesthe units right! Units for g =

221

G distance

massmassGF

Newton’s (N)

The Acceleration Due to Gravity:

So what is g for the moon, on the surface of the moon?

2moon

moonmoon

d

mGg

2

222211

moonm 1737100

kg 107.3477/kgmN106.673g

2moon m/s 1.62g

QUESTION????Which has more gravitational force…the earth and you or you and your neighbor?Why? (Use the formula… FG = G x m1 x m2

d2

You (60.4 kg) and your neighbor (70.5 kg) and d=1.5 m

You (60.4 kg) and the earth (5.9742×1024 kg)

But what is your weight?

2

2211

G1.5m

kg 60.4kg 70.5/kgmN106.673F

2

242211

Gm 6378100

kg 60.4kg 105.97kgmN106.673F

/

2m/s 9.8kg 60.4gmwThey’re the same!

… but how?!?!?!

1.26 x 10 -7 N

591 N

591 N

The Acceleration Due to Gravity:

We can calculate it! At ground level:

2

242211

Gm 6378100

kg 60.4kg 105.97/kgmN106.673F

What’s this? 9.80 m/s2 = g

2dmG

g

But we can calculate “g” anywhere, caused by any object with mass!

RFA 18 #4D (6.673x10-11Nxm2/kg2) (61kg) (63kg) (4m)

U kg, m, N x m2/kg2

F FG = G x m1 x m2 d2

A FG= (6.673x10-11Nxm2/kg2) (61kg) (63kg)(4m)2

S FG = 1.60 x 10-8 N

Gravity Practice Problems

Use DUFASRemember G=6.673x10-11Nxm2/kg2

1) m1 = 15 kg

m2 = 996 kg

d = 596 m

3) m1 = 600 kg

m2 = 72,684 kg

d = 30 m

2) m1 = 232 kg

m2 = 9,456 kg

d = 56 m

4) m1 = 7.35x1022 kg

m2 = 5.97x1025 kg

d = 3.84x108 m

1. D U F A

m1 = 15 kg

m2 = 996 kg

d = 596 m G=6.67x10-11N

Kg, m

N x m2/kg2

S

221

G dmmG

F

221

G dmmG

F

N102.80Fm 355216

mN109.97F

m 596

kg 996kg 15/kgmN106.673F

12G

2

27

G

2

2211

G

2. D U F A

m1=

232kg

m2=

9,456 kg

d = 56 m G=6.67x10-11N

Kg, m

N x m2/kg2

S

221

G dmmG

F

221

G dmmG

F

N104.66Fm 3136

mN101.46F

m 56

kg 9456kg 232/kgmN106.673F

8G

2

24

G

2

2211

G

3. D U F A

m1 =

600 kg

m2 =

72,684 kg

d = 30 mG=6.67x10-11N

Kg, m

N x m2/kg2

S

221

G dmmG

F

221

G dmmG

F

N103.23Fm 900

mN102.91F

m 30

kg 72684kg 600/kgmN106.673F

6G

2

23

G

2

2211

G

4. D U F A

m1=

7.351022 kg

m2 =

5.971025 kg

d =

3.84108 m

G=6.67x10-11N

Kg, m

N x m2/kg2

221

G dmmG

F

221

G dmmG

F

N101.98Fm 101.47mN103.84

F

m 103.84

kg 105.97kg 107.35/kgmN106.673F

21G

217

28

G

28

25222211

G

S

Gravity Practice ProblemsG= 6.673x 10-11 N x m2/kg2

In DUFAS

1. What is the gravitational attraction between an object with a mass of 10 kg and another object with a mass of 20 kg if they are separated by 0.01 meters?

Fg= 0.0001334 N 2. What is the gravitational attraction between my 10 kg killer Poodle and his 2kg bone that is 20m away from him?

Fg= 3.335 x 10-12 N

Gravity Practice1. Explain why a tree’s leaf falls to

the earth.

2. Who has more gravity pulling on them…a 350 lb offensive lineman or a 215 lb wide receiver on a football field? And why?

3. Why is there less gravity on the moon than here on Earth?

Gravity Answers1. The leaf falls to the Earth b/c the

Earth has a larger mass than the tree so it has a greater gravitational pull on the leaf.

2. The 350 lb lineman b/c he has more mass. More mass= more gravity pulling on that mass.

3. There is less gravity on the moon because you are very far from the center of the Earth so the gravitational pull is only 1/6th of Earth.