Explain why you don’t feel pulled to the stars in the sky even though they have gravity.

Universal Gravitation

Gravity

What we know:Gravity pulls all things down and pulls everything at the same rate of 9.8 m/s2.

What we don’t know:How gravity works if we’re not on Earth.

Notes

Gravity

Let’s pretend we live in a Universe where this one asteroid is the only thing in existence. No people, no stars, no dust, nothing except this asteroid. What can we say about gravity?

Gravity

Since gravity is a force, it has to be between two things.

Gravity

Since gravity is a force, it has to be between two things.

These weights want to pull together because of gravity. What if we replace the left one with a 20 lb weight?

Gravity

Since gravity is a force, it has to be between two things.

These weights want to pull together because of gravity. What if we replace the left one with a 20 lb weight?

The force pulling them together will double.

Increase mass of objects

Increase force of gravity between objects

“directly proportional”

Gravity

What if we keep both of them at the same mass, but double the distance?

Gravity

What if we keep both of them at the same mass, but double the distance?

The force between them is cut down to ¼.

Increase distance between objects

Decrease force of gravity between objects by alot!

“Inversly proportional”

Equations

Universal Gravitation

more mass F = G

more distance

F: The force of gravity in m/s2

m: Masses in kg. More mass means

stronger gravity!r: The distance between the

objects in meters. More distance means

a lot less gravity!

G: gravity constant 6.674 x 10-11 N x m2/kg2

Used to calculate the force of gravity between two objects

Equations

m1 • m2

r2

Gravity

Gravity is what keeps the Earth going around the sun. Imagine we could take the sun and squeeze it down into a tiny ball like this without changing anything else:

What would happen to the Earth?

Notes

Gravity

Gravity is what keeps the Earth going around the sun. Imagine we could take the sun and squeeze it down into a tiny ball like this without changing anything else:

What would happen to the Earth?

Nothing! The distance from the middle of the sun to the middle of the earth didn’t change, so gravity is the same!

Gravity

What if you were standing on the Earth and the Earth began to shrink? Would you feel gravity change?

Gravity

What if you were standing on the Earth and the Earth began to shrink? Would you feel gravity change?

Yes! The distance between you and the middle of Earth got smaller and mass didn’t change, so gravity goes way up!

GravityLastly, we can use Newton’s second law to link mass and weight.

F = m • a

Force (weight) mass of person acceleration of gravity

GravityLastly, we can use Newton’s second law to link mass and weight.

F = m • a

Force (weight) mass of person acceleration of gravity

The moon has only about 1/6 the acceleration of gravity Earth has. What does this do to your weight?

GravityLastly, we can use Newton’s second law to link mass and weight.

F = m • a

Force (weight) mass of person acceleration of gravity

The moon has only about 1/6 the acceleration of gravity Earth has. What does this do to your weight?

Divides it by 6. If “a” goes down, “F” goes down!

Gravity practiceclasswork

1)

2) If the earth somehow grew larger without gaining in mass, what would happen to your weight? What if the earth got smaller?

3) Jupiter has a mass more than 300 times that of earth, but the acceleration of gravity there is only 3 times more than on earth. What’s going on here?

Calculating gravity

1. Calculate the force of gravity in a 1kg mass at the Earth’s surface. The mass of the Earth is 6 x 1024 kg and it’s radius is 6.4 x 106 m.

2. Calculate the force of gravity on the same 1kg mass if it were 6.4 x 106 m above the Earth’s surface. (two of Earth’s radii from the Earth’s center)

3. Calculate the force of gravity between Earth and the moon (mass = 7.4 x 1022 kg). (average Earth – moon distance is 3.8 x 108m)

4. How about the Earth and the Sun? (Sun’s mass = 2.0 x 1030 kg and average Earth Sun distance = 1.5 x 1011 m )

5. Calculate the force of Gravity between a newborn baby (mass= 3kg) and the planet Mars (mass = 6.4 x 1023 kg ) when Mars is at it’s closest distance to the Earth. (5.6 x 1010 m )

6. Calculate the force of gravity between the newborn baby and the obstetrician with a mass of 100kg when he/she is .5m from the baby. Who exerts more gravitational force on the baby, Mars or the doctor? By how much?

Exit Question #204.14a

What happens to the pull of gravity as your distance from the Earth increases?

a. It gets less and eventually goes away

b. It stays the same no matter how far

c. It gets less but never totally goes away

d. It suddenly stops right where the air does

e. As soon as you're off the Earth, there is no gravity

f. It gets weaker and fades where the air ends