Aim: How can we explain Newton’s Universal Law of Gravitation?

Do Now:

Why does the moon orbit the Earth?

You will know by the end of the lesson

GravityGravityGravity is a universal law.  Gravity is a universal law.  Should behave the same where Should behave the same where

ever you are in the universe. ever you are in the universe. It's a force of attraction that It's a force of attraction that

exists between any two objects exists between any two objects that have mass.  that have mass. 

Gravity: similar to another Gravity: similar to another force of attractionforce of attraction

Love… awwLove… aww

The greater the The greater the mass, the more force mass, the more force of attraction  of attraction 

The closer the The closer the objects, the greater objects, the greater the force of attractionthe force of attraction

m1 & m2 = masses of 2 objects

r = distance between the centers of the 2 objects

What about G?

G is the Gravitational G is the Gravitational ConstantConstant

What does G equal and where can we find it?What does G equal and where can we find it?

How does G compare to g?

For an object on the Earth:

= 9.8 m/s2

Hence, Fg = mg

Problems

What is the force of attraction between two football players (m1 = 80 kg and m2 = 100 kg) that are standing 1 m apart?

Fg = 5.4 x 10-7 N Not much attraction!

What is the force of attraction between the Earth and the Moon?

Fg = 2.0 x 1020 N

Now that’s a big attraction!

So why doesn’t the Moon fall to the Earth?

It is falling!

It also has a velocity vector tangent to the Earth

What is the resultant?

An orbit!

Why Doesn’t the Moon Fall Down

How is Fg proportional to m1 and m2?

They are directly proportional

If one goes up, the other goes up

What happens to the force if:

mass force double m1

triple m2

double m1 and m2

triple m1 and m2

½ m1

½ m1 and m2

2F

3F

4F

9F

½ F

¼ F

The trick:

•Start with easy numbers

•Manipulate

•Solve

How is Fg proportional to r?

They are indirectly proportional

If one goes up, the other goes down

What happens to the force if:distance force double r

triple r

½ r

1/3 r

¼ F

1/9 F

4F

9F

The gravitational force of attraction between two objects is 100 N. What will be the new force if you double m1, quadruple m2, and triple r?

The force will be 8/9 F

Fg = (8/9)(100 N)

Fg = 88.9 N