Conceptual Physics

1
Conceptual Physics

• Notes on Chapter 12
• Universal
• Gravitation

2
Universal Gravitation

• Isaac Newton and the falling apple ---
• Newton was under a apple tree when a apple hit
  him on the head and he got an idea that changed
  the world.
• What was his GREAT idea? .

3
Universal Gravitation

• Isaac Newton and the falling moon ---
• Newton compared a falling
• apple to the moon in orbit around
• the Earth. He said that the
• moon fell toward the earth as
• fast as it moved in a straight line - tangential
  velocity.

4
Universal Gravitation

• Newton generalized
• his moon findings to
• ALL objects, and said that ALL objects
  in the universe attract each other.
• (Law of Universal Gravitation)

5
Universal Gravitation

• NEWTONS LAW OF UNIVERSAL GRAVITATION
• Every object attracts every other object with a
  force directly proportional to the mass of each
  object, but decreases as the square of the
  distance between masses.
• F m1m2
• d2

6
Universal Gravitation

• We want the force to equal, so we have to add a
  constant to the equation.
• G 6.67 x 10-11 kg
• (Note that the force of gravity is very weak)
• Henry Cavendish measured G 150 years after
  Newtons Law listed above and was said to have
  WEIGHED the world!

7
Universal Gravitation
8
Universal Gravitation

• So the equation becomes
• F G m1m2
• d2

9
Universal Gravitation

• Inverse-square Law
• The brightness of an object decreases
  dramatically as we move farther from the source.
  The result is due to a larger "sphere" of
  influence. Light from a source like a star shines
  in all directions, to the area of illumination
  increases with increasing distance so the total
  brightness thins out.

10
Universal Gravitation
11
Universal Gravitation

• EXAMPLES
• 32 9 but 1/32 1/9
• 2d2 4d but 1/2d2 1/4d
• (Note that as the distance gets LARGER, the
  overall force gets smaller)