Gravity

1
Gravity

• Dr. Don Franceschetti
• Physics 1010
• February 25, 2005

2
Aristotle

• Heavy objects (predominantly earth and water)
  seek to come to rest on earth surface.
• Earth Center of Universe
• Celestial objects execute orbits around center of
  Earth

3
Newton realized

• Every mass in the universe attracts every other
  with a force given by

4
Law of Universal Gravitation

• Everything attracts everything else.
• Earth not special.
• An inverse square force. (Double distance, force
  reduced to ¼)

5
Determining big G

• Use balanced masses and bring large mass near.
• Done by Cavendish and Jolly.
• G6.67x10-11 Nm2/kg2

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Comparing G and g

• MgGMME/RE2
• ME g RE2 /G
• And so we now know the mass of the Earth! About
  6x1024kg

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And we know a lot more

• Mass of sun from radius of orbit and length of
  year.
• Mass of planets that have moons.
• Locate undiscovered planets Uranus and Pluto
  through gravitational effects

8
Weight and weightlessness

• You feel weightless in free fall.
• Force of gravity on the space shuttle in orbit is
  about as much as weight of shuttle on earth
  surface.
• Satellites fall around the Earth.

9
Tides

• Tides occur because force of gravity decreases
  with distance.
• There are tides in the atmosphere and in the
  moon, too.

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Homework due 3/4

• RQ 8.9 How does the force of gravity between two
  bodies change when the distance between them is
  doubled?
• RQ 8.17 Give an example of when your (apparent)
  weight is more than mg.

11
Homework due 3/4

• 9E2 Gravitational forces act on all bodies in
  proportion to their masses. Why then doesnt a
  heavy body fall faster than a light body?
• 9E21 Is gravitational force acting on a person
  who falls off a cliff? On an astronaught inside
  the orbiting space shuttle?