Planetary Motions

1
Planetary Motions

• Newtons Laws
• The New Physics

2
Newtons Laws

• LAW 1 A body is in natural motion when it is at
  rest or when moving along at a constant speed
  along a straight line
• Definition of natural motion

3
Newtons Laws

• LAW 2 A net force applied to a mass results in
  an acceleration
• OR
• Acceleration is directly proportional to the net
  force and inversely proportional to the mass

4
Newtons Laws

• Speed is the rate of motion (km/s)
• Velocity is speed with direction
• Acceleration is a change in velocity (speed,
  direction, or both!)
• Accelerations result from net forces (add them
  all!)

5
Newtons Laws

• Circular motion is accelerated motion (even if
  speed constant)
• Why? Direction of motion is constantly changing!
• Planetary motions are accelerated implies a force

6
Newtons Laws

• LAW 3 Every force involves another equal but
  opposite force
• OR
• Forces act in simultaneous pairs

7
Falling Bodies

• Observation All masses near surface of earth
  fall with same acceleration
• Inference Gravity acts so that the greater mass
  is pulled by a greater force

8
Gravitation

• Interaction at a distance between masses (any
  size, shape)
• More mass gt more force (directly proportional
  to mass)
• Greater distance gt less force(inversely
  proportional to square of distance inverse
  square law)

9
Newtons Law of Gravitation

• Force is proportional to (M1 x M2)/Distance2
• Outside of a body, acts as if all mass is
  concentrated at center
• Weight is a force!
• Predicts escape speed of one object leaving
  another (space travel!)

10
Newtons Law of Gravitation

• Results in Keplers laws (new version of 3rd
  masses!)
• Predicts/explains motions of planets with great
  precision
• Physical basis of heliocentric model (earths
  rotation, revolution)
• Powerful theory of gravitation

11
Newtons Law of Gravitation

• Predicts interactions of masses even outside
  solar system a universal physical law!
• Erased separation of earth and sky
• Newton thought cosmos infinite otherwise all
  masses would have already collapsed together