Newtons Law of Gravity

1
Newtons Law of Gravity

• One of the four fundamental forces in nature
• Gravitational
• Electromagnetic
• Weak Nuclear
• Strong Nuclear

2

• Force is proportional to the mass of the object
• Force is symmetric between objects
• Force is inversely proportional to the square of
  the distance between the objects
• Force acts on a line between the two objects

• G is Newtons Gravitational Constant
• G 6.67 x 10-11 N-m2/kg2

3
Orbits

• Two objects orbit each other if they have angular
  momentum
• One object is pulled towards the other, but also
  moves perpendicular to the pull
• Orbiting occurs when second objects surface does
  not intersect the curve of the first objects
  motion

4

• All orbits are about common center of mass
• All orbits are elliptical
• Center of mass is at one of the foci

5
Keplers Laws

• All planets move in elliptical orbits with the
  Sun at one of the focal points.
• A line drawn from the Sun to any planet sweeps
  out equal areas in equal time intervals.
• The square of the orbital period of any planet is
  proportional to the cube of the average distance
  from the planet to the Sun.

6

• Can derive Keplers 3rd law from Newtons laws of
  motion

• Can use orbital motion to determine mass inside
  orbit

7
Tides

• Force decreases with distance
• Stronger on side facing the Moon
• Weaker on side away from the Moon

8

• Motion viewed as relative to center of Earth
• Near side pulls away towards Moon
• Far side pulls away from Moon

9
Electricity

• Electricity is part of another fundamental force
  electromagnetism
• Two aspects
• Electrostatics
• Electrodynamics
• All matter is in one of three electrical states
• Positively charged
• Negatively charged
• Not charged

10
Maxwells Equations

• Will see that all electric and magnetic
  interactions can be described by Maxwells
  Equations

11
Electrostatics

• Charges can exert a force even if they are not
  moving
• Strength of force depends on
• Strength of charge exerting the force
• Strength of charge being acted upon
• Distance between the two charges
• Relative sign between the two charges
• Acts on a direct line between charges