Newton, Einstein and Gravity

1
Newton, Einstein and Gravity

• Chapter 5
• Do not read section 5-3

2
Gravity is an Acceleration

• Does not depend on mass.
• Earths gravity at sea level g 9.81 m/s2
• Moons gravity at the surface g 1.6 m/s2
• Jupiters gravity at cloud level g 24.5m/s2
• Mars gravity at the surface g 3.63 m/s2

3
Momentum and Force

• Linear Momentum a quantity of motion p mv
  mass x velocity
• Ex. throwing a baseball vs. throwing a bowling
  ball at same speed.
• Bowling ball has more momentum at the same speed
• Force how momentum changes F ma
  mass x acceleration
• Ex. accelerating an empty storage box vs.
  accelerating a full storage box
• Empty box takes less force at the same
  acceleration

4
Mass and Weight

• They are not the same thing.
• Mass is a property of an object, no matter where
  in the universe it is located. Mass is how much
  stuff?
• Weight is dependent upon the local gravity (g).
  The Moons gravity is 6 times less on than
  Earths.
• Weight mg mass x gravity
• Notice that this is a force (Fma), where a g.

5
Speed, Velocity, Acceleration

• Speed how fast your location changes
• Velocity your speed in a certain direction
• Acceleration how fast velocity changes
• speed or direction can change
• You can feel acceleration, but not velocity

6
Newtons Laws of Motion

• 1st Law An object moves with a constant velocity
  in the absence of a net force.
• 2nd Law In the presence of a net force, the
  force is how fast your momentum changes, or force
  mass x acceleration.
• F ma
• 3rd Law For any force there is an equal and
  oppositely directed reactionary force.

7
Examples
8
Gravitation

• Newtons Law of Universal Gravitation Masses
  attract each other with a force that is directly
  proportional to their masses and inversely
  proportional to their separation distance.

9
Orbits and Escape Velocity

• Orbiting the Earth is NOT getting away from
  gravity - it is a constant freefall.
• If you can go fast enough, you can escape the
  Earths gravitational influence.

10
Freefall and Weightlessness

• Freefall is when your acceleration is solely due
  to gravity.
• Exs. Jump from an airplane. Stand in an elevator
  whose cable has been cut.
• This gives the appearance of having no weight. A
  scale would read zero.

11
Angular Momentum

• Momentum in circular motion.
• Includes rotations (spins) and revolutions
  (orbits)
• L mvr mass x velocity x radius of the circle

12
Newton and Kepler

• Newtons form of Keplers Third Law Is more
  general than K3, and applies in any situation
  where two masses orbit each other.
• Ex. Two stars, two galaxies, a star and its
  galactic center, a star and planet, a planet and
  a moon, etc.