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G r a v i t y Chapter 9 Isaac Newton (1642 - 1727) m M d Newton’s Universal Gravitation Two masses separated by a distance Newton’s Universal Gravitation Newton’s Universal Gravitation Inverse Square Law Separation Force
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G r a v i t y
Chapter 9
Newton’s Universal Gravitation
The force of gravity cannot be made zero.
- Mass causes gravity
- Only one kind of mass
- Contrast with the electric force
- G is small
- 6.67 X 10-11 N m2/kg2
Apparent Weightlessness
- All objects fall at the same rate in a gravitational field.
- Apparent weightlessness
- Lack supporting force
- Examples:
- Space Shuttle
- Elevator
- Amusement Park Rides
Ocean Tides
- Differences in gravity on different parts of the Earth produce tidal bulges
Low Tide
High Tide
High Tide
Low Tide
Ocean Tides
- Should expect two high tides, two low tides per day
- Moon moves in its orbit so that it returns to the same place in your sky every 24h 50 m
- Sun has one-half the influence of the moon
- The gravitational force is greater but tides depend on the gravitation difference on opposite sides of the Earth
Ocean Tides
- Complications
- Friction with ocean bottom
- Irregular shape of continents
- Bay of Fundy (Video)
- Tides can come in very fast and exceed 15 m
Crustal & Atmospheric Tides
- Tides in ionosphere produce electric currents and alter the magnetic field
- Affects penetration of cosmic rays
- Can have effects on living things
- Tides in the crust mean that volcanoes and earthquakes are more common at new or full moon
- Measured for the moon
Gravitational Field
- Alternate view of gravitational influence
- Properties of space surrounding object are changed so that another mass entering the region experiences a force.
Gravitational Field
- Objects accelerate in the direction of the arrows
- Field is stronger where field lines are closer together
Distance
g
Distance
Gravitational Field- Uniform Sphere
- Linear inside
- like Earth outside
- Hollow Sphere
- Zero inside
- like Earth outside
R
R
General Relativity
- Mass distorts space-time
- Objects move in the straightest path they can in the distortions caused by mass
Low Gravity
Very small amount of bending
Stronger Gravity
Light at an angle is bent noticeably
Exit Cone and Photon Sphere
Photon Sphere
What Can We Know?
- Mass
- gravity
- Charge
- Electric Fields
- Rotation Rate
- Co-rotation
Distance
Raisin Cake ModelVelocity
1 cm/hr
2 cm/hr
3 cm/hr
Distance Distance
from Home from Home
(before baking) (afterbaking)
1 1 cm 2 cm
2 2 cm 4 cm
3 3 cm 6 cm
Universal Expansion
- Space-Time itself is expanding
- Carries galaxies along with it
- Conflicting cosmologies
- Big Bang
- Draw the expansion back in time until space-time occupies a point (12 - 15 billion years ago)
- Steady State
- Requires continual creation of matter
