1. Lunar Tides: Why on both sides of the Earth?

2. Tides: Why do they occur?
Why do high tides occur on both sides of the earth, the side closest to the moon and the side farthest from the moon ?
Newton applied the law of gravitation to understand high and low tides twice each day
Tides are mainly due the differential pull of gravity by the moon across the earth, which varies with distance from the moon

3. Spring and Neap Tides

4. Tides
The side closer to the moon accelerates (tends to move) towards it faster than the farther side, which lags behind. Relative to the center, the earth is stretched in opposite directions
The Moon does NOT “lift” the water up on one side of the Earth closest to it!
High and low tides ~ 12hrs; alternate ~ 6 hrs
Solar tides are about half as strong as lunar tides
When the sun and the moon are in line with the earth (line of nodes), at new and full moon, the tidal effects add up and we have the strongest tides called Spring Tides
When the moon is in quarter positions the effects tend to cancel out and we have the weakest tides, called Neap Tides
Tidal forces slow down Earth’s rotation by ~ sec/century

6. Earth’s Rotation
The Sun sets in the “West” because the Earth rotates “East”, counterclockwise
How do we know the Earth rotates?
Foucault’s Pendulum (COSI)
Experiment: The pendulum swings independent of the motion of supporting platform
Foucault’s experiment with the long pendulum suspended from the dome of the Pantheon in Paris  traced a full circle on the floor in one day
The period depends on the latitude; 24 hrs on the north pole but 34 hrs at 45 degrees N

7. Effects of the Earth’s Rotation
Movement along a meridian (N-S) is affected, like trying to walk straight on the rotating platform of a carousel  experience a sideways force
Coriolis Force – due to linear motion relative to rotational motion
Winds blowing north from the Equator veer rightward as they move closer to the Earth’s axis at the North Pole
At the Equator the eastward velocity is 1700 Km/hr; at the poles it is 0
As they move north, winds circle and spiral around low-pressure areas  Cyclones

9. Coriolis Force
Object moving north from the equator moves rapidly eastward (right) due to:
1. An eastward velocity which gets higher relative to the Earth’s surface as it moves northward
2. Gets closer to the Earth’s axis; therefore the rotational speed increases, like that of an ice skater pulling arms inward
Conservation of angular momentum:
L = m v r (mass x velocity x radius)
remains constant  if r decreases then v must increase

10. Angular Momentum
Conservation of angular momentum says that product of radius r and momentum mv must be constant  radius times rotation rate (number of rotations per second) is constant

11. Angular Momentum All rotating objects have angular momentum
L = mvr ; acts perpendicular to the plane of rotation
Examples: helicopter rotor, ice skater, spinning top or wheel (experiment)
Gyroscope (to stabilize spacecrafts) is basically a spinning wheel whose axis maintains its direction; slow precession like the Earth’s axis along the Circle of Precession

12. Conservation of Angular Momentum
Very important in physical phenomena observed in daily life as well as throughout the Universe. For example,
Varying speeds of planets in elliptical orbits around a star
Jets of extremely high velocity particles, as matter spirals into an accretion disc and falls into a black hole

13. Relativistic1 Jet “From” Black Hole
1. “Relativistic velocities are close to the speed of light