Transcript Earth`s Rotation and Effects

Lunar Tides:
Why on both sides of the Earth?
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
Spring and Neap Tides
Tides
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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 ~ 0.0015
sec/century
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
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
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
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
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
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
1
Relativistic
Jet “From” Black
Hole
1. “Relativistic velocities are close to the speed of light