Lec 7 Flashcards
(19 cards)
Q: When, in the highly eccentric orbit of a comet, is the speed of the comet the greatest?
A: When the comet is closest to the sun
Q: When, in the highly eccentric orbit of a comet, is the angular momentum of the comet the greatest?
A: It’s always the same, because angular momentum is conserved
Q: When, in the highly eccentric orbit of a comet, is the force of gravity between the comet and the Sun the greatest?
A: (remember the eqn- bigger distance=smaller force) When the comet is closest to sun
Q: When, in the highly eccentric orbit of a comet, is the comet’s acceleration the greatest?
A: When the comet is closest from the sun (speed changing the least and direction is changing)
Newtons 1st law
1) An object in motion remains in motion unless acted upon by an outside force
Expresses the principle of inertia (moves straight line at constant speed)
newtons 2nd law
2) Something changes speed if force is applied to it, heavier=less quickly it accelerates (A=F/m)
Acceleration is directly proportional to force and inversely proportional to mass (more force=more A and more mass=less A)
newtons 3rd law
3) For every force there is always an equal and opposite reaction force
Momentum (p=mv, more velocity=more momentum, in a closed system=conserved)
freefall
When the only force acting on an object is gravity, you’re in freefall
tides
The varying gravitational force squeezes both the earth and the moon
Oceans respond to the tidal forces from the moon
There are 2 high tides and 2 low tides per day
tidal locking
Friction with the rotating earth causes tidal bulge(on both sides) to lag behind (lags bc earth spins)
This lag applies a force on the earth, causing its rotation to slow down
The tidal force on the moon by the earth is much larger than the tidal force of the moon on the earth
So the moon has already stopped relative to the earth
It’s tidally locked
spring tide
In a full moon and a new moon, the tidal forces from the moon and the earth add
Called spring tide (large tides in full moon)
neap tide
In a quarter moon, the tidal forces from the sun partially cancel the tidal forces from the moon
Called neap tide (small tides in full moon)
Explain why there are two high tides and two low tides within 24 hours at any given location on Earth
There are two high tides and two low tides each day at most locations on Earth due to the gravitational interaction between Earth and the Moon
Why There Are Two High and Two Low Tides Every 24 Hours
- Gravitational Pull of the Moon
The Moon’s gravity pulls strongest on the side of Earth facing it, drawing the ocean water into a bulge—this creates the first high tide.
On the opposite side of Earth, the Moon’s gravitational pull is weaker. Here, Earth is pulled slightly more than the water is, causing the ocean to “lag behind” and form a second bulge—this is the second high tide - Low Tides Between the Bulges
In areas between these two bulges, the ocean level is lower, resulting in low tides. - Earth’s Rotation
As Earth rotates once every 24 hours, any given coastal location passes through:
the bulge facing the Moon (first high tide),
then a low tide,
then the bulge on the far side (second high tide),
and another low tide.
This cycle repeats approximately every 24 hours, giving two high tides and two low tides per day.
Summary: The Tidal Cycle
Tides are caused by the difference in gravitational force across Earth.
The Moon is the primary cause; the Sun also contributes, but its effect is smaller than the Moon’s.
Two bulges form: one on the side facing the Moon and one on the opposite side.
Earth’s daily rotation causes locations to move through both bulges and both low points—producing two high and two low tides every day.
Correctly describe how tides arise from differences in the force of gravity between two points
Tides arise due to differences in the force of gravity exerted by the Moon across Earth’s diameter—not because of the Moon’s absolute gravitational pull alone
How Tides Arise from Differences in Gravity
1. The Moon’s Gravitational Pull Is Stronger on the Near Side
The side of Earth closest to the Moon experiences a stronger gravitational pull, so water is pulled more toward the Moon, forming a tidal bulge—this is high tide on the near side.
2. The Moon’s Pull Is Weaker on the Far Side
On the far side of Earth, the Moon’s gravitational pull is weaker. Earth itself is pulled slightly more than the water on the far side. This causes the water to “lag behind”, forming a second tidal bulge—which is another high tide on the far side.
3. Low Tides Form Between the Bulges
In the areas between the two bulges, water is “squeezed out,” leading to low tides.
Why Two High Tides per Day?
As Earth rotates, any given coastal location passes through:
One bulge (first high tide),
Then a region of low water (low tide),
Then the opposite bulge (second high tide),
Then a second low tide.
This creates two high tides and two low tides roughly every 24 hours at most locations.
how do tides result
Tides result from the difference in gravitational force (called the tidal force) that the Moon exerts on different parts of Earth
This stretching effect causes Earth’s oceans to bulge on both the side facing the Moon and the side facing away, leading to two high tides and two low tides each day
Explain why we can never see the far side of the Moon from Earth
We can never see the far side of the Moon from Earth because of a phenomenon called synchronous rotation, which is the result of tidal locking
Synchronous Rotation (Tidal Locking)
The Moon rotates on its axis in the same amount of time that it takes to orbit Earth: approximately 27.3 days.
Because of this perfect match, the same hemisphere of the Moon always faces Earth, and the far side remains hidden.
How This Happened
In the past, the Moon likely rotated at a different rate.
Over time, Earth’s gravitational pull created tidal bulges on the Moon.
The gravitational interaction between these bulges and Earth created a torque that gradually slowed the Moon’s rotation.
Eventually, the rotation slowed enough to match the Moon’s orbital period—a stable configuration known as tidal locking.
Why It Matters
From Earth, we always see the near side of the Moon.
The far side, sometimes inaccurately called the “dark side,” is not always dark—it experiences day and night just like the near side.
It’s simply never visible from Earth because of this gravitational synchronization.
Summary
The Moon shows only one face to Earth because its rotation period is exactly equal to its orbital period, a result of gravitational tidal forces over billions of years. This is called synchronous rotation, and it ensures the far side of the Moon always faces away from us
tidal locking
Tidal locking is a phenomenon in which an object’s rotational period matches its orbital period, causing it to always show the same face to the object it is orbiting
Why Tidal Locking Happens
Tidal locking occurs because of gravitational tidal forces between two bodies (e.g., Earth and the Moon):
Gravitational pull varies across the object:
The side of the Moon facing Earth feels a stronger gravitational force than the far side.
This difference creates tidal bulges—stretching the Moon slightly along the Earth-facing axis.
Tidal friction slows rotation:
As the Moon rotated, Earth’s gravity pulled on the bulges, creating a torque.
This gradually slowed the Moon’s rotation until it synchronized with its orbital period.
Stable configuration:
Once the Moon’s rotation matched its orbital period, the bulges no longer shift.
The system becomes tidally locked and remains stable unless disturbed by external forces.
Consequences of Tidal Locking
The Moon always shows the same hemisphere to Earth—we never see the far side without spacecraft.
Tidal locking is common in the solar system—many moons are tidally locked to their planets.
Summary
Tidal locking is when an object’s rotation period equals its orbital period, due to gravitational interactions and tidal friction. This causes one side of the object to always face the body it orbits, as in the case of the Moon and Earth
