Transcript Cooneyclass914HC_JCx

9/14/12
• Jim out today – e-mail if needed.
• Remember – exam on Monday.
• Extra office hour Monday at 10am for last
minute questions
Gravity
• Take a look around you….in our everyday
experience we see gravity acting on all objects.
• Newton’s great
insight – the force
that causes an apple
to drop near the
surface of the Earth
and the force that
causes the Moon to
circle the Earth are
one and the same!
Universal Law of Gravitation
Between every two objects there is an attractive
force, the magnitude of which is directly
proportional to the mass of each object and
inversely proportional to the square of the
distance between the centers of the objects.
• Question : Why do astronauts in the space
station feel weightless?
Remember: We live on a sphere
• When you drop
something it heads
toward the center of
the Earth
• IF you put a bit of
horizontal velocity
on that something….
Mastering
Astronomy:
Study area: Ch 4
Interactive Fig.
4.18
© 2005 Pearson Education Inc.,
publishing as Addison-Wesley
Weight
• W=mg
• g=acceleration due to gravity at Earth’s
surface = 9.8 m/s/s
• How does your weight change if you are on a
different planet? If you are in free-fall?
How is mass different from weight?
• mass – the amount of matter in an object
• weight – the force that acts upon an object
You are weightless
in free-fall!
© 2005 Pearson Education Inc.,
publishing as Addison-Wesley
• How is mass different from
weight?
• Mass = quantity of matter
• Weight = force acting on mass
• Objects are weightless when in free-fall
© 2005 Pearson Education Inc.,
publishing as Addison-Wesley
Tides
• Gravitational force decreases with (distance)2
– The Moon’s pull on Earth is strongest on the side facing the Moon,
and weakest on the opposite side.
• The Earth gets stretched along the Earth-Moon line.
• The oceans rise relative to land at these points.
Tides
• Every place on Earth passes through
high tides twice per day as the Earth
rotates.
• High tides occur every 12 hours
25minutes
– remember, the Moon moves!
• The Sun’s tidal effect on Earth is not as
strong.
– the ratio Earth’s diameter : distance to
Sun is much less than ratio Earth’s
diameter : distance to Moon
Spring and Neap Tides
• When the Sun & Moon
pull in the same
direction (new & full
phases)
–
high tide is higher than usual (spring)
• When the Sun & Moon
pull at right angles (first
& last quarter phases)
–
high tide is lower than usual (neap)
Tidal Friction
• This fight between Moon’s pull & Earth’s rotation
causes friction.
• Earth’s rotation slows down (1 sec every 50,000
yrs.)
• The Moon moves farther away from Earth.
Tidal Friction
– It adds up! 4 billion years
ago 1 day may have only
been 5 or 6 hours long.
– The moon may have been
1/10 the distance: 22 Earth
radii away instead of 221
Why does the Moon always show the same face to Earth?
Moon rotates in the same amount of time that it orbits…
But why?
© 2005 Pearson Education Inc.,
publishing as Addison-Wesley
Synchronous
Rotation
• …is when the rotation period of a
moon, planet, or star equals its
orbital period about another object.
• Tidal friction on the Moon (caused
by Earth) has slowed its rotation
down to a period of one month.
• The Moon now rotates
synchronously.
– We always see the same side of the
Moon.
• Tidal friction on the Moon has
ceased since its tidal bulges are
always aligned with Earth.
Conservation Laws
• Momentum = mv
• Angular Momentum = r x mv
• Energy
– Holy cow, this is a very fine law.
– Energy comes in many forms.
– Even when Newton’s Laws fail, conservation of
energy holds.
Types of Energy
• Kinetic
– The energy of movement.
• Potential
– Gravitational
– Chemical
– Nuclear
• Radiative
– Light
An Aside on Temperature
• Temperature measures the average kinetic
energy of the particles.
• Temperature scales
– Fahrenheit (bad)
– Celsius (better)
– Kelvin (best)
Temperature Scales
© 2005 Pearson Education Inc.,
publishing as Addison-Wesley