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….

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Mastering Astronomy:

Study area: Ch 4

Interactive Fig. 4.18

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?

© 2005 Pearson Education Inc., publishing as Addison-Wesley

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

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

• 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)

Spring and Neap Tides

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

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Why does the Moon always show the same face to Earth?

Moon rotates in the same amount of time that it orbits… But why?

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)

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Temperature Scales