Download Today`s topic: Gravity…

Today’s topic: Gravity…
Reminder: midterm on Wed Oct 8.
I will do a review session second half of today’s class
Midterm will cover everything through last week.
Remember to bring you star wheel for the midterm!
Next Wed Oct 15: Kitt Peak trip. Leave at 4 pm, return about 10 pm;
TOCC van will continue to Sells. Let me know if you have
transportation questions, problems!
Last week: how stars form… gravity was mentioned a lot
Gravity pulls inward, gas pressure pushes outward
•  Gravity is responsible
for the different sizes of
stars
Mathematical calculations show how different masses interact
Calculation of forces
new positions
but how does gravity affect our daily life? What is the “law of gravity”?
First, a few questions:
Besides the force of gravity, which of the following
hold us to Earth’s surface?
a. Air pressure from Earth’s atmosphere.
b. Forces from Earth’s spinning motion.
c. Magnetism from Earth’s magnetic field.
d. More than one of the above factors.
e. No other significant factors; only gravity.
Which of the following would make you weigh half
as much as you do right now?
a. Take away half of Earth’s atmosphere.
b. Take away half of Earth’s mass.
c. Take away half of Earth’s magnetic field.
d. Make Earth spin half as fast.
e. Move Earth twice as far from the Sun.
An astronaut standing on the Moon’s surface has a pen in his
hand and releases it. What happens to the pen?
a. It falls to the surface at a slower rate than it would on Earth.
b. It falls to the surface at a faster rate than it would on Earth.
c. It doesn’t fall : floats where it is.
d. It doesn’t fall and slowly drifts away.
Why do things fall to the ground?
Why does it do more damage to drop something from a
greater height than a smaller height?
If you drop a heavy object, and a lighter object, does the
heavier object fall faster than the lighter object?
Sir Isaac Newton,
1642-1727
Why does an apple fall? What holds you to the
surface of the earth? (your weight) What keeps the
moon in orbit around the earth?
From observations of falling objects, Newton proposed a force between any
two objects described by the law of gravity. The force between two bodies is
directed along a line joining them:
• The force increases directly with the mass of either body
• It decreases with the square of the distance between the two bodies
This force is what makes us weigh what we do.
Let’s represent gravity as an equation
G is a constant number, depending on the units you are using
Examples:
1. Suppose mass Mb doubles. What happens to the force?
2. Suppose distance, r, doubles. What happens to the force?
(which is larger? 1/2, 1/3, 1/4 )
Do the astronauts in the space station
feel gravity?
Do the astronauts in space station feel
gravity? Let’s explore this.
What is the diameter of the
Earth?
How far above the Earth does
the space shuttle orbit?
The space shuttle, and the
astronauts inside, are falling
around the earth!
…And the moon is
falling around the
earth
And the earth-moon
system is…
How far from a body (earth, sun, etc)
does gravity exist?
Suppose an astronaut lands on the
moon. What happens to the force
between the astronaut and the
moon- what we call his weight?
A calculation shows that
the astronaut’s weight
(the force pulling him
down) is only 0.17 what it
was on earth.
Another effect of law of gravity:
Bodies in orbit follow a relation (an equation) between their
masses, their period (“year”) and the distance between them.
If you know two of these quantities, you can find the third:
let P = period, R= distance between two bodies, and M be the
mass of each: P2 (Ma + Mb) = R3
P is period of moon around
earth
R is distance to moon
Then we can find the mass of
earth-moon system!
Another example: From APOD, last week, Albireo, a double star
We can find the masses of the two stars:
- We watch the stars revolve
around each other to get
their period, P:
- We measure the angle
between them, (and if we
know the distance) –Then
we can calculate the mass of
the two stars from
P2 (Ma + Mb) = R3
And a final example, using this equation:
P2 (Ma + Mb) = R3
We can calculate how fast the sun is
moving around the Milky Way
galaxy: its galactic period, P
We can calculate the distance to the
center of our galaxy , R
…and then we can solve for the mass
of our galaxy: how many stars in
the galaxy !
So, try these again:
Besides the force of gravity, which of the following
hold us to Earth’s surface?
a. Air pressure from Earth’s atmosphere.
b. Forces from Earth’s spinning motion.
c. Magnetism from Earth’s magnetic field.
d. More than one of the above factors.
e. No other significant factors; only gravity.
Which of the following would make you weigh half
as much as you do right now?
a. Take away half of Earth’s atmosphere.
b. Take away half of Earth’s mass.
c. Take away half of Earth’s magnetic field.
d. Make Earth spin half as fast.
e. Move Earth twice as far from the Sun.