Transcript Gravitational Interactions

Gravitational Interactions
Newton Again
• Sir Isaac Newton did not discover gravity.
– He discovered that gravity is universal
– All matter attracts all other matter.
Can you name a few ways gravity
affects things on our planet??
Have you ever heard of a force field?
• A force field surrounds things with mass. We
call it a gravitational field.
Remember this picture…
What is your acceleration as you fall?
a=9.8m/s2
a=4.9m/s2
a=0m/s2
a=4.9m/s2
a=9.8m/s2
What would your weight be if you were in a cavity
at the center of the Earth?
Your
weight
would
be zero!
Imagine a journey to the center of the Earth!
What motion would you experience?
Weight & Weightlessness
• What would a scale read (higher or lower than
normal) if your were on an elevator:
– Accelerating up?
• Higher!
– Accelerating down?
• Lower!
– In freefall after the cable breaks?
• Zero!
Weight & Weightlessness
• Weightlessness is not the absence of gravity. It
is the absence of a support force.
Why do astronauts feel weightless?
Why do astronauts feel weightless?
• If you had a ladder that could reach as high as a
shuttle's orbit, your weight would be 11% less at the
top.
• A person who weighs 100 pounds on the Earth's
surface would weigh about 89 pounds at the top of
the ladder.
• The astronaut, the spaceship and everything inside it
are falling towards the Earth.
• They are always falling towards the Earth, but they
never get there.
Let’s revisit good old “g”
Weight on Earth
m=object’s mass on Earth

r=Earth’s radius

F
F  mg  g 
m
m1m2
FG 2
d
mM
FG 2
r
M=Earth’s mass
Let’s revisit good old “g”
F
g
m

GM
g 2
r
mM
G 2
r
g
m
The value of “g”
GM
g 2
r
11
24
(6.67 10 )(5.98 10 kg)
g
6
2
(6.37 10 m)
g  9.8m/s How can we figure out the
2
acceleration due to gravity on
any other planet?
The moon’s radius is 1737000m and its mass is
7.36x1022kg. How long would it take for a rock to
reach the ground if dropped from a height of 2m?
GM
g 2
r
1 2
d  gt
2
(6.67 1011)(7.36 10 22 kg)
g
2
1737000
 2
g  1.63m/s
2d
2(2m)
t

1.57s
2
g
1.63m/s
A neutron star has a radius of only 6000m, but its
mass is 6x1030kg! How much would you weigh if
you were able to stand on its surface??
A neutron star has a radius of 6000m and its mass
is 6x1030kg. How much would you weigh if you
were able to stand on its surface??
GM
g 2
r
11
(6.67 10 )(6 10 kg)
g
2
6000
g162,400,000,000,000
 1.111013 m/s2
pounds!!!!
weight
F  mg
F  65kg(1.1110
m/s )

13
F  7.22 10 N
14
2
30
OCEAN TIDES
• What causes them?
– Give your best explanation on a whiteboard.
OCEAN TIDES
Low Tide
High Tide
High Tide
Low Tide
Why are there two per day?
Because the Earth spins
under this bulge!
Largest and smallest tides
• Neap tide- sun is at 90 degrees relative to
Earth and moon, tide is smallest
• Spring tide- Sun, Moon, and Earth are Aligned,
tide is largest.
Do you know what big is?
• Astronomical Scale
Our Galaxy
There is an ongoing
battle between
thermonuclear fusion
and gravity…
Gravity
Watch out if gravity
wins!
Fusion
Red Giant- the swelling of a star
Supernova
• The explosion
of a star
Supernova Remnant
• White Dwarf
Star
• More mass
stars end up
as Black Holes
Black Holes
Black Holes
Same gravitational
field strength
because neither
mass nor distance
changes.
Black Holes:
•
•
•
•
Alters the space around it
Enormously dense
Very strong gravitational field
Escape velocity is greater than the speed of
light hence…BLACK hole
How do you think black holes might be discovered?