Post on 29-Jan-2016
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=4.9m/s2
a=0m/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”
F GmM
r2
F mg g F
m
F Gm1m2
d2
Weight on Earth
m=object’s mass on EarthM=Earth’s mass
r=Earth’s radius
g F
m
Let’s revisit good old “g”
g G
mM
r2
m
g GM
r2
The value of “g”
g GM
r2
g (6.67 10 11)(5.98 1024 kg)
(6.37 106m)2
g 9.8m/s2How can we figure out the
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?
g (6.67 10 11)(7.36 1022kg)
17370002
g 1.63m/s2
g GM
r2
d 1
2gt 2
t 2d
g
2(2m)
1.63m/s2 1.57s
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??
g (6.67 10 11)(6 1030kg)
60002
g 1.111013m/s2
g GM
r2
weight
F mg
F 65kg(1.111013m/s2)
F 7.22 1014 N
162,400,000,000,000 pounds!!!!
OCEAN TIDES
• What causes them?– Give your best explanation on a whiteboard.
High TideHigh Tide
Low Tide
Low Tide
Why are there two per day? Because the Earth spins under this bulge!
OCEAN TIDES
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
FusionGravity
There is an ongoing battle between thermonuclear fusion and gravity…
Watch out if gravity wins!
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?