Transcript Inverse Square Law - Hutto High School

This week in Physics
• 3/02 Monday – Universal Gravitation Lecture (notes)
• 3/03 Tuesday – Computer Lab 9-121
• 3/04 Wednesday – Universal Gravitation Lecture (as
needed) Worksheet
• 3/05 Thursday – Universal Gravitation
• 3/06 Friday – Individual Review – Review Sheets
• Progress Reports due
• 3-09 Monday – Class Review
• 3/10 Tuesday – Rotational Motion, Torque & Universal
Gravitation Exam
MOST COMMON QUIZ MISTAKES
-2.5 Incorrect or no Unit!
Frequency = 1/sec =Hertz
Period = sec/1 = seconds
Velocity = m/s
Acceleration = m/s2
Force = N
Torque = N-m
Using t (time)
instead of
T (period)in formula
for
Velocityc = 2πr
T
USE WTF!!!!
set your problem up,
then plug in numbers
March 2: Pioneer 10
• Launched In 1972
• It passed close by Jupiter and Neptune
• 1st man-made object to leave the Solar System and travel
on a flight path into interstellar space.
• Heading in the direction of the red star Aldeberan.
• That's the main star seen as the eye of The Bull in the
constellation Taurus in Earth's night sky.
• Aldebaran is about 68 light years away.
• Pioneer 10 should arrive at Aldeberan in 2 million
years.
Where is Pioneer 10 today?
Law of Universal
Gravitation
Newton's Discovery
Newton explains gravity
• Remember• Law of Inertia: An object in motion stays in
motion unless acted upon by an outside force
• Also applied to objects traveling in a circular
motion
• So… His breakthrough was to explain how the
same rules apply to little things like apples and
big things like the moon.
Gravitational Force
• the mutual force of attraction between particles
of matter.
• always exists between two masses, regardless of
what separates them.
• It is between all objects.
• you are pulled to the object, & the object is
pulled to you.
What Newton knew…
• An apple will accelerate at about 10.0 m/s2
downward.
• The moon accelerates toward the Earth at
0.00272 m/s2.
Gravity of Moon =
0.00272 m/s2 = 1
=1
Gravity of Apple
10 m/s2
3600 602
•
The moon is 60 times further from the center of
the earth than an apple on the surface of the
earth.
Gravitational Relationship
Gravity of the apple
10 m/s2
Gravity of the moon
0.00272 m/s2
Radius of Moon to Earth
60 x Radius of Apple to Earth
Fgrav ~ 1/d2
Fgrav represents the force of
gravity between 2 objects
~ means “proportional to”
d = distance separating 2 objects
Newton’s Law of Universal
Gravitation
• Distance and mass, of both objects effect the
force of gravity
• Inverse Square Law (Example)
• As distance increases , Force decreases
m1m2
Fg  G 2
r
Newton’s Law of Universal
Gravitation
• Radius, r, is the distance between the center of gravity
in one object to the center of gravity in the other
object.
• g = 10 m/s2
• On Earth
• G= 6.67 x 10-11 m/s2
• Universal Gravitation Constant*
* Newton calculated the constant mathematically, not
until 1798 when Henry Cavendish built a torsion
balance and was able to observe masses pulling toward
each other.
Newton’s Law of Universal
Gravitation & Tides
• Newton used Universal gravitation to explain the
tides.
• Caused by the gravitational attraction of the Sun
and moon occurring unequally on different parts
of the earth.
• The moon is the main cause of the tides because it
has a stronger gravitational pull, and it is closer to
the Earth than the Sun.
Tides
High & Low tides
• High tides will occur on
the side facing the moon
& the opposite side
(180o)
• Low tides will occur on
the sides that are at a
90o & 270o angle facing
the moon
Spring & Neap Tides
• Spring tides are tides that
are even higher than high
tides caused by the
alignment of the sun &
the moon
• Neap tides are lower than
the normal high tide &
higher than the normal
low tide, not much
change between high &
low in other words.
Practice: Example #1
m1m2
Fg  G 2
r
• Find the force of gravity between a 30 kg girl
and her 10 kg cat if they are 2 meters apart.
Practice: Example #1
m1m2
Fg  G 2
r
• Find the distance between a 0.300 kg billiard
ball and a 0.400 kg billiard ball if the
magnitude of the gravitational force is 8.92 x
10-11 N.