March 28 - Do NOW - - Take your
clickers, and write this down and solve
in your notes:
Action: Earth pulls moon.
- What is the reaction?
- What is stronger – action,
reaction, or same?
- What Law defends your answer?
Action: Earth pulls moon.
What is the reaction?
1.
2.
3.
4.
Sun Pulls Earth
Moon pulls Earth
Sun pulls moon
No reaction
0%
1.
0%
2.
0%
3.
0%
4.
Action: Earth pulls moon.
What is stronger – action, reaction, or
same?
1. Action(Earth pulls
moon)
2. Reaction (moon pulls
Earth)
3. Same!
0%
1.
0%
2.
0%
3.
What law defends your answer?
1. 1st Law
2. 2nd Law
3. 3rd Law
0%
1.
0%
2.
0%
3.
PHET Computer Lab
• 15 minutes to complete the entire packet –
go!
Lab Review
•
•
•
•
Clickers ready!
We are not reviewing individual questions…
Rather, we are reviewing MAIN CONCEPTS
Feel free to take note/ make corrections on
your lab OR write stuff down…but work fast!!!
We are rapidly completing!
Choose the picture you think shows
the gravity forces on the Earth and
A.
the Sun.
B.
C.
D.
E.
80%
20%
0%
0%
0%
How would these gravity forces
change if the Sun got more
massive?
70%
A. Increase
30%
B. Decrease
0%
C. Stay the same
A.
B.
C.
How would these gravity forces
change if the Earth was much
closer to the Sun?
56%
44%
A. Increase
B. Decrease
0%
C. Stay the same
A.
B.
C.
How would these gravity forces
change if the Earth got much less
massive?
78%
A. Increase
B. Decrease
22%
0%
C. Stay the same
A.
B.
C.
How would these gravity forces
change if the Earth and Sun were
moved farther apart?
90%
A. Increase
B. Decrease
10%
0%
C. Stay the same
A.
B.
C.
The force of gravity increases as objects move
closer together.
88%
A. True
B. False
13%
A.
B.
The force of gravity increases as an object’s
mass increases.
63%
A. True
B. False
38%
A.
B.
If two objects have different masses, the more
massive object pulls with a greater force.
67%
A. True
B. False
33%
A.
B.
An object with more mass is able to exert more
gravitational force than an object with a
smaller mass.
60%
40%
A. True
B. False
A.
B.
Based on those last questions…
• Think about PROPORTIONS
between…
– MASS and FORCE
– DISTANCE and FORCE
• Are they directly or inversely
proportional?
What is the relationship between
mass and force?
80%
A. Directly proportional
B. Inversely proportional
20%
A.
B.
What is the relationship between
distance and force?
67%
A. Directly proportional
B. Inversely proportional
33%
A.
B.
Gravitational force is always attractive /
repulsive.
100%
A. Attractive
B. Repulsive
0%
A.
B.
Gravitational force exists everywhere / only in
some places in the universe
75%
A. everywhere
B. only in some places
25%
A.
B.
IF a gravitational force exists between two objects, one
very massive and one less massive, then the force on
the less massive object will be greater than/ equal to /
less than the force on the more massive object.
55%
A. greater than
B. Less than
C. Equal to
27%
18%
A.
B.
C.
As the distance between masses decreases,
force increases / decreases.
91%
A. Increases
B. Decreases
9%
A.
B.
12.3 The Falling Earth
What would happen to the paths of the
planets if GRAVITY was reduced to zero?
Would they…
• Continue to orbit the sun as usual?
• Crash straight into the sun?
• Take the tangent path away from the sun?
Use what you know about forces and circular motion to
DEFEND your answer using scientific reasoning
What would happen if the
paths of the planets if GRAVITY
was reduced to zero?
91%
9%
..
aw
en
tp
ta
ng
th
e
st
ra
Ta
ke
sh
Cr
a
Co
n
tin
ue
to
ig
or
b
ht
i
it
nt
o
th
e
th
e
su
n.
..
su
n
0%
at
h
A. Continue to orbit the
sun as usual
B. Crash straight into
the sun
C. Take the tangent
path away from the
sun?
12.3 The Falling Earth
• What would happen if the paths of the
planets if GRAVITY was reduced to zero?
• Their motion would be TANGENT, away from the
sun and they would stop orbiting it.
• All objects in the solar system have long ago
taken on some kind of rotation or revolution
(result of the Big Bang)…because GRAVITY was
one of the first things to emerge from the Big
Bang, and GRAVITY provides the centripetal
force on planets, moons, etc.!
Choose the picture of how the Earth
would move if you “turned off” the
gravity forces.
A.
B.
C.
D.
E.
100%
0%
0%
0%
0%
Take out your HW…
• #1-7 (Ch 12)
• Checking now with
clickers
• #8-12 (Ch 12)
• Checking later with
peers
#1 –Why did Newton think there must
be a force acting on the moon?
A. It moved in a straight line, not a circle
B. It moved in a circle, not a straight line
C. I had no clue how to answer this question
0%
...
0%
a.
..
in
..
0%
#2 What did Newton conclude about the force
that pulls apples to the ground and the force
that holds the moon orbit?
A. They were different forces
B. They were the same force
C. I had no clue how to answer this question
0%
0%
0%
#3 If the moon falls, why doesn’t it get closer to
Earth?
A. It has a large tangential speed, so it falls around
Earth and not into it
B. It has a small tangential speed, so it falls around
Earth and not into it
C. I had no clue how to answer this question
0%
0%
0%
#4
• What is meant by tangential speed?
– Check with your neighbor
#5
• How did Newton check his hypothesis that
there is an attractive force between Earth and
moon?
– Check with your neighbor …
#6
• What is required before a hypothesis can
become a theory?
– Check with your neighbor
#6
• What is required before a hypothesis can
become a theory?
– Check with your neighbor …
#7 Since there is gravity between the planets
and the sun, why don’t the planets crash into
the sun?
A. They have small tangential speeds, so they fall
around sun and not into it
B. They have large tangential speeds, so they fall
around sun and not into it
C. I had no clue how to answer this question
0%
0%
0%
#7 continued… What would happen to the
tangential velocities of the planets were
reduced to zero?
Would they…
• Continue to orbit the sun as usual?
• Crash straight into the sun?
• Take the tangent path away from the sun?
Use what you know about forces and
circular motion to DEFEND your answer
using scientific reasoning
What would happen if the
tangential velocities of the
planets were reduced to zero?
A. Continue to orbit
the sun as usual
B. Crash straight
into the sun
C. Take the tangent
path away from
the sun?
55%
45%
0%
12.3 The Falling Earth
• What would happen if the tangential velocities of
the planets were reduced to zero?
• Their motion would be straight toward the sun
and they would indeed crash into it.
• Any objects in the solar system with insufficient
tangential velocities have long ago crashed into
the sun.
•
(Same concept applies to the moons and other objects orbiting planets!!)
Which picture would show the path the planets
would take towards the sun if tangential
velocity was reduced to zero?
A.
B.
C.
D.
E.
55%
36%
9%
0%
0%
Open Notes Post Lab
1. Return your clickers
2. Normal procedures:
– Cell phones out and on silent
– Open notes, not open text
– You will NOT need a calculator
3. When you are done, you can start tonight’s HW:
– Read and study CH 12
– Complete # 21-24, 26
– 1 point each, and 1 point for random problem graded on
correctness = 6 points total
4. Finish your COMPUTER LABS! Just because we did
the Post Lab does not mean we are DONE with them!
You still have a CLOSED NOTES TEST on Ch. 9 & 12
(April 11th), and you could have a CH. 12 open notes
quiz at any time!
Isaac Newton
• Initial calculations didn’t work
Isaac Newton
• Initial calculations didn’t work
• Placed theory in a drawer for 20 years
Isaac Newton
• Initial calculations didn’t work
• Placed theory in a drawer for 20 years
• When the comet of 1670 came, Halley
convinced him to re- look at it. (the comet’s
namesake)
Isaac Newton
• Initial calculations didn’t work
• Placed theory in a drawer for 20 years
• When the comet of 1670 came, Halley
convinced him to re- look at it. (the comet’s
namesake)
• Produced the Law of Universal Gravitation
12.4 Newton’s Law of Universal Gravitation
Newton’s law of universal gravitation states that every
object attracts every other object with a force that is
directly proportional to the mass of each object, and
inversely proportional to the distance squared
Newton deduced that the force decreases as the square
of the distance between the centers of mass of the
objects increases.
12.4 Newton’s Law of Universal Gravitation
The force of gravity between objects depends on
the distance between their centers of mass.
Gravitational Force
• The force is extremely small and for
most tangible things it is too
sensitive to measure
• Attraction of person to another
person is hardly measurable….but
the attraction of person to the earth
is what we call weight
12.4 Newton’s Law of Universal Gravitation
Your weight is less
at the top of a
mountain because
you are farther
from the center of
Earth.
Is this a new diet
infomercial???
12.5 Gravity and Distance: The Inverse-Square Law
Main Concept #5
Gravity decreases according to the
inverse-square law. The force of gravity
weakens as the square of distance.
12.5 Gravity and Distance: The Inverse-Square Law
• This law applies to the weakening of gravity with
distance.
• Gravity weakens by the INVERSE SQUARE of the
distance.
• For example, if we were to make earth 9 times as far
from the sun….
 The square of 9 is 81.
 The inverse square of 9 is 1/81
 Therefore, if Earth was 9 times as far from the
sun, then gravity would be 1/81 as much.
Inverse Square Law
12.5 Gravity and Distance: The Inverse-Square Law
How does the force of gravity
change with distance?
12.4 Newton’s Law of Universal Gravitation
The Universal Gravitational Constant, G
The law of universal gravitation can be expressed as an exact
equation when a proportionality constant is introduced.
The universal gravitational constant, G, in the equation for
universal gravitation describes the strength of gravity… the
only problem was, people couldn’t figure out the value of G!
12.4 Newton’s Law of Universal Gravitation
Measuring G
G was first measured 150 years after Newton’s
discovery of universal gravitation by an English
physicist, Henry Cavendish.
Cavendish accomplished this by measuring the tiny
force between lead masses with an extremely
sensitive torsion balance.
12.4 Newton’s Law of Universal Gravitation
Measuring G
G was first measured 150 years after Newton’s discovery of universal
gravitation by an English physicist, Henry Cavendish.
Cavendish accomplished this by measuring the tiny force between lead
masses with an extremely sensitive torsion balance.
12.4 Newton’s Law of Universal Gravitation
Measuring G
A simpler method was developed by Phillip Von Jolly
• Mercury Flask and
a 6 ton lead ball
• Measured the
shifts in the Force
readings when he
rolled the massive
lead ball under the
set up
• Used that to
determine G
12.4 Newton’s Law of Universal Gravitation
Cavendish’s first measure of G was
called the “Weighing the Earth”
experiment.
When G was first measured in the
1700s, newspapers everywhere
announced the discovery as one that
measured the mass of Earth. (because
that was the missing variable in the
equation!)
Once “G” was known, the mass of
Earth was easily calculated.
Try to calculate G!
• View at least 2 rows of data from your
activity
• Calculate G!
– You have m1, m2, and the distance…
– You also have the force of gravity
– Plug in solve for G
– WRITE DOWN YOUR WORK ON LOOSE
LEAF!!!
– Solve for G for at least 2 rows!
12.4 Newton’s Law of Universal Gravitation
• The value of G tells us that gravity is a
very weak force.
• It is the weakest of the presently known
four fundamental forces.
• The 4 forces are:
•Gravity
•Electromagnetic
•Nuclear (weak)
•Nuclear (strong)
• We sense gravitation only when BIG
masses like that of Earth are involved.
12.4 Newton’s Law of Universal Gravitation
What did Newton discover about gravity?
We are now going to try the math…
1. Take a worksheet
2. READ the front
3. Think about the
SOLVED PROBLEMS
on the front
4. Try the rest of the
problems on the
front and back.
5. NOT SURE? CHECK
IN W/TEACHER!
6. When you are done,
review the HW #8-12
with your peers.
7. After practicing the
math, you should very
easily be able to check
and correct this HW.
8. NOT SURE? CHECK
IN W/TEACHER!
1=1.1
12
??? = 4
1
??? = 2 . 2
12
??? = 4
12
4=4
1…
gravity is 4x stronger!!! (or 4x
as strong, 4x more, etc)
.
1=1 1
12
??? = _1_
(1/4)
??? = 1 . 1
(½ ) 2
??? = _1_
(½) (½)
4 = _1_
(1/4)
gravity is 4x stronger!!! (or 4x
as strong, 4x more, etc)
We are now going to try the math…
1. Take a worksheet
2. READ the front
3. Think about the
SOLVED PROBLEMS
on the front
4. Try the rest of the
problems on the
front and back.
5. NOT SURE? CHECK
IN W/TEACHER!
6. When you are done,
review the HW #8-12
with your peers.
7. After practicing the
math, you should very
easily be able to check
and correct this HW.
8. NOT SURE? CHECK
IN W/TEACHER!