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Improvements
• Needs reordered according to gravity
notes for real.
• Look up common misconceptions &
address them
Today you are going to…take notes on gravity!
So you can…
You’ll know you’ve got it when you can
• Accurately answer the questions
Bell Work 4/8/13
New Bell Work Sheet!
1. How many Newtons would a 885.0 kg object
weigh on Earth’s surface? (Use F = ma, a =
9.81 m/s2)
2. Convert this to lbs.
The 4 Fundamental Forces
Gravity!
• What does it do?
– Makes objects fall
– Keeps objects on Earth
– Keeps objects in orbit!
Gravity!
• Gravity is the attraction between any two
objects that have mass.
• It’s actually the weakest force!
• It’s magic!!
– We don’t really understand it (but then, that’s
true of most fundamental stuff)
• Sir Issac Newton wrote the law of
universal gravitation.
• Universal gravitation equation 
G=
• The force of gravity is
___________________ related to mass.
• The force of gravity is
___________________ related to distance.
• 2 things affect the force of gravity:
___________ and _____________
The Law of Universal Gravitation!
What happens to the force of gravity if the mass
doubles? If the distance doubles? So gravity
depends more on ___.
• Gravity is the attraction between any two
objects that have mass. Why don’t we
notice the force of gravity between all
objects?
• Would your mass or weight be any different
on the moon? Why?
• Would your mass or weight be any different
on the top of a tall mountain? Why?
Magnets!
• The force of magnetism
• Which is stronger, gravity or magnetism?
• How are gravity and magnetism similar?
• How are gravity and magnetism different?
Freefall!
• An object is in freefall when gravity is the
only force acting on it.
• All things on Earth’s surface fall with same
acceleration (_______). This means if
they were both dropped from the same
height, they would hit the ground at the
same time. Why don’t we notice this?
• Strobe light & drops of water!
The Law of Universal Gravitation!
What all is the same on the surface of Earth?
Figure out that everything else must be equal to
acceleration due to gravity.
Air Resistance!
• Air resistance keeps all objects from falling
with the same acceleration on Earth.
• Terminal velocity – the fastest an object
can fall due to air resistance
– Happens when force of air resistance = weight
• FNET = 0, therefore a = _______!
• Moon Hammer Drop
• Feather in vacuum
• Elephant and feather on physics classroom
Freefall!
• Why do all things fall with the same
acceleration? Shouldn’t heavier objects
fall faster?
• What do we call Earth’s force of gravity
on an object?
• On the surface of Earth, 1.0 kg weighs
about 9.81 Newtons. A 5.0 kg object
weighs 44.05 Newtons. Calculate its
acceleration when dropped.
• A 10.0 kg object weighs 98.1 Newtons.
Calculate its acceleration.
Why is the acceleration for
everything the same?
• If something has twice the weight, it also
has twice the inertia, therefore it’s twice as
hard to accelerate.
• Gravity is like horse drawn carriages, or a
bunch of tiny magnets!
• Would two pieces of clay fall faster if
combined?
Bell Work
• Which falls faster, a bullet that’s fired or a
bullet that’s dropped?
Today you are going to…take notes on projectiles,
circular motion, and weightlessness.
So you can…
You’ll know you’ve got it when you can
• Accurately answer the questions
Monday
Schedule
Gravity Notes & Questions
Tuesday
Projectile & Circular Motion Notes & Questions
Wednesday
Orbits!
Thursday
Review Questions & Begin Egg Drop Project
Friday
Build Egg Drop Project
Projectile Motion
Projectile Motion
• Projectile motion - curved path objects follow
when projected (thrown, shot, launched, etc.)
near surface of Earth
– Ex. Thrown football, bullet shot, cow launched
from catapult.
• Projectiles follow curved path called a
parabola.
Projectile Motion
• Projectiles are in freefall!
• On Earth, 2 forces act on objects in projectile
motion.
– gravity and air resistance.
• Air resistance is usually ignored.
Vertical Projectile Motion
• When you throw something straight up into
the air:
– It takes as long to go up as it does to come down
to the same point at which you released it.
– It will be going the same speed in the opposite
direction when it reaches its original height.
– At its maximum height:
• v=
• t=
Projectile Motion
• has horizontal (left & right) & vertical (up &
down) motion.
• The horizontal and vertical motion are
independent.
– They don’t affect each other!
• Which will hit the ground first, a bullet fired from
a gun or a bullet dropped from the same
height?
• Mythbusters!!
• http://www.youtube.com/watch?v=D9wQVIEdK
h8
Projectile Motion
• What 2 things affect the distance of a
projectile?
• What angle will launch a projectile farthest?
• What angles will launch projectiles the same
distance?
Projectile Motion
Shoot a Monkey!
• http://www.youtube.com/watch?v=cxvsHN
RXLjw
• Can an object be moving upwards if
nothing is pushing it?
• Can an object be moving horizontally if
nothing is pushing it?
• Is force required to keep an object in
motion? (Newton’s ______ Law)
Weight
Weight Weight don’t tell me!
• Weight - force of gravity between objects & a
planet.
• Mass measures:
– how much matter something has
– How much inertia something has
• (how hard it is to accelerate)
• 1 kg (mass) weighs 9.8 N = 2.2 lbs
• 1 N = 0.224 lbs (about ¼ lb)
• 1 lb (weight) = 4.46 N
•
•
•
•
•
F = ma
Fweight = ma
a = 9.81 m/s2 on Earth’s surface
What is the weight of a 2.0 kg object?
What is the weight of a 9.0 kg object?
Bell Work
• Which direction do you think the doughnut
will travel when the string breaks?
Today you are going to…take notes on circular motion.
So you can…understand the direction of acceleration &
velocity in circular motion and “centrifugal” force.
You’ll know you’ve got it when you can
• Accurately answer the circular motion questions.
Circular Motion
Circular Motion
• Objects in circular motion are accelerating
towards the center
• Centripetal force – force acting on an
object in circular motion; pulls towards
center of circle
Circular Motion
• Shape of horizontally thrown projectile
Centripetal Force
• Force diagram of object in circular motion
The forbidden F word!
• Centrifugal force – imaginary center
fleeing force on an object in circular
motion
• NOT REAL
• Caused by inertia…the object wants to
keep traveling in a straight line
Car driving in circles
The forbidden F word!
• Circular motion demos
• Why does it stay there? (a law)
• What force acts on it? What direction?
Demos & Puzzles
What provides the force
• Tom’s puzzle
• Penny in balloon
• Marble & tape
• Ball on string
• Color changing ball
• Ball on string through tube with weight
• Question: A ball is being whirled around by
a string. If the string suddenly breaks (or is
let go), what direction will the ball travel; A,
B, C, or D? (Neglect gravity) HAMMER
THROW!!!
Doughnut on a string!
• http://www.youtube.com/watch?v=TniQtoa
pVrQ&feature=related
• Answer: Path ___
• While attached to the string, the ball is
forced to follow a curved path. But once
the string is gone, no force is exerted and
(law of inertia) it follows a straight-line
path.
Today you are going to…take notes on orbits!
So you can…understand how satellites, moons, and
planets orbit things.
You’ll know you’ve got it when you can
• Accurately answer the questions.
Projectile Motion & Orbits
Projectile Motion & Orbits
• Force diagram of object in circular motion
– (this should be earlier in slide show, when
circular motion first mentioned)
Projectile Motion & Orbits
• Objects orbiting Earth are in free fall!
– They’re falling!
– Gravity acts as the centripetal force
– Cannonball!
Satellites!
Earth Satellites
Earth satellite - projectile moving fast enough to fall
continually around Earth rather than into it.
Why can’t we put a satellite into orbit at low altitudes on
Earth?
Earth Satellites
In the curvature of Earth, the surface drops a vertical
distance of nearly 5 meters for every 8000 meters
tangent to its surface.
A satellite in circular orbit close to Earth moves
tangentially at 8 km/s (5 mi/s). During each second, it
falls 5 m.
Earth Satellites
• Orbital speed for close orbit about Earth is 8 km/s
or 29,000 km/h (18,000 mi/h).
• At that speed, atmospheric friction would burn an object to a
crisp.
• A satellite must stay 150 kilometers (94 miles) or more above
Earth’s surface—to keep from burning due to the friction.
For a satellite close to Earth, the time for a complete orbit
around Earth, its period, is about 90 minutes.
Escape Velocity
To escape Earth’s gravity, an projectile must be
launched at a velocity of 11.186 km/s = 25,000 mph.
Rockets aren’t projectiles…they don’t need to go this fast.
Pioneer 10, launched from Earth in 1972, escaped from
the solar system in 1984 and is wandering in interstellar
space.
Space Junk!
Bell Work 4/8/11 – 4 minutes
1. Convert the following weight to Newtons.
A. 16 lb bowling ball
Find the mass of the above in kilograms.
2. Explain how satellites orbit Earth.
Weightlessness!
Weight & Weightlessness
• How would it feel to be weightless?
• How do you feel your weight?
Circular Motion & Gravitation
65
Weight & Weightlessness
• What you feel as your “weight” is a
sensation due to the normal force
• Just because you don’t feel your weight,
doesn’t mean you’re weightless!
• Astronauts in space are not really
weightless.
Circular Motion & Gravitation
66
Weight & Weightlessness
• Astronauts are just in constant freefall!
• Since there’s nothing holding them up,
they feel weightless!
Circular Motion & Gravitation
67
Would you like to be weightless?
Zero-G
• People On Vomit Comet
• http://www.youtube.com/watch?v=N6ZJnIJ
RUCI&feature=related
• Water balloons in zero gravity
http://www.youtube.com/watch?NR=1&fea
ture=fvwp&v=gTqLQO3L4Ko
end
Bell Work 4/11/12
1. What is a satellite?
2. What are the forces acting on a satellite?
3. How long does it take a satellite to orbit
Earth?
4. Can we see satellites with the naked eye?
5. How fast do you have to throw a ball to
throw it off of the planet?
• Pluto has a mass of 1125 x 1022 kg and an
equatorial radius of 1195 km. (a) Calculate
the acceleration due to gravity on the surface
of Pluto. (b) A python weighs 980 N on Earth.
How much would it weigh on Pluto?
MP = 1024 kg
rP = 1195 km = 1195000 m = 1.195 x 106 m
G = 6.67 x 10-11 N m2/kg2
WE = 980 N
gE = 9.8 m/s2
gP = 0.59 m/s2
WP = ? N
g = G MP/rP2
g = (6.67 x 10-11)(1024)/(1.195 x 106 )2
g = 4.78 x 10-20 m/s2
WE = mgE
980 = m(9.8)
100 kg = m
W = mg
= (100)(4.78 x 10-18m/s2
= 4.78 x 10-18 N
• The elephant and the feather each have the same force of gravity.
• The elephant has more mass, yet both elephant and feather
experience the same force of gravity.
• The elephant experiences a greater force of gravity, yet both the
elephant and the feather have the same mass.
• On earth, all objects (whether an elephant or a feather) have the
same force of gravity.
• The elephant weighs more than the feather, yet they each have the
same mass.
• The elephant clearly has more mass than the feather, yet they each
weigh the same.
• The elephant clearly has more mass than the feather, yet the
amount of gravity (force) is the same for each.
• The elephant has the greatest acceleration, yet the amount of
gravity is the same for each.
Bell Work 4/8/11 – 5 minutes
1. Convert the following weights to Newtons.
A. 16 lb bowling ball
B. 17 lb cat
C. 20,000 lb truck
2. Find the mass of the above in kilograms.
3. If an object has reached terminal velocity, what
is the net force on the object? What’s its
acceleration?
4. T/F An object in orbit is actually falling.
Bell Work 4/4/11 – 5 minutes
START NEW MONDAY BELL RINGER
FOR 4TH QUARTER 
1. Define mass according to chemistry and
according to physics.
2. W5SAYW
Bell Work 3/30/12 – 5 min
1. Robbie can throw a 3.0 kg medicine ball with
an acceleration of 10.0 m/s2. What
acceleration could he give a 9.0 kg medicine
ball? Show your work!
2. If you were in a plane, would your weight or
mass change? Why?
The moon has a radius of 1.74 million meters and a mass of
7.32x1022 kg. In both MKS and FPS units, determine the
acceleration due to gravity on the lunar surface.
M = 7.32 x 1022 kg
r = 1,740,000 m = 1.74 x 106 m
a=?
a = GM
r2
a = (6.67 x 10-11)(7.32 x 1022 kg )
(1.74 x 106)2
48.824 x 1011 16.17 x 10-1
3.02 x 1012
1.62 m/s2
Bell Work 4/11/11 – 5 minutes
Mass (kg)
1.
1 kg
2.
50 kg
3.
0.102 kg
4.
_____ kg
5.
_____ kg
6.
_____ kg
7. W5SAYW
Weight on Earth
Newtons
Pounds
____N
____N
____N
500 N
50 N
____N
____lbs
____lbs
____lbs
____lbs
____lbs
2,000 lbs
Bell Work 3/29/12 – 5 min
1. A force of 250.0 lbs acts on an object with a
mass of 8.0 kg. What’s it’s acceleration?
2. What is gravity?
Bell Ringer 4/1/11 – 2 min
1. If the mass of an object magically
doubled, the force of gravity between it
and other objects __________.
2. If the distance between two objects
doubles, the force of gravity between
them is ____ what it was.
Bell Work 4/7/11 – 5 minutes
Use the conversions between lbs, kg, and N in your notes to
answer the following questions.
1. What is the difference between mass and
weight? Give 2 units that measure weight and
1 unit that measures mass.
2. Change the following weights to Newtons.
12 lb bowling ball
117 lb teen
1600 lb car
3. Find the weight of the following masses in
Newtons AND pounds.
1 kg
5 kg
1000 kg
If Pandora has a mass of 5.00 x 1024 kg and a radius of
6.00 x 103 km, find the weight of a 110 kg Na’vi on Pandora’s
surface. (Pay attention to the units!)
MP = 5.00 x 1024 kg
mN = 110 kg
r = 6.00 x 103 km = 6.00 x 106 m
W = Fg =?
Fg = GMPm
r2
Fg = (6.67 x 10-11)(5.00 x 1024 kg )(110 kg)
(6.00 x 106)2
Fg = 1019 N
Tips for using the G equation.
• Make sure you square r!
• You’ll be dealing with big numbers…don’t
forget the exponents! “x 10-11”!
• Double check your answers.
Tips for using the G equation.
• Make sure you square r!
• You’ll be dealing with big numbers…don’t
forget the exponents! “x 10-11”!
• Double check your answers.
Satellite Motion
1. What is a satellite?
2. What are the forces acting on a satellite?
3. How long does it take a satellite to orbit
Earth?
4. Can we see them with the naked eye?
5. How fast do you have to travel to get
escape this planet?
14.2 Circular Orbits
The speeds of the bowling ball and the satellite are
not affected by the force of gravity because there is
no horizontal component of gravitational force.
14.2 Circular Orbits
The satellite is always moving at a right angle (perpendicular)
to the force of gravity.
• It doesn’t move in the direction of gravity, which would
increase its speed.
• It doesn’t move in a direction against gravity, which
would decrease its speed.
• No change in speed occurs—only a change in direction.
14.4 Energy Conservation and Satellite Motion
For a satellite in circular orbit, no force acts along the
direction of motion. The speed, and thus the KE,
cannot change.
What force or forces hold satellites in orbit?
14.2 Circular Orbits
A satellite in circular orbit close to Earth moves
tangentially at 8 km/s (5 mi/s). During each second, it
falls 5 m beneath each successive 8-km tangent.
What happens to the velocity of a satellite as the
distance from Earth increases?