Transcript Class 5 F10 (Ch 4a)

Chapter 4
Making Sense of the Universe:
Understanding Motion, Energy, and Gravity
“If I have seen farther than others, it is
because I have stood on the shoulders of
giants.” — Sir Isaac Newton (1642 – 1727)
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Outline of Ch 4 Motion and Gravity
(soap opera’s final episode)
4.1 and 4.2 Describing Motion, Newton and Galileo
Speed, velocity and acceleration (skip momentum)
Galileo’s experiments with falling objects:
g = 9.8 m/sec2
Objects fall together
Inertia (motion in absence of force)
Newton’s Laws:
1.
2.
3 laws of motion: a. Inertia b. F=ma c. Action = Reaction
Gravitation: F= GM1M2/R2 (Inverse-square law)
4.3 (Thermal Energy only ignore the rest of 4.3)
4.4 The force of Gravity

The Strength of Gravity
■ Newton and Kepler

Orbits: 1. Closed: circles (circular velocity) & ellipses (v > v c)
2. Open: parabolas and hyperbolas (escape velocity, v > v e)

Tides: Lunar and Solar
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4.1 Describing Motion
• Our goals for learning:
• How do we describe motion?
• How is mass different from weight?
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How do we describe motion?
Precise definitions to describe motion:
• speed: rate at which object moves

speed = distance units of m

s

time

example: speed of 10 m/s
• velocity: speed and direction
example: 10 m/s, due east
• acceleration: any change in velocity
units of speed/time (m/s2)
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•
The Acceleration of Gravity (some not in
book)
Galileo was one of the
giants on who’s shoulders
Newton stood. Galileo’s
experiments with falling
objects showed that:
• Objects fall together (not
counting friction of air
resistance).
• On Earth, g ≈ 10 m/s2:
speed increases 10 m/s with
each second of falling (g =
9.8 m/s2).
• Inertia (motion in absence
of force: uniform rectilinear
or at rest)
• g = 9.8 m/sec2
• Objects fall together
• Inertia (motion in
absence of force)
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The Acceleration of Gravity (g)
• Galileo showed that
g is the same for all
falling objects,
regardless of their
mass.
Apollo 15 demonstration
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Question:
Is there a net force? Y/N
1.
2.
3.
4.
A car coming to a stop.
A bus speeding up.
A bicycle going around a curve.
A moon orbiting Jupiter.
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Is there a net force? Y/N
1.
2.
3.
4.
A car coming to a stop. Y
A bus speeding up. Y
A bicycle going around a curve. Y
A moon orbiting Jupiter. Y
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How is mass different from weight?
• mass – the amount of matter in an object
• weight – the force that acts upon an object
You are weightless
in free-fall!
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Question
On the Moon:
A.
B.
C.
D.
My weight is the same, my mass is less.
My weight is less, my mass is the same.
My weight is more, my mass is the same.
My weight is more, my mass is less.
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On the Moon…
A.
B.
C.
D.
My weight is the same, my mass is less.
My weight is less, my mass is the same.
My weight is more, my mass is the same.
My weight is more, my mass is less.
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Why are astronauts weightless in space?
• There IS gravity in space…
• weightlessness is due to a constant state of free-fall:
• Ask me about
another way to
explain this
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What have we learned?
•How do we describe motion?
•Speed = distance/time
•Speed + direction => velocity (v)
•Change in velocity => acceleration (a)
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What have we learned?
• How is mass different from
weight?
• Mass = quantity of matter
• Weight = force acting on mass
• Objects are weightless when in
free-fall
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What have we learned?
• Galileo’s experiments with
falling objects:
• On Earth g = 9.8 m/sec2
(m/sec/sec)
• Objects fall together
• Inertia (motion in absence of
force)
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4.2 Newton’s Laws of Motion
Our goals for learning:
• How did Newton change our view of the
universe?
• What are Newton’s three laws of motion?
• What is Newton’s laws of gravitation?
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How did Newton change our view of the Universe?
• Realized the same physical
laws that operate on Earth
also operate in the heavens
 one universe
• Discovered 3 laws of
motion and law of
gravitation
• Much more: experiments
with light; first reflecting
telescope, calculus…
Sir Isaac Newton
(1642-1727)
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What are Newton’s three laws of motion?
Newton’s first law of motion: An object moves at
constant velocity unless a net force acts to change its
speed or direction (this he adopted from Galileo).
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Newton’s second law of motion:
Force = mass  acceleration (F= ma)
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Newton’s third law of motion:
For every force, there is always an equal and opposite
reaction force (action = reaction).
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The Universal Law of Gravitation
1. Every mass attracts every other mass.
2. Attraction is directly proportional to the product of
their masses.
3. Attraction is inversely proportional to the square of
the distance between their centers..
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Question:
Is the force the Earth exerts on you larger, smaller, or
the same as the force you exert on it?
A. Earth exerts a larger force on you.
B. I exert a larger force on Earth.
C. Earth and I exert equal and opposite forces on
each other.
D. There is no force between Earth and any object
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Is the force the Earth exerts on you larger, smaller, or
the same as the force you exert on it?
A. Earth exerts a larger force on you.
B. I exert a larger force on Earth.
C. Earth and I exert equal and opposite forces
on each other.
D. There is no force between Earth and any object
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Question:
A compact car and a Mack truck have a head-on
collision. Are the following true or false?
1. The force of the car on the truck is equal and
opposite to the force of the truck on the car.
2. The change of velocity (acceleration) of the car is
the same as the change of velocity of the truck.
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Question:
A compact car and a Mack truck have a head-on
collision. Are the following true or false?
1. The force of the car on the truck is equal and
opposite to the force of the truck on the car. T
2. The change of velocity of the car is the same as
the change of velocity of the truck. F
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Question:
A compact car and a Mack truck have a head-on
collision. Are the following true or false?
1. The force of the car on the truck is equal and
opposite to the force of the truck on the car. True
2. The change of velocity of the car is the same as
the change of velocity of the truck. False
(remember F = ma, if “F” is the same and the
masses are very different then “a”, which is the
change in velocity must also be very different)
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What have we learned?
•
How did Newton change our view of the universe?
•
•
•
He discovered laws of motion & gravitation.
He realized these same laws of physics were identical in the
universe and on Earth.
What are Newton’s Three Laws of Motion?
1)
2)
3)
Object moves at constant velocity if no net force is acting.
Force = mass  acceleration
For every force there is an equal and opposite reaction force.
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4.3 Ignore all except Thermal Energy
• Relation temperature  motion of atoms :
• The higher the temperature the faster the atoms in a
substance will be moving
• As atoms collide the electrons collide and their motion is
disturbed
• When the motion of electrons gets disturbed they produce
photons
• The higher the temperature, the more collisions, the more
photons (more about this in Ch. 5)
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Temperature Scales
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Outline of Ch 4 Motion and Gravity
(soap opera’s final episode)
4.1 and 4.2 Describing Motion, Newton and Galileo
Speed, velocity and acceleration (skip momentum)
Galileo’s experiments with falling objects:
g = 9.8 m/sec2
Objects fall together
Inertia (motion in absence of force)
Newton’s Laws:
1.
2.
3 laws of motion: a. Inertia b. F=ma c. Action = Reaction
Gravitation: F= GM1M2/R2 (Inverse-square law)
4.3 (Thermal Energy only)
4.4 The force of Gravity


The Strength of Gravity
■ Newton and Kepler
Orbits: 1. Closed: circles (circular velocity) & ellipses (v > v c)
2. Open: parabolas and hyperbolas (escape velocity, v > v e)

Tides: Lunar and Solar
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4.4 The Force of Gravity
Our goals for learning:
•What determines the strength of gravity?
•How does Newton’s law of gravity extend
Kepler’s laws?
•How do gravity and energy together allow us
to understand orbits?
•How does gravity cause tides?
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What determines the strength of gravity?
The Universal Law of Gravitation
1. Every mass attracts every other mass.
2. Attraction is directly proportional to the product of
their masses.
3. Attraction is inversely proportional to the square of
the distance between their centers..
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How does Newton’s law of gravity extend Kepler’s laws?
(some not in book)
• Ellipses are not the only
orbital paths. Orbits can be:
bound
• Circle (v = vc)
• Ellipse (v > vc)
unbound
• Parabola (v = ve)
• Hyperbola (v > ve)
• Circular and Escape
velocities (vc and ve)
 vc = GM/R
 ve = 2GM/R
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circular and
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• Newton generalized Kepler’s Third Law:
Newton’s version of Kepler’s Third Law:
If a small object orbits a larger one and you
measure the orbiting object’s
orbital period AND average orbital distance
THEN you can calculate the mass of the larger object.
Examples:
• Calculate mass of Sun from Earth’s orbital period (1 year) and
average distance (1 AU).
• Calculate mass of Earth from orbital period and distance of a
satellite.
• Calculate mass of Jupiter from orbital period and distance of
one of its moons.
•What about asteroids?
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Newton’s version of Kepler’s Third Law
p2 
4 2
a3
G(M1M2)

p = orbital period
a=average orbital distance (between centers)
(M1 + M2) = sum of object masses
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How do gravity and energy together explain orbits?
• Orbits cannot change spontaneously.
• An object’s orbit can only change if it somehow
gains or loses orbital energy =
kinetic energy + gravitational potential energy
(due to orbit).
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 So what can make an object gain or lose orbital
energy?
• Friction or atmospheric drag
• Rocket engine
• A gravitational encounter.
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• If an object gains enough orbital energy, it may
escape (change from a bound to unbound orbit)
•escape velocity from Earth ≈ 11 km/s from sea
level (about 40,000 km/hr, 25,000 mph)
•What is Earth’s circular
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•
How does Newton’s law of gravity extend Kepler’s laws?
(some not in book)
Ellipses are not the only orbital paths.
Orbits can be:
bound
• Circle (v = vc)
• Ellipse (v > vc)
unbound
• Parabola (v = ve)
• Hyperbola (v > ve)
• Circular and Escape
velocities (vc and ve)
 vc = GM/R
 ve = 2GM/R
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circular and
Mastering
Astronomy:
Study area: Ch 4
Interactive Fig.
4.18
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Escape and
orbital velocities
do NOT depend
on the mass of
the cannonball
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Outline of Ch 4 Motion and Gravity
(soap opera’s final episode)
4.1 and 4.2 Describing Motion, Newton and Galileo
Speed, velocity and acceleration (skip momentum)
Galileo’s experiments with falling objects:
g = 9.8 m/sec2
Objects fall together
Inertia (motion in absence of force)
Newton’s Laws:
1.
2.
3 laws of motion: a. Inertia b. F=ma c. Action = Reaction
Gravitation: F= GM1M2/R2 (Inverse-square law)
4.3 (Thermal Energy only)
4.4 The force of Gravity


The Strength of Gravity
■ Newton and Kepler
Orbits: 1. Closed: circles (circular velocity) & ellipses (v > v c)
2. Open: parabolas and hyperbolas (escape velocity, v > v e)

Tides: Lunar and Solar
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Question
The tides due to the Moon affect:
a) Only the Oceans
b) The whole Earth
c) Only the night side of Earth
d) None of the other answers is correct
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Question
The tides due to the Moon affect:
a) Only the Oceans
b) The whole Earth
c) Only the night side of Earth
d) None of the other answers is correct
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Tides
• Gravitational force decreases with (distance)2
– The Moon’s pull on Earth is strongest on the side facing the Moon,
and weakest on the opposite side.
• The Earth gets stretched along the Earth-Moon line.
• The oceans rise relative to land at these points.
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Tides vary with
the phase of the
Moon:
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Special Topic: Why does the Moon always show the
same face to Earth?
Moon rotates in the same amount of time that it orbits…
But why?
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Tidal friction…
• Tidal friction gradually slows Earth rotation (and makes Moon
get farther from Earth).
• Moon once orbited faster (or slower); tidal friction caused it to
“lock” in synchronous rotation with its orbit around Earth.
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What have we learned?
•What determines the strength of gravity?
•Directly proportional to the product of the masses (M x m)
•Inversely proportional to the square of the separation d
• How does Newton’s law of
gravity allow us to extend
Kepler’s laws?
• Applies to other objects, not
just planets.
• Includes unbound orbit
shapes: parabola, hyperbola
• We can now measure the
mass of other systems.
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What have we learned?
• How do gravity and
energy together allow us
to understand orbits?
• Gravity determines orbits
• Orbiting object cannot
change orbit without
energy transfer
• Enough energy -> escape
velocity -> object leaves.
•How does gravity cause tides?
•Gravity stretches Earth along Earth-Moon line because
the near side is pulled harder than the far side.
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