Download Our last class Class Exercise: Multi

1/26/2015
Today’s Class: Energy
January 26, 2015
Homework:
1. Read about Gravity at http://www2.jpl.nasa.gov/basics/bsf3-1.php
2. Homework #2 is due on Friday.
Also, Astr 2020 Final Exam is now scheduled for May 4 at 1:30 pm
Our last class
• Why do objects move at constant
velocity if no force acts on them?
–Conservation of momentum
Astronomy 2020 – Space Astronomy & Exploration
Astronomy 2020 – Space Astronomy & Exploration
Class Exercise: Multi-stage rockets
Conservation of Angular Momentum
Discuss how a 2-stage
rocket might be more
effective in placing a
payload into Earth orbit.
Angular momentum L = mass x velocity x radius
(mvr).
• The angular momentum of an object cannot
change unless an external twisting force (torque)
is acting on it.
• Earth experiences no twisting force as it orbits
the Sun, so its rotation and orbit will continue
indefinitely.
Konstantin Tsiolkovsky, 1857-1935
Russian rocket pioneer
Astronomy 2020 – Space Astronomy & Exploration
Astronomy 2020 – Space Astronomy & Exploration
Angular momentum conservation also explains why
objects rotate faster as they shrink in radius
What keeps a planet rotating and
orbiting the Sun?
Astronomy 2020 – Space Astronomy & Exploration
Astronomy 2020 – Space Astronomy & Exploration
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1/26/2015
Today’s Learning Goals
• Where do objects get their energy?
–Energy makes matter move.
–Energy is conserved, but it can:
• transfer from one object to another
• change in form
Basic Types of Energy
• Kinetic (motion)
• Radiative (light)
• Potential (stored)
Energy can change
type, but cannot be
created or destroyed.
Astronomy 2020 – Space Astronomy & Exploration
Astronomy 2020 – Space Astronomy & Exploration
Kinetic Energy (KE)
Thermal Energy
• Kinetic Energy depends upon both the mass (m) and the
square of the velocity (v). KE = ½ mv2.
• Class Exercise: Why did a piece of foam thermal insulation
breaking off the main fuel tank and hitting the left wing
result in the destruction of the Columbia Space Shuttle?
• The collective kinetic energy of many particles
(for example, in a rock, in air, in water)
– Thermal energy is related to temperature but it is
NOT the same.
– Temperature is the average kinetic energy of the
many particles in a substance (T ~ v2).
Space Shuttle external
tank foam block
Astronomy 2020 – Space Astronomy & Exploration
Thermal Energy
• Thermal energy is a measure of the total
kinetic energy of all the particles in a
substance. It therefore depends on both
temperature AND density.
Example:
Astronomy 2020 – Space Astronomy & Exploration
Astronomy 2020 – Space Astronomy & Exploration
Gravitational Potential Energy
• On Earth, depends on:
– object's mass (m)
– strength of gravity (g)
– distance object could
potentially fall
Astronomy 2020 – Space Astronomy & Exploration
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1/26/2015
Clicker Question: The energy of Earth's
rotation is an example of
Gravitational Potential Energy
• In space, an object
or gas cloud has
more gravitational
energy when it is
spread out than
when it contracts.
– A contracting cloud
converts
gravitational
potential energy to
thermal energy.
a)
b)
c)
d)
e)
a)
b)
c)
d)
e)
twisting energy.
potential energy.
thermal energy.
radiative energy.
kinetic energy.
Astronomy 2020 – Space Astronomy & Exploration
Astronomy 2020 – Space Astronomy & Exploration
Clicker Question: The energy of Earth's
rotation is an example of
Mass-Energy
twisting energy.
potential energy.
thermal energy.
radiative energy.
kinetic energy.
• Mass itself is a form of potential
energy (E):
E = mc2
where m = mass, c = speed of light.
• A small amount of mass can release
a great deal of energy (for example,
an H-bomb).
• Concentrated energy can
spontaneously turn into particles (for
example, in particle accelerators).
Astronomy 2020 – Space Astronomy & Exploration
Astronomy 2020 – Space Astronomy & Exploration
Energy Problems for Interstellar Travel
Conservation of Energy
• Travel in interstellar space means spacecraft
(S/C) has v > 0.9c. The required KE = ½
m(0.9c)2 = 0.4 mc2 or 40% conversion of mass
into energy!
– Fusion efficiency is only 0.1%.
– Assume S/C is Orion with a mass of 9000 kg. Total
KE = 3x1020 Joules if it travels at 0.9c. Entire U.S.
population energy consumption in 1 year is 9x1016
Joules!
Astronomy 2020 – Space Astronomy & Exploration
• Energy can be neither created nor destroyed.
• It can change form or be exchanged between
objects.
• The total energy content of the universe was
determined in the Big Bang and remains the
same today.
Astronomy 2020 – Space Astronomy & Exploration
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1/26/2015
What have we learned?
• Where do objects get their energy?
–Conservation of energy: energy
cannot be created or destroyed but
only transformed from one type to
another.
–Energy comes in three basic types:
kinetic, potential, radiative.
Astronomy 2020 – Space Astronomy & Exploration
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