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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 1 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 2 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 3 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 4