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The Scientific Method 1. Observe some aspect of the universe. 2. Invent a tentative description, called a hypothesis, that is consistent with what you have observed. 3. Use the hypothesis to make predictions. 4. Test those predictions by experiments or further observations and modify the hypothesis in the light of your results. 5. Repeat steps 3 and 4 until there are no discrepancies between theory and experiment and/or observation. 1 Science versus Pseudoscience The scientific method is unprejudiced. A theory is accepted based only the results obtained through observations and/or experiments which anyone can reproduce. The results obtained using the scientific method are repeatable. a theory must be ``falsifiable''. Pseudoscience, in contrast, does not employ the scientific method and is constructed in such a fashion that its claims are not falsifiable 2 System of Units Scientific International (SI) Based on multiples of 10 meter (m) light in of second. kilogram (kg) prototype of distance mass second (s) time ampere (A) electric current length of the path travelled by vacuum during a time interval 1/299 792 458 of a mass of the international the kilogram. the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of Cs 133 current, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce 3 Scalars and Vectors A scalar quantity has magnitude only (how much?) Examples Mass Volume Distance Speed A vector quantity has magnitude and direction Examples Displacement Velocity Acceleration Force 4 Aristotle’s Universe (Geocentric) Aristotle proposed that the heavens were literally composed of concentric, crystalline spheres to which the celestial objects were attached. Each object rotated at different velocities, with the Earth at the center. The ordering of the spheres to which the Sun, Moon, and visible planets were attached are shown to the right. 5 Stellar Parallax Stars should appear to change their position with the respect to the other background stars as the Earth moved about its orbit, because they are viewed from a different perspective. 6 Planetary Motion Most of the time, planets move from west to east relative to the background stars. This is direct motion. Occasionally, however, they change direction and temporarily undergo retrograde motion before looping back. (retrograde-move) 7 Planetary Motion-2 Retrograde motion was first explained using the following model devised by Ptolemy: The planets were attached, not to the concentric spheres themselves, but to circles attached to the concentric spheres, as illustrated in the adjacent diagram. These circles were called "Epicycles",and the concentric spheres to which they were attached were termed the "Deferents". 8 Planetary Motion-3 In actual models, the center of the epicycle moved with uniform circular motion, not around the center of the deferent, but around a point that was displaced by some distance from the center of the deferent. 9 Heliocentric Model Copernicus proposed that the Sun, not the Earth, was the center of the Solar System. Such a model is called a heliocentric system. The ordering of the planets known to Copernicus in this new system is illustrated in the following figure, which we recognize as the modern ordering of those planets. (copernicanmove) 10 Galileo Galilei Galileo used his telescope to show that Venus went through a complete set of phases, just like the Moon. This observation was among the most important in human history, for it provided the first conclusive observational proof that was consistent with the Copernican system but not the Ptolemaic system. 11 Kepler- Elliptical orbits The amount of "flattening" of the ellipse is the eccentricity. In the following figure the ellipses become more eccentric from left to right. A circle may be viewed as a special case of an ellipse with zero eccentricity, while as the ellipse becomes more flattened the eccentricity approaches one. (eccentricity-anim) 12 Kepler’s Laws Kepler’s 1st Law: The orbits of the planets are ellipses, with the Sun at one focus of the ellipse. 13 Kepler’s Laws Kepler’s 2nd Law: The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse. 14 Kepler’s Laws Kepler’s 3rd Law: The ratio of the squares of the revolution periods (P) for two planets is equal to the ratio of the cubes of their semimajor axes (R). 15 Speed “How fast/ How slow is it going?” Time rate of change of motion: Speed = distance time 16 Constant Motion vs. Changing Motion Object’s motion is constant: its speed and direction are not changing Object’s motion is changing: its speed and/ or direction are changing 17 Acceleration “Is it speeding up, is it slowing down, How fast is its speed changing?” More important: “How is its motion changing?” 18 Acceleration (con’t) Acceleration = change in velocity time a = vf - vi t or a = v t 19 Acceleration Due to Gravity Some everyday . .as . it falls ~observatios an object’s velocity increases ~ how an object’s velocity changes depends on air resistance If there is o air resistace, the object will fall We call it a freely . . . Freely Fallig 20 Motion with Constant Acceleration Recall: a = vf - vi t Whe the objects iitial velocity is We say that it zero . . . started from rest Or was dropped from rest vf = a t d = ½ at2 and 21 Motion in a Circle Recall that acceleration is defined as a change in velocity with respect to time. Since velocity is a vector quantity, a change in the velocity’s direction , even though the speed is constant, represents an acceleration. This type of acceleration is known as Centripetal acceleration ac = v2/r 22 Force & Motion Force everyday words: Push or Pull Examples: (Earth’s Gravity pulls down on objects) Forces are Vectors 23 Recognizing Forces Note: The Force due to Gravity is always pulling down on us! At-a-Distance Forces & Contact Forces 24 Net Force: The sum of all the forces acting on an object Net force zero: Balanced forces Net force non-zero: Unbalanced forces 25 Newton’s Laws of Motion (Net) force causes change in motion Net Force Change in Motion Cause Effect 26 Newton’s 1st Law of Motion: The Law of Inertia An object at rest, or in motion, will stay at rest, will continue in the same (straight –line) motion, unless a net, external force acts on it. unbalanced outside 27 Newton’s 3rd Law of Motion “For every action there is an equal and opposite reaction.” Better!: For every force one object exerts onto another, there is an equal & opposite force exerted back. 28 Newton’s 2nd Law of Motion Force = mass x acceleration F = ma 29 Relationship between Mass & Weight Weight is the force due to gravity on an object w = Fg = mag w = mg ag 30 Momentum Product of the mass and velocity of an object Momentum = mass x velocity p = mv 31 Conservation of Momentum When two objects collide (exerting forces on each other), their total momentum is conserved Law of Conservation of Linear Momentum: The total linear momentum of an isolated system remains the same: if there is no external, unbalanced force acting on the system. 32 Conservation of Angular Momentum Angular Momentum: The momentum of rotation Law of Conservation of Angular Momentum The angular momentum of an object remains constant, if there is no external, unbalanced torque acting on it. 33 Gravity Does the rotation of the Earth cause gravity? Does the our atmosphere/ air pressure push us down to keep us on the ground? Does the Moon’s orbit about the Earth cause gravity? Is there gravity on the Moon? On the Sun? Are the astronauts in the space shuttle really weightless? 34 Law of Gravity: Every mass in the universe attracts (and is attracted by) every other mass in the universe by a force that we call the force of gravity. Equation form of this law: m1m2 F G 2 r (where G = 6.67 x 10-11 Nm2/kg2) 36 Weightlessness (Sometimes called “microgravity”) Apparent weightlessness: the sensation you experience when there is no floor pushing up on you. 37 “It takes energy to do work” Work the process by which energy is transferred or changed from one form into another Work is done when you apply a force over a distance W=Fxd 40 Gravitational Potential Energy: - the energy an object has because of its location (in a force field) “position energy” Energy sugar in muscles PotentialEnergy of ball Work I did lifting = PEball maF d == PE gg PE mgh 41 Kinetic Energy, KE - the energy an object has because of its motion “moving energy” KE = ½ mv2 Ex: Basketball Ball Work I do pushing KineticEnergy of ball 42 Law of Conservation of Mechanical Energy In the absence of friction: the sum of the kinetic energy and the potential energy of a system is constant. (I.e., total energy is constant!) 43 Power - time rate of energy usage “How fast was the work done?” Power = work time P= W t Units: Watts = Joule/sec 44 Thermal Energy (Heat) Heat is simply thermal energy; i.e., a measure of the kinetic energy of the atoms or molecules that make up a substance. Heat Energy is measured in calories— defined as the heat required to raise 1 gram of water by 1 o C. The mechanical equivalent(in joules) of a calorie is : 1 calorie = 4.186 Joules 45 Mass Energy Every object contains the (mass)potential energy equivalent : = m c2 where c is the speed of light E c = 3 x 108 m/s 46 Relativity Revealed 1′ Lecture: The Special Theory of Relativity tells us that time, distance and mass are not what we think they are. The General Theory of Relativity shows us that mass warps space. 47 Relativity Revealed Special Theory Applies to non-accelerated “frames of reference.” Makes two (2) assumptions: 1. 2. The is no preferred inertial frame of reference. The velocity of light, c is a constant. 48 Relativity Revealed What about assumptions? 1. 2. No preferred inertial frame. O.K. ☑ Speed of light is constant. (c = 300,000,000 m/s) ?????? 49 Relativity Revealed It’s everywhere! It’s everywhere! The Lorentz Factor: 1/√[1 - v 2/c 2 ] Not important until v ≈ c, then VERY important. 50 Fundamental Principles of Temperature & Heat • Matter is made up of particles and these particles are in motion • Heat energy naturally “flows” from warmer parts to cooler parts of a system • Conservation of (Heat) Energy (First Law of Thermodynamics) Heat Energy lost + Heat Energy gained = 0 51 “Particles are in motion” Microscopic Properties of Substances • Particles are moving in all directions! • Particles are colliding with each other & the walls of the container! Temperature, T, is a relative measure of the average KE of the particles. 52 Heat Transfer Mechanisms 1. Conduction: Transfer of heat by individual particles colliding with each other 2. Convection: Transfer of heat by the large scale movement of heated regions of a fluid to cooler regions 3. Radiation: Heat transfer by the absorption or emission of EM radiation (mainly infrared radiation) 53 Measuring Heat Heat energy , Q: - thermal energy that is transferred between objects at different temperatures Units joules (J) or calories (cal) What happens when something “gains” or “loses” heat?: 1. Temperature can change or 2. Phase can change 54 1. To change Temperature of a substance: Heat energy must be “lost” or “gained” Q = mcT m = mass c = specific heat capacity of substance T = change in temperature 55 Heat & Changes of Phases 2. To change the Phase of a substance: Heat energy must be given off or absorbed Q = mL Liquid - gas changes: Lv = Latent Heat of Vaporization Solid – liquid changes: Lf = Latent Heat of Fusion 56 Phase Change Chart 120 Water is boiling Temperature (ºC) 100 80 60 During phases changes, temperature is constant! 40 0 Ice is gaining heat Ice is melting -20 Ice Ice & only Water 20 Water only Heat added Water & Vapor Vapor only 57 2nd Law of Thermodynamics One statement of the Second Law of Thermodynamics: Heat does not spontaneously flow from a lowtemperature region to a hightemperature region. 58 2nd Law of Thermodynamics Another form of the Second Law of Thermodynamics It is not possible to make a heat engine whose only effect is to absorb heat from a high-temperature region and turn all that heat into work. 59 nd 2 Law- Continued If we could design such a 100% efficient heat engine, we could then use that heat engine to power a refrigerator. The net result of that combination would be to cause heat to flow from a cold temperature to a high temperature. 60 Electricity Electric Charge, q [Unit: coulomb, C] Recall Structure of the Atom: Nucleus : Positively charged Electrons: Negatively charged q (one proton) = + 1.6 x 10-19 C q (one electron) = - 1.6 x 10-19 C 61 “Like charges repel/ unlike charges attract” Coulomb’s Law The force law that describes: charge-charge interaction F=kQq/r2 Electric Field: The region of space around an electric charge 62 Ohm’s Law: Voltage is proportional to current voltage = current x resistance V = IR I = V/R R = V/I 63 Series circuits: Circuit with one path. The current must flow through the resistors one at a time. Therefore: • The total resistance in the circuit is the sum of the individual resistances. • Current is the same throughout the circuit. • When one resistor breaks the current can no longer flow through any of the resistors. 64 Parallel circuits: Circuit with many paths. The current splits to flow through the resistors. Therefore: • The total current in the circuit is the sum of the currents in each path. • The same voltage is provided to each path of the circuit. • When one device/ resistor is turned off, or breaks, the current can continue to flow through other paths. 65 Power, P Recall: Power is the time rate of energy usage P = E/t [Units: watt, W] Electric Power = current x voltage P = IV 66 Magnetism “Like poles repel, Unlike pole attract” • Magnetic Force Field Direction Strength • Ferromagnetic Materials Just what makes a ferromagnetic material magnetic? 67 Electromagnetism A moving charge creates a magnetic field Example: The Electromagnet - coils of current carrying wire producing a magnetic field w A magnetic field can exert a force on a moving charge 68 Electromagnetic Induction The induction of an electric current in a wire when a nearby magnetic field changes Example: The Hand-held Flashlight 69 Electromagnetic Devices: 1. Motor: A device that converts electrical energy into mechanical energy 2. Generator: A device that converts mechanical energy into electrical energy 3. Transformers: A device that increases or decreases the voltage of an alternating current 70 Wave Types Transverse Wave: A wave which consists of a series of “up and down” disturbances of a medium. Examples: Water waves Rope waves Light waves Longitudinal Wave: A wave which consists of a series of compressions and expansions disturbances of a medium. Examples: Slinky Sound waves 71 Wave Characteristics Amplitude - the ‘height’ of the wave, the distance from equilibrium to the maximum displacement of the wave • Wavelength, the distance between corresponding points on a wave • Frequency, w the number of wave disturbances that occur per second • Wave speed, v the speed of the wave: v = w 72 Longitudinal Waves Longitudinal Waves Wavelength Constructive Interference http://www.colorado.edu/physics/2000/applets/fourier.html 75 Destructive Interference http://www.colorado.edu/physics/2000/applets/fourier.html 76 Standing Waves A standing or stationary wave is produced when two waves of the same wavelength but travelling in opposite directions interfere constructively Longitudinal Wave http://home.a-city.de/walter.fendt/physengl/stlwaves.htm 77 Electromagnetic Waves Electromagnetic waves are produced by an oscillating or accelerated charge The changing electric field produces a changing magnetic field http://www.Colorado.EDU/physics/2000/applets/fieldwaves. html http://www.phy.ntnu.edu.tw/~hwang/emWave/emWave.html http://home.a-city.de/walter.fendt/physengl/emwave.htm 78 Doppler Effect A change in pitch resulting from the relative motion of the source of the sound and the observer. When a source of sound is moving toward you, the wave crests are closer together and the pitch sounds higher. When the source of sound is moving away from you, the wave crests are farther apart and the pitch sounds lower. http://home.a-city.de/walter.fendt/physengl/dopplerengl.htm http://www.mohawk.net/~viking/physics/doppler.html 79 Electromagnetic Radiation 80 Radio Waves-- Amplitude Modulation http://www.colorado.edu/physics/2000/applets/fourier.html 81 Infrared Radiation Light whose wavelength is longer than visible light “Heat” Radiation-- produced by objects whose temperature is ~ 300 K 82 Ultraviolet Radiation Light whose wavelength is shorter than visible light Higher in energy than visible light--produces damage in organic material (e.g., sunburn) 83 X-Ray Radiation Electromagnetic radiation of short wavelength and high-energy. Produced by rapid deceleration of electrons or other high energy processes 84 Gamma Ray Radiation The highest energy, shortest wavelength electromagnetic radiation. Produced by nuclear decay or highly energetic processes. 85 Structure of the Atom For at least 25 centuries, matter believed to be made of tiny particles -- atoms. Newton thought that atoms were hard and indivisible. Complex structure of the atom not observed until 20th century. In 1897, J.J. Thomson discovered the electron. In 1911, Ernest Rutherford detected the atomic nucleus. 86 Bohr Model of the Atom “Planetary model” of the atom. Neutrons and protons occupy a dense central region called the nucleus. Electrons orbit the nucleus much like planets orbiting the Sun. Modifications Only certain select radii are possible for the electron orbits. If an electron moves in an allowed orbit, it radiates no energy. The amount of energy required to move from one orbit to another is fixed. 87 Photon: A particle of light • The photon is a unit packet of electromagnetic radiation • The photon has an energy that depends on its frequency: E = hw (h = 6.626 x 10-34 Js) The energy of one photon! 88 Photons Electrons may exist only in orbitals having certain specified energies. Atoms can absorb only specific amounts of energy as their electrons are boosted to excited states; atoms emit only specific amounts of energy when their electrons fall back down to lower energy states. The light absorbed or emitted must be in “packets” of electromagnetic radiation containing a specific amount of energy. These packets are called PHOTONS. The energy of a photon is related to the frequency of the electromagnetic energy absorbed or emitted. 89 Frequency and Energy Frequency is very important in physics and in astronomy, where we are very often interested in such things as energy and temperature. This is because energy is related to the = hfbywhere h = Planck’s constant frequency of E light When writing about light, people often use the Greek symbol (pronounced “noo”) for frequency, and c for the speed of light. So in astronomy you will often see the symbols and c for frequency and speed. Waves in general v = f E = hf Light c= E=h 90 Three Types of Spectra 91 Emission Spectra Pattern of bright spectral lines produced by an element. 92 Photoelectric effect 93 94 Band of Stability Chart of the Isotopes As the atomic number increases, more neutrons are needed to make the nucleus stable Clues to radioactivity: Atomic number of 83 and above Fewer neutrons than protons in the nucleus Odd-Odd nuclide 95 U-238 Decay 234 U 90Th 238 92 234 Th 91 Pa 234 90 234 91 234 Pa 92U 230 U 90Th 234 92 226 Th 88 Ra 230 90 226 88 222 84 222 84 218 80 Ra Rn Rn Po 96 Nuclear Fission When a nucleus fissions, it splits into several smaller fragments. Two or three neutrons are also emitted. The sum of the masses of these fragments is less than the original mass. This 'missing' mass (about 0.1 percent of the original mass) has been converted into energy. Fission can occur when a nucleus of a heavy atom captures a neutron, or it can happen spontaneously. 97 Fission-Continued... A chain reaction occurs when neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats. 98 Control of Fission To maintain a sustained controlled reaction, for every 2 or 3 neutrons released, only one must be allowed to strike another uranium nucleus. Nuclear reactions are controlled by a neutron-absorbing material, such as cadmium or graphite. 99 Nuclear Fusion Fusion is combining the nuclei of light elements to form a heavier element. In a fusion reaction, the total mass of the resultant nuclei is slightly less than the total mass of the original particles. 100 Fusion In order for fusion reactions to occur, the particles must be hot enough, in sufficient number and well contained. These simultaneous conditions are represented by a fourth state of matter known as plasma. In a plasma, electrons are stripped from their nuclei. A plasma, therefore, consists of charged particles, ions and electrons. 101 Fusion Magnetic confinement utilizes strong magnetic fields, typically 100,000 times the earth's magnetic field. Inertial confinement uses powerful lasers or high energy particle beams to compress the fusion fuel. The enormous force of gravity confines the fuel in the sun and stars. 102 Nuclear Scales 103 Nuclear Scales--cont. 104 Fundamental Particles 105 Fundamental Particles Quarks make up protons and neutrons, which, in turn, make up an atom's nucleus. Each proton and each neutron contains three quarks. There are several varieties of quarks, as seen to the right. Protons and neutrons are composed of two types: up quarks and down quarks. The sum of the charges of quarks that make up a nuclear particle determines its electrical charge. 106 Building an Atom Protons contain two up quarks and one down quark. +2/3 +2/3 -1/3 = +1 Neutrons contain one up quark and two down quarks. +2/3 -1/3 -1/3 = 0 The nucleus is held together by the "strong nuclear force," which is one of four fundamental forces The strong force counteracts the tendency of the positivelycharged protons to repel each other. It also holds together the quarks that make up the protons and neutrons. http://cgi.pbs.org/wgbh/aso/tryit/atom/ 107 Antimatter Antimatter is matter with a charge opposite to that of what we think of as normal matter, such as: Electron, Positron, Proton, Anti-proton, and Neutron, Anti-neutron, etc. Antiparticles act in much the same way as do ordinary particles Each has the same mass as their counterparts, but the charge is opposite. If any particle touches it's corresponding antiparticle both would be totally 108 annihilated leaving only energy.
