A100 Movie Special Tuesday, March 23 Swain West 119 7:00 pm (153 minutes) Winner of several awards 20 activity points! BYOP (Bring Your Own Popcorn) Homework #5 due Wednesday, March 10, 2:30 pm Absorption & Emission Line spectra Electron Orbits / Absorption & Emission Electrons can gain or lose energy while they orbit the nucleus. When electrons have the lowest energy possible, we say the atom is in the ground state. When electrons have more energy than this, we say the atom is in an excited state. When electrons gain enough energy to escape the nucleus, we say the atom is ionized. Since energy must be conserved, transitions between energy levels involve the absorption or emission of energy Emission/Absorption Spectra Each electron is only allowed to have certain energies in an atom. Electrons can absorb light and gain energy or emit light when they lose energy. Only photons whose energies (colors) match the jump in electron energy levels can be emitted or absorbed. Hydrogen Energy levels of Hydrogen. 1 eV = 1.60 x joules A hot, low density gas emits light of only certain wavelengths, determined by the composition of the gas. Emission line spectrum. Absorption line spectrum. When light with a continuous spectrum passes through a cool gas, dark lines appear in the continuous spectrum at wavelengths determined by the composition of the absorbing gas. Photons at specific wavelengths are are absorbed while passing through the relatively cool photospheric gas. As a result, we see an absorption spectrum when observing stars. The interior of a star is hot and dense, producing a continuous spectrum. However, the light emerging from the interior of a star must pass through the stars cooler atmosphere before we can see it. Absorption Spectra If light shines through a gas, each element will absorb those photons whose energy match their electron energy levels. The resulting absorption line spectrum has all colors minus those that were absorbed. We can determine which elements are present in an object by identifying emission & absorption lines. Molecules have rotational & vibrational energy levels. These levels are less energetic than electron energy levels. Therefore energy level transitions involve photons with less energy that visible photons. Energies correspond with photons in the infrared, microwave, and radio portion of the electromagnetic spectrum. The Doppler Shift: A shift in wavelength due to a wave emitter moving towards (shorter wavelength) or away (longer wavelength) from an observer. v c = The Doppler Effect 1. Light emitted from an object moving towards you will have its wavelength shortened. 2. Light emitted from an object moving away from you will have its wavelength lengthened. 3. Light emitted from an object moving perpendicular to your line-of-sight will not change its wavelength. BLUESHIFT REDSHIFT Measuring Radial Velocity We can measure the Doppler shift of emission or absorption lines in the spectrum of an astronomical object. We can then calculate the velocity of the object in the direction either towards or away from Earth. (radial velocity) v c = Measuring Rotational Velocity Phases of Matter the phases solid liquid gas plasma depend on how tightly the atoms and/or molecules are bound to each other As temperature increases, these bonds are loosened: In thinking about phases of matter, recall that temperature measures the average kinetic energy of particles. Faster particles can escape electrical bonds easier. Matter, Forces and Motion Scalars and Vectors Scalar: a quantity described solely by its size (and units) Vector: a quantity described by its size AND direction speed rate at which an object moves [e.g., m/s]. A scalar quantity. velocity an objects speed AND direction, [e.g.,10 m/s east]. A vector quantity. acceleration a change in an objects velocity, i.e., a change in speed OR direction [m/s 2 ]. A vector quantity. Momentum (p) the mass of an object times its velocity (p=mv) Force (f) anything that can cause a change in an objects momentum As long as the objects mass does not change, a force causes a change in velocity, or an acceleration (a) Force, momentum, and acceleration are all vectors Forces and Newtons Laws of Motion Newtons First Law of Motion A body in motion remains in motion and a body at rest remains at rest unless acted upon by an outside force. If the net force acting on an object is zero, then there is no change in the objects motion. OR What happens when there are forces? The change in a bodys velocity due to an applied force is in the same direction as the force, and is proportional to the force, but is inversely proportional to the bodys mass. F = ma Or F = rate of change of momentum Newtons Second Law of Motion Because force is a vector, forces only affect motion in the direction of the force. Motion perpendicular to the force is unchanged. F = ma can be rewritten to show that for a given force, the acceleration is inversely proportional to the mass: a = F / m Do not confuse mass and density Mass = amount of matter Density = amount of matter per volume Higher density means more matter packed into same volume Momentum: p = mv Both p and v are vector quantities Law of Conservation of Momentum If the net force acting on an object is zero, then the total momentum of a system remains constant. Newtons Third Law of Motion For every applied force, a force of equal size but opposite direction arises or For every action there is an equal and opposite reaction A body in motion remains in motion and a body a rest remains at rest unless acted upon by an outside force. F = ma (= rate of change of momentum) For every applied force, a force of equal size but opposite direction arises. Newton's Laws of Motion Conservation of energy Conservation of momentum Conservation of angular momentum Major Conservation Laws Angular Momentum angular momentum the momentum involved in spinning /circling = mass x velocity x radius torque anything that can cause a change in an objects angular momentum (twisting force) Conservation of Angular Momentum In the absence of a net torque, the total angular momentum of a system remains constant. Forces change momentum Torques change angular momentum A planet is always changing its direction of motion. Newtons second law therefore states that a force must be acting on the planet. Gravity provides this force. Gravity & Orbits The angular momentum of an orbiting planet is conserved, i.e., it is always the same. This provides yet another reason why planets move fastest at perihelion and slowest at aphelion. Angular Momentum & Orbits The Acceleration of Gravity (a force) As objects fall, they accelerate (a = g = F grav /m). We use the special symbol g to represent the acceleration due to the force of gravity. At sea level on the Earth, g = 9.8 m/s each second, or g = 9.8 m/s 2. The higher you drop the ball, the greater its velocity will be at impact (force will be acting on it longer). Weight is the force of gravity acting upon an object : W = F g = mg Galileo demonstrated that g is the same for all objects, regardless of their mass! Is Mass the Same Thing as Weight? mass the amount of matter in an object weight a measurement of the force due to gravity acting upon an object When in free-fall, you still have weight! weightless is a misnomer W = mg (weight) F = ma Objects do have weight in space Free-fall often confused with weightlessness