Transcript Ch.8
8 Rotation • tangential speed changes with radius from axis • mass distribution (and amount) determines rotational inertia • torque = force x lever-arm • angular momentum = rotational inertia x rotational speed • conservation of angular momentum can be used to interpret rotational collisions and interactions 9 Gravity • Newton's law of gravity: universality, mass dependence, distance dependence • light propagation and the inverse square law • origin and effect of tidal forces Intensity • intensity = power delivered per unit area in watts/m2 • point source intensity is inverse-square with distance Universal Law of Gravity • all matter is weakly attracted • the force is an inverse-square law m1m2 F G 2 d • where G = 6.67x10-11 N·m2/kg2 • Example: Two 100kg persons stand 1.0m apart F 6.67 10 11 (100)(100) 7 6 . 67 10 N 2 (1) Gravity causes differential (rather than constant) pull on large objects. This causes: orbital lock of moon with earth ocean tides on earth Spring Tides (largest) Neap Tides (smallest) Eclipses Solar Eclipse Solar Eclipse, moon blocks light heading toward earth (shown above) Lunar Eclipse, earth blocks light from sun heading toward moon Einstein modeled gravity as stretched space-time 10 Projectile Motion • Projectile motion is a combination of horizontal motion at constant velocity and vertical free fall • satellite motion is projectile motion • gravity does work on objects in elliptical orbits • kinematic equations used to calculate range or distance of fall for horizontally released projectiles • Kepler's laws: • elliptical orbits • equal areas in equal times • period2 ~ average radius3 Projectile Motion is motion when only gravity acts Horizontal force is zero Horizontal velocity constant. Vertical force is weight Vertical velocity is decreasing. Each component affects range vertical component affects time of flight Range = (horizontal component) x (time of flight) as angle increases (time of flight) increases as angle increases (horizontal component) decreases Distance of Fall • • • • d = ½ gt2 with g = 10m/s/s t = 1s d = 5m Example: h = 15m, projectile fired horizontally at 6 m/s. t = 2s d = 20m Calculate the Range. t = 3s d = 45m Another way to see “distance of fall” in projectile motion 11 Atomic Physics • • • • protons, neutrons, electrons and atoms atomic number and isotopes nucleus size and Rutherford’s Experiment elements, molecules, compounds, and mixtures Atomic History • Hypothesized in 5th Century B.C. (Greece) • Aristotle 4th Century B.C. did not believe in atoms. Taught 4-element idea (earth, air, water, fire) • Dalton 1800s England revived atomic idea • Brown 1827 hypothesized atoms caused oscillation of microscopic particles • Today we know 88 naturally occurring atoms and about 30 more lab created Atomic Imagery • resolving power = size of smallest feature we can see • electron microscopy ~ 0.0005 um • visible light microscopy ~ 0.2um • size of atom ~ 0.0002 um What do Atoms Look Like? • “middle” of the atom is nucleus which is composed of protons and neutrons • “outside” of atom is an electron shell • atom is mostly empty space • this was confirmed by Rutherford’s Experiment Elements and the Atomic Number • Atomic Number is the number of protons in a nucleus, • this number determines which element the atom is. • Isotopes are atoms with the same number of protons but different number of neutrons The Periodic Table of The Elements (the distinct atoms) Molecules Molecules are groups of atoms. Here are a few examples. oxygen ammonia methane water A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. Terminology • Element – group of atoms with the same number of protons. • Compound – chemical substance formed from 2 or more elements and which has a definite chemical formula. Compounds do not have to be formed from molecules since compounds can be crystal structures such as table salt. • Mixture – substance with components that have not combined chemically. • Chemical Substance - an element, compound, or mixture. • Molecule – smallest particle of chemical substance that has the same composition and chemical properties, e.g. water molecule. Anti-matter anti-proton: same mass as proton but oppositely charged anti-electron (positron): same mass as electron but with opposite charge. a particle and its antiparticle annihilate each other in a burst of energy Dark Matter Universe appears to be about 4% visible and 96% dark. There are various theories about what makes up this mass. End