Transcript L - People
Homework Work on Lab Project paper and presentation due this week presentation may be done next week Work Reading Assignment for Wednesday Work Mastering Physics Assignments Study for Exam Review Session? Gravitational Force always attractive (never repulsive) between all pieces of things with mass on average acts between centers of objects F = GMm/r2 G = 6.67 x 10-11 Nm2/kg2 force is the same size on both objects Where does mg come from? If the distance between the centers of the objects is about the radius of the earth, the force is about equal to mg F = m (GM/r2) g = GMearth/Rearth2 g = (6.67 x 10-11 Nm2/kg2 )(5.97 x 1024kg) (6.38 x 106 m)2 g = 9.8 m/s2 directed toward the center of the earth Between all objects Estimate the gravitational force between two people standing at arms length? Does weight change? What is the weight of a 70 kg man at the surface of the earth? at the top of a tall (100 m) building? in an airplane (10000 m)? orbiting the earth (350000 m)? On the moon What is the weight of a 70 kg man on the moon? m = 7.35 x 1022 kg R = 1.74 x 106 m In-class exercise A satellite orbits the earth at a height of 2 earth radii. How does its weight on earth compare with its weight in space? 1) The two weights are the same. 2) The weight on earth is 2 times its weight in space. 3) The weight on earth is 1/2 its weight in space. 4) The weight on earth is 3 times its weight in space. 5) The weight on earth is 1/3 its weight in space. 6) The weight on earth is 4 times its weight in space. 7) The weight on earth is 1/4 its weight in space. 8) The weight on earth is 9 times its weight in space. 9) The weight on earth is 1/9 its weight in space. Gravitational Potential Energy (all cases: not necessarily near the earth’s surface) -U = F dr -U = (-GMm/r2) dr ri = choosing zero for U For h <<Rearth reduces to: rf = r this U -U =(GMm/r) - (GMm/) = mgh - C -U = GMm/r where C is a very large constant for U = 0 at . -(Ug - U0) = GMm/r Ug = -GMm/r always negative Example A 150 kg satellite is in circular orbit of radius 7.3 Mm about the earth. Determine the potential energy. Determine the orbital speed. Determine the kinetic energy. What is the escape speed from this altitude? Exercise Why are rocket launch sites located nearer to the equator (rather than the poles)? Angular Momentum in the Bohr Model of the Atom In the Bohr model: The electron orbits like a planet. The electron can only have certain values of angular momentum. L = n (h/2) where n is an integer Angular Momentum in Quantum Mechanics L = l (l 1) (h/2) gives length of angular momentum vector Lz = ml (h/2) gives orientation of the angular momentum vector The Bohr quantum number Lz L Quantum Numbers determine the shape of the state 1s 2s 2p l=0 for s-state l=1 for p-state l=2 for d-state L = mv x r Only s-states (L = 0) can have electrons at the origin.