John is pushing his daughter Rachel in a wheelbarrow when it is stopped by a brick of height h (Fig. P12.21). The handles make an angle of u with the ground. Due to the weight of Rachel and the wheelbarrow, a downward force mg is exerted at the center of the wheel, which has a radius R. (a) What force F must John apply along the handles to just start the wheel over the brick? (b) What are the components of the force that the brick exerts on the wheel just as the wheel begins to lift over the brick? In both parts, assume the brick remains fixed and does not slide along the ground. Also assume the force applied by John is directed exactly toward the center of the wheel.
1/11/16 Phys 20A WEEK TWO 1. inner planets rotate faster than outer planets 2. Kepler’s laws a. 1. orbits are ellipse with sun at one focus 3. planets from stars a. stars twinkle, planets don’t b. bc planets have bigger mass, so twinkling of particles makes light more constant c. planets shine: because reflect sun’s light d. planets can be brighter than most stars (venus and jupiter) or as faint as other stars (uranus and neptune) depending on distance and size