Simple Nature - Light and Matter

is straight down, which is perpendicular to the line connecting theshoulder to the center of mass, soF ⊥ = |F| = 29 N .Continuing to pretend that the force acts at the center of the arm,r equals 30 cm = 0.30 m, so the torque iss / Discussion question D.τ = r F ⊥ = 9 N · m .Discussion QuestionsA This series of discussion questions deals with past students’ incorrectreasoning about the following problem.Suppose a comet is at the point in its orbit shown in the figure. Theonly force on the comet is the sun’s gravitational force. Throughout thequestion, define all torques and angular momenta using the sun as theaxis.(1) Is the sun producing a nonzero torque on the comet? Explain.(2) Is the comet’s angular momentum increasing, decreasing, or stayingthe same? Explain.Explain what is wrong with the following answers. In some cases, theanswer is correct, but the reasoning leading up to it is wrong.(a) Incorrect answer to part (1): “Yes, because the sun is exerting a forceon the comet, and the comet is a certain distance from the sun.”(b) Incorrect answer to part (1): “No, because the torques cancel out.”(c) Incorrect answer to part (2): “Increasing, because the comet is speedingup.”t / Discussion question E.u / Discussion question A.B You whirl a rock over your head on the end of a string, and graduallypull in the string, eventually cutting the radius in half. What happens tothe rock’s angular momentum? What changes occur in its speed, the timerequired for one revolution, and its acceleration? Why might the stringbreak?C A helicopter has, in addition to the huge fan blades on top, a smallerpropeller mounted on the tail that rotates in a vertical plane. Why?D Which claw hammer would make it easier to get the nail out of thewood if the same force was applied in the same direction?E The photo shows an amusement park ride whose two cars rotate inopposite directions. Why is this a good design?258 Chapter 4 Conservation of Angular Momentum