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Presentation on theme: "© 2007 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their."— Presentation transcript:

1 © 2007 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. Clicker Questions ConcepTests Chapter 11 Physics, 3 rd Edition James S. Walker

2 ConcepTest 11.1Using a Wrench You are using a wrench to loosen a rusty nut. Which arrangement will be the most effective in loosening the nut? ) all are equally effective

3 You are using a wrench to loosen a rusty nut. Which arrangement will be the most effective in loosening the nut? largest lever armcase #2 largest torque Since the forces are all the same, the only difference is the lever arm. The arrangement with the largest lever arm (case #2) will provide the largest torque. 5) all are equally effective Follow-up: What is the difference between arrangement 1 and 4? ConcepTest 11.1Using a Wrench

4 Two forces produce the same torque. Does it follow that they have the same magnitude? 1) yes 2) no 3) depends ConcepTest 11.2Two Forces

5 Two forces produce the same torque. Does it follow that they have the same magnitude? 1) yes 2) no 3) depends Because torque is the product of force times distance, two different forces that act at different distances could still give the same torque. ConcepTest 11.2Two Forces Follow-up: If two torques are identical, does that mean their forces are identical as well?

6 ConcepTest 11.3Closing a Door In which of the cases shown below is the torque provided by the applied force about the rotation axis biggest? For all cases the magnitude of the applied force is the same. 1) F 1 2) F 3 3) F 4 4) all of them 5) none of them

7 ConcepTest 11.3Closing a Door 1) F 1 2) F 3 3) F 4 4) all of them 5) none of them In which of the cases shown below is the torque provided by the applied force about the rotation axis biggest? For all cases the magnitude of the applied force is the same.  = F d sin  90° largest torque perpendicular The torque is:  = F d sin  and so the force that is at 90° to the lever arm is the one that will have the largest torque. Clearly, to close the door, you want to push perpendicular!! Follow-up: How large would the force have to be for F 4 ?

8 When a tape is played on a cassette deck, there is a tension in the tape that applies a torque to the supply reel. Assuming the tension remains constant during playback, how does this applied torque vary as the supply reel becomes empty? 1) torque increases 2) torque decreases 3) torque remains constant ConcepTest 11.4Cassette Player

9 When a tape is played on a cassette deck, there is a tension in the tape that applies a torque to the supply reel. Assuming the tension remains constant during playback, how does this applied torque vary as the supply reel becomes empty? 1) torque increases 2) torque decreases 3) torque remains constant As the supply reel empties, the lever arm decreases because the radius of the reel (with tape on it) is decreasing. Thus, as the playback continues, the applied torque diminishes. ConcepTest 11.4Cassette Player

10 ConcepTest 11.5aDumbbell I 1) case (a) 2) case (b) 3) no difference 4) it depends on the rotational inertia of the dumbbell A force is applied to a dumbbell for a certain period of time, first as in (a) and then as in (b). In which case does the dumbbell acquire the greater center-of- mass speed ?

11 ConcepTest 11.5aDumbbell I 1) case (a) 2) case (b) 3) no difference 4) it depends on the rotational inertia of the dumbbell A force is applied to a dumbbell for a certain period of time, first as in (a) and then as in (b). In which case does the dumbbell acquire the greater center-of- mass speed ? Because the same force acts for the same time interval in both cases, the change in momentum must be the same, thus the CM velocity must be the same.

12 1) case (a) 2) case (b) 3) no difference 4) it depends on the rotational inertia of the dumbbell A force is applied to a dumbbell for a certain period of time, first as in (a) and then as in (b). In which case does the dumbbell acquire the greater energy ? ConcepTest 11.5bDumbbell II

13 1) case (a) 2) case (b) 3) no difference 4) it depends on the rotational inertia of the dumbbell A force is applied to a dumbbell for a certain period of time, first as in (a) and then as in (b). In which case does the dumbbell acquire the greater energy ? If the CM velocities are the same, the translational kinetic energies must be the same. Because dumbbell (b) is also rotating, it has rotational kinetic energy in addition. ConcepTest 11.5bDumbbell II

14 ConcepTest 11.6Moment of Inertia 1) 1) solid aluminum 2) hollow gold 3) same same mass & radius solid hollow Two spheres have the same radius and equal masses. One is made of solid aluminum, and the other is made from a hollow shell of gold. Which one has the bigger moment of inertia about an axis through its center?

15 ConcepTest 11.6Moment of Inertia same mass & radius solid hollow Two spheres have the same radius and equal masses. One is made of solid aluminum, and the other is made from a hollow shell of gold. Which one has the bigger moment of inertia about an axis through its center? Moment of inertia depends on mass and distance from axis squared. It is bigger for the shell since its mass is located farther from the center. 1) 1) solid aluminum 2) hollow gold 3) same

16 ConcepTest 11.7Figure Skater 1) the same 2) larger because she’s rotating faster 3) smaller because her rotational inertia is smaller A figure skater spins with her arms extended. When she pulls in her arms, she reduces her rotational inertia and spins faster so that her angular momentum is conserved. Compared to her initial rotational kinetic energy, her rotational kinetic energy after she pulls in her arms must be

17 ConcepTest 11.7Figure Skater 1) the same 2) larger because she’s rotating faster 3) smaller because her rotational inertia is smaller A figure skater spins with her arms extended. When she pulls in her arms, she reduces her rotational inertia and spins faster so that her angular momentum is conserved. Compared to her initial rotational kinetic energy, her rotational kinetic energy after she pulls in her arms must be KE rot =1/2 I  2 = 1/2 L  (used L= I  ). Since L is conserved, larger  means larger KE rot. The “extra” energy comes from the work she does on her arms. Follow-up: Where does the extra energy come from?

18 ConcepTest 11.8Two Disks ConcepTest 11.8 Two Disks 1) disk 1 2) disk 2 3) not enough info Two different spinning disks have the same angular momentum, but disk 1 has more kinetic energy than disk 2. Which one has the bigger moment of inertia ? L L Disk 1 Disk 2

19 ConcepTest 11.8Two Disks ConcepTest 11.8 Two Disks 1) disk 1 2) disk 2 3) not enough info Two different spinning disks have the same angular momentum, but disk 1 has more kinetic energy than disk 2. Which one has the bigger moment of inertia ? L L Disk 1 Disk 2 KE=1/2 I  2 = L 2 /(2 I) (used L= I  ). Since L is the same, bigger I means smaller KE.

20 ConcepTest 11.9 Spinning Bicycle Wheel You are holding a spinning bicycle wheel while standing on a stationary turntable. If you suddenly flip the wheel over so that it is spinning in the opposite direction, the turntable will 1) remain stationary 2) start to spin in the same direction as before flipping 3) start to spin in the same direction as after flipping

21 ConcepTest 11.9 Spinning Bicycle Wheel You are holding a spinning bicycle wheel while standing on a stationary turntable. If you suddenly flip the wheel over so that it is spinning in the opposite direction, the turntable will The total angular momentum of the system is L upward, and it is conserved. So if the wheel has -L downward, you and the table must have +2L upward. 1) remain stationary 2) start to spin in the same direction as before flipping 3) start to spin in the same direction as after flipping

22 ConcepTest 11.10Balancing Rod 1kg 1m A 1 kg ball is hung at the end of a rod 1 m long. If the system balances at a point on the rod 0.25 m from the end holding the mass, what is the mass of the rod ? 1) 1/4 kg 2) 1/2 kg 3) 1 kg 4) 2 kg 5) 4 kg

23 1 kg X CM of rod same distance m ROD = 1 kg A 1 kg ball is hung at the end of a rod 1 m long. If the system balances at a point on the rod 0.25 m from the end holding the mass, what is the mass of the rod ? The total torque about the pivot must be zero !! 0.25 m to the right of the pivot same distance to the left of the pivot l The total torque about the pivot must be zero !! The CM of the rod is at its center, 0.25 m to the right of the pivot. Since this must balance the ball, which is the same distance to the left of the pivot, the masses must be the same !! 1) 1/4 kg 2) 1/2 kg 3) 1 kg 4) 2 kg 5) 4 kg ConcepTest 11.10Balancing Rod

24 ConcepTest 11.11Mobile 1 kg 1 m3 m 1 m2 m ? ? A (static) mobile hangs as shown below. The rods are massless and have lengths as indicated. The mass of the ball at the bottom right is 1 kg. What is the total mass of the mobile ? 1) 5 kg 2) 6 kg 3) 7 kg 4) 8 kg 5) 9 kg

25 1 kg 1 m3 m 1 m2 m ? ? A (static) mobile hangs as shown below. The rods are massless and have lengths as indicated. The mass of the ball at the bottom right is 1 kg. What is the total mass of the mobile ? Finally, add up all the masses. Use torques in two steps: (1) find the big mass on the bottom left (lower rod only). (2) use the entire lower rod assembly (with two masses) to find the mass on top right. Finally, add up all the masses. ConcepTest 11.11Mobile 1) 5 kg 2) 6 kg 3) 7 kg 4) 8 kg 5) 9 kg

26 ConcepTest 11.12aTipping Over I 123 1) all 2) 1 only 3) 2 only 4) 3 only 5) 2 and 3 A box is placed on a ramp in the configurations shown below. Friction prevents it from sliding. The center of mass of the box is indicated by a blue dot in each case. In which case(s) does the box tip over ?

27 ConcepTest 11.12aTipping Over I 123 1) all 2) 1 only 3) 2 only 4) 3 only 5) 2 and 3 A box is placed on a ramp in the configurations shown below. Friction prevents it from sliding. The center of mass of the box is indicated by a blue dot in each case. In which case(s) does the box tip over ? If the box can rotate such that the CM is lowered, it will !! The torque due to gravity acts like all the mass of an object is concentrated at the CM. Consider the bottom right corner of the box to be a pivot point. If the box can rotate such that the CM is lowered, it will !!

28 ConcepTest 11.12bTipping Over II 1) case 1 will tip 2) case 2 will tip 3) both will tip 4) neither will tip Consider the two configurations of books shown below. Which of the following is true? 1/2 1/4 1/2 1/4 12

29 ConcepTest 11.12bTipping Over II The CM of the system is midway between the CM of each book. Therefore, the CM of case #1 is not over the table, so it will tip CM of case #1 is not over the table, so it will tip. 1) case 1 will tip 2) case 2 will tip 3) both will tip 4) neither will tip Consider the two configurations of books shown below. Which of the following is true? 1/2 1/4 1/2 1/4 12


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