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Rotational Motion

Rotational Motion. October 31, 2005 and November 2, 2005. 60. 5. 0. 0. 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 1 F 3 F 4 All of them None of them.

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Rotational Motion

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  1. Rotational Motion October 31, 2005 and November 2, 2005

  2. 60 5 0 0 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. • F1 • F3 • F4 • All of them • None of them

  3. 60 5 0 0 In the demo, how does the applied force change in order to achieve the same angular acceleration? • The force is zero in both cases. • The force must be larger when the arms are out. • The force must be larger when the arms are drawn in. • The force is the same in both cases.

  4. 60 5 0 0 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? • Solid aluminum • Hollow gold • The same hollow solid same mass & radius

  5. 60 5 0 0 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 • The same • Larger because she is rotating faster • Smaller because her moment of inertia is smaller

  6. 60 5 0 0 You are holding a spinning bicycle wheel while standing on a stationary turntable (as in class demo). If you suddenly flip the wheel over so that it is spinning in the opposite direction, the turntable will • Remain stationary • Start to spin in the same direction as before flipping • Start to spin in the opposite direction as before spinning

  7. Answers • The torque is: t = F d sin q and so the force that is at 90° to the lever arm is the one that will have the largest torque which is F1. • If the same angular acceleration is desired, then the applied force cannot be constant between the two cases because the mass distribution has changed (the moment of inertia I). The force must INCREASE if I increases, so the correct answer is (2). • The hollow gold – the mass is concentrated further away from the axis of rotation when compared to the solid sphere. • Larger. The product of her moment of inertia and her angular speed is constant, but the speed is SQUARED in finding rotational energy. • Answer (2) – the direction of L has changed from up to down, and the total angular momentum must be conserved, so some angular momentum that is “up” must be created.

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