Angular Momentum

1)        An object with more  _______________________  will be more difficult to spin, but once it gets spinning it will be harder to stop.

2)        A(n)  _______________________  is needed to change an object’s rotation.

3)        Moving an object’s mass farther from its rotational axis increases its  _______________________  .

4)        Imagine you were to look down upon a spinning child’s toy, and it is rotating counter-clockwise. What is the direction of its angular velocity? If the toy were slowing in its rotation, what direction is its angular acceleration?

5)        If a ball spins 3.0 times in 1.2 seconds, what is its angular speed? Give your answer in both rotations/second and radians/second.

6)        A solid disc has a moment of inertia equal to (1/2)MR2. If a disc with a radius of 0.35 m and a mass of 0.95 kg spins 2.0 times in 0.75 seconds, what is its angular momentum? Assume it’s spinning clockwise in a horizontal plane, and be very careful with your units.

7)        Take the disc from Problem 6 and decrease its radius by half while it is spinning. If there are no external torques, what is its new angular velocity?

8)        A neutron star forms when the remains of a massive star collapses after a supernova explosion. Before collapse, the star has a radius of 7 × 108 m. After collapse it has a radius of 104 m. If the initial angular speed is 7.3 × 10-6 radians/second, what is the angular speed after the collapse? The mass is the same before and after collapse. Model the rotating star as a uniform sphere – so its moment of inertia equals (2/5)MR2 – and be careful with precision.

9)        Convert the speed found in Problem 8 from radians/second to rotations/second. Again, be careful with precision.

10)      Picture a vertical rotational axis. A 0.45 m horizontal lever is perpendicular to the axis, pointed east. Apply a 25 N force, due north. Calculate the torque. Don’t forget the direction.