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### Physics for Scientists and Engineers, 6e

Chapter 10 – Rotation of a Rigid Object about a Fixed Axis

A rigid object is rotating in a counterclockwise sense around a fixed axis. Each of the following pairs of quantities represents an initial angular position and a final angular position of the rigid object. Which of the sets can only occur if the rigid object rotates through more than 180°?

- 3 rad, 6 rad
- -1 rad, 1 rad
- 1 rad, 5 rad

For a rotation of more than 180°, the angular displacement must be larger than π = 3.14 rad. The angular displacements in the three choices are (1) 6 rad – 3 rad = 3 rad; (2) 1 rad – (-1) rad = 2 rad; (3) 5 rad – 1 rad = 4 rad.

Suppose that the change in angular position for each of the pairs of values in question 1 occurs in 1 s. Which choice represents the lowest average angular speed?

- 3 rad, 6 rad
- -1 rad, 1 rad
- 1 rad, 5 rad

Because all angular displacements occur in the same time interval, the displacement with the lowest value will be associated with the lowest average angular speed.

A rigid object is rotating with an angular speed ω < 0. The angular velocity vector ω and the angular acceleration vector α are antiparallel. The angular speed of the rigid object is

- clockwise and increasing
- clockwise and decreasing
- counterclockwise and increasing
- counterclockwise and decreasing

The fact that ω is negative indicates that we are dealing with an object that is rotating in the clockwise direction. We also know that when ω and α are antiparallel, ω must be decreasing – the object is slowing down. Therefore, the object is spinning more and more slowly (with less and less angular speed) in the clockwise, or negative, direction.

Consider again the pairs of angular positions for the rigid object. If the object starts from rest at the initial angular position, moves counterclockwise with constant angular acceleration, and arrives at the final angular position with the same angular speed in all three cases, for which choice is the angular acceleration the highest?

- 3 rad, 6 rad
- -1 rad, 1 rad
- 1 rad, 5 rad

In Equation 10.8, both the initial and final angular speeds are the same in all three cases. As a result, the angular acceleration is inversely proportional to the angular displacement. Thus, the highest angular acceleration is associated with the lowest angular displacement.

Andy and Charlie are riding on a merry-go-round. Andy rides on a horse at the outer rim of the circular platform, twice as far from the center of the circular platform as Charlie, who rides on an inner horse. When the merry-go-round is rotating at a constant angular speed, Andy\'s angular speed is

- twice Charlie’s
- the same as Charlie’s
- half of Charlie’s
- impossible to determine

The system of the platform, Andy, and Charlie is a rigid object, so all points on the rigid object have the same angular speed.

Consider again the merry-go-round situation. When the merry-go-round is rotating at a constant angular speed, Andy\'s tangential speed is

- twice Charlie’s
- the same as Charlie’s
- half of Charlie’s
- impossible to determine

The tangential speed is proportional to the radial distance from the rotation axis.

A section of hollow pipe and a solid cylinder have the same radius, mass, and length. They both rotate about their long central axes with the same angular speed. Which object has the higher rotational kinetic energy?

- the hollow pipe
- the solid cylinder
- the have the same rotational kinetic energy
- impossible to determine

Almost all of the mass of the pipe is at the same distance from the rotation axis, so it has a larger moment of inertia than the solid cylinder.

If you are trying to loosen a stubborn screw from a piece of wood with a screwdriver and fail, you should find a screwdriver for which the handle is

- longer
- fatter

The fatter handle of the screwdriver gives you a larger moment arm and increases the torque that you can apply with a given force from your hand.

If you are trying to loosen a stubborn bolt from a piece of metal with a wrench and fail, you should find a wrench for which the handle is

- longer
- fatter

The longer handle of the wrench gives you a larger moment arm and increases the torque that you can apply with a given force from your hand.

You turn off your electric drill and find that the time interval for the rotating bit to come to rest due to frictional torque in the drill is Δt. You replace the bit with a larger one that results in a doubling of the moment of inertia of the entire rotating mechanism of the drill. When this larger bit is rotated at the same angular speed as the first and the drill is turned off, the frictional torque remains the same as that for the previous situation. The time for this second bit to come to rest is

- 4Δt
- 2Δt
- Δt
- 0.5Δt
- 0.25Δt
- impossible to determine

With twice the moment of inertia and the same frictional torque, there is half the angular acceleration. With half the angular acceleration, it will require twice as long to change the speed to zero.

A rod is attached to the shaft of a motor at the center of the rod so that the rod is perpendicular to the shaft, as in the figure below. The motor is turned on and performs work W on the rod, accelerating it to an angular speed ω. The system is brought to rest, and the rod is attached to the shaft of the motor at one end of the rod as in Figure 10.23b. The motor is turned on and performs work W on the rod. The angular speed of the rod in the second situation is

- 4ω
- 2ω
- ω
- 0.5ω
- 0.25ω
- impossible to determine

When the rod is attached at its end, it offers four times as much moment of inertia as when attached in the center (see Table 10.2). Because the rotational kinetic energy of the rod depends on the square of the angular speed, the same work will result in half of the angular speed.

A ball rolls without slipping down incline A, starting from rest. At the same time, a box starts from rest and slides down incline B, which is identical to incline A except that it is frictionless. Which arrives at the bottom first?

- the ball
- the box
- both arrive at the same time
- impossible to determine

All of the gravitational potential energy of the box-Earth system is transformed to kinetic energy of translation. For the ball, some of the gravitational potential energy of the ball-Earth system is transformed to rotational kinetic energy, leaving less for translational kinetic energy, so the ball moves downhill more slowly than the box does.

Two solid spheres roll down an incline, starting from rest. Sphere A has twice the mass and twice the radius of sphere B. Which arrives at the bottom first?

- sphere A
- sphere B
- Both arrive at the same time
- impossible to determine

In Equation 10.30, ICM for a sphere is 2/5 MR2. Thus, MR2 will cancel and the remaining expression on the right-hand side of the equation is independent of mass and radius.

Two spheres roll down an incline, starting from rest. Sphere A has the same mass and radius as sphere B, but sphere A is solid while sphere B is hollow. Which arrives at the bottom first?

- sphere A
- sphere B
- Both arrive at the same time
- impossible to determine

The moment of inertia of the hollow sphere B is larger than that of sphere A. As a result, Equation 10.30 tells us that the center of mass of sphere B will have a smaller speed, so sphere A should arrive first.

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