Rotation Quiz Key

Rotation Quiz Answers ­For class review.notebook
March 17, 2015
Rotational Motion Quiz Key
1) When a rigid object rotates about a fixed axis, what is true about all the points in the object? (There could be more than one correct choice.)
A) They all have the same angular speed.
B) They all have the same tangential speed.
C) They all have the same angular acceleration.
D) They all have the same tangential acceleration.
E) They all have the same radial acceleration.
Answer: A and C
1
Rotation Quiz Answers ­For class review.notebook
March 17, 2015
2) The figure shows scale drawings of four objects, each of the same mass and
uniform thickness, with the mass distributed uniformly. Which one has the greatest
moment of inertia when rotated about an axis perpendicular to the plane of the
drawing at point P?
A) A
B) B
C) C
D) D
E) the same for all I mr2
Answer: B
3) A disk, a hoop, and a solid sphere are released at the same time at the top of an inclined plane. They are all uniform and roll without slipping. In what order do they reach the bottom?
A) disk, hoop, sphere
B) hoop, sphere, disk
C) sphere, disk, hoop
D) sphere, hoop, disk
E) hoop, disk, sphere
Does an object with high inertia accelerate quickly or slowly?
What IS inertia??
Answer: C
2
Rotation Quiz Answers ­For class review.notebook
March 17, 2015
4) Two equal­magnitude forces are applied to a door at the doorknob. The first force is applied perpendicular to the door, and the second force is applied at 30° to the plane of the door. Which force exerts the greater torque about the door hinge?
A) the first force (applied perpendicular to the door)
B) the second force (applied at an angle)
C) Both forces exert equal non­zero torques.
D) Both forces exert zero torque.
Answer: A
5) Five forces act on a rod that is free to pivot at point P, as shown in the figure.
Which of these forces is producing a counter­clockwise torque about point P?
(There could be more than one correct choice.)
CW
A) force A
B) force B
C) force C
D) force D
E) force E
CW
CCW
CW
Answer: C
3
Rotation Quiz Answers ­For class review.notebook
March 17, 2015
6) The rotating systems shown in the figure differ only in that the two identical
movable masses are positioned a distance r from the axis of rotation (left), or a
distance r/2 from the axis of rotation (right). If you release the hanging blocks
simultaneously from rest,
A) the block at the left lands first.
B) the block at the right lands first.
C) both blocks land at the same time.
Answer: B
7) A person sits on a freely spinning lab stool that has no friction in its axle. When this person extends her arms,
A) her moment of inertia decreases and her angular speed increases.
B) her moment of inertia decreases and her angular speed decreases.
C) her moment of inertia increases and her angular speed increases.
D) her moment of inertia increases and her angular speed decreases.
E) her moment of inertia increases and her angular speed remains the same.
More Inertia
Less Inertia
Answer: D
4
Rotation Quiz Answers ­For class review.notebook
March 17, 2015
8) What is the angular speed of a flywheel turning at 813.0 rpm?
A) 85.14 rad/s
B) 13.53 rad/s
C) 63.84 rad/s
D) 95.33 rad/s
Answer: A
9) Through how many degrees does a 33 rpm turntable rotate in
A) 63°
B) 35°
C) 46°
D) 74°
Answer: A
5
Rotation Quiz Answers ­For class review.notebook
March 17, 2015
10) When a fan is turned off, its angular speed decreases from 10 rad/s to 6.3 rad/s in 5.0 s. What is the magnitude of the average angular acceleration of the fan?
A) 0.86 rad/s2
B) 0.74 rad/s2
C) 0.37 rad/s2
D) 11 rad/s2
E) 1.2 rad/s2
Answer: B
11) A child is riding a merry­go­round that is turning at 7.18 rpm. If the child is standing 4.65 m from the center of the merry­go­round, how fast is the child moving?
A) 5.64 m/s
B) 3.50 m/s
C) 0.556 m/s
D) 1.75 m/s
E) 1.80 m/s
Answer: B
6
Rotation Quiz Answers ­For class review.notebook
March 17, 2015
at a uniform rate of
12) A wheel accelerates from rest to
Through what angle (in radians) did the wheel turn while accelerating?
A) 30 rad
B) 24 rad
C) 60 rad
D) 38 rad
Answer: A
13) A uniform solid cylinder with a radius of 10 cm and a mass of 3.0 kg is rotating about its center with an angular speed of 33.4 rpm. What is its kinetic energy?
A) 0.18 J
B) 0.092 J
C) 0.96 J
D) 1.1 J
E) 17 J
Answer: B
7
Rotation Quiz Answers ­For class review.notebook
March 17, 2015
14) A solid uniform sphere of mass 120 kg and radius 1.7 m starts from rest and rolls without slipping down an inclined plane of vertical height 5.3 m. What is the angular speed of the sphere at the bottom of the inclined plane?
A) 5.1 rad/s
B) 8.7 rad/s
C) 9.7 rad/s
D) 6.1 rad/s
Answer: A
15) A wheel having a moment of inertia of 5.00 kg ∙ m2 starts from rest and accelerates under a constant torque of 3.00 N ∙ m for 8.00 s. What is the wheel's rotational kinetic energy at the end of 8.00 s?
A) 122 J
B) 64.0 J
C) 78.8 J
D) 57.6 J Answer: D
8
Rotation Quiz Answers ­For class review.notebook
March 17, 2015
16) A pencil that is 15.7 cm long is released from a vertical position with the eraser end resting on a table. The eraser does not slip as it tips over. Treat the pencil like a uniform rod. What is the angular speed of the pencil just before it hits the table?
A) 13.7 rad/s
B) 7.23 rad/s
C) 3.70 rad/s
D) 24.5 rad/s
E) 16.8 rad/s
Answer: E
17) A solid uniform disk is rolling without slipping along a horizontal surface with a speed of 4.5 m/s when it starts up a ramp that makes an angle of 25° with the horizontal. What is the speed of the disk after it has rolled 3.0 m up as measured along the surface of the ramp?
A) 4.0 m/s
B) 1.9 m/s
C) 2.1 m/s
D) 6.8 m/s
E) 8.0 m/s
Answer: B
9
Rotation Quiz Answers ­For class review.notebook
March 17, 2015
18) The drive chain in a bicycle is applying a torque of 0.850 N ∙ m to the wheel of the bicycle. You can treat the wheel as a thin uniform hoop (or ring) with a mass of 0.750 kg and a radius of 33.0 cm. What is the angular acceleration of the wheel?
A) 1.06 rad/s2
B) 20.8 rad/s2
C) 5.20 rad/s2
D) 3.43 rad/s2
E) 10.4 rad/s2
Answer: E
19) A cinder block of mass m = 4.0 kg is hung from a nylon string that is wrapped around a frictionless pulley having the shape of a cylindrical shell, as shown in the figure. If the cinder block accelerates downward at 4.90 m/s2 when it is released, what is the mass M of the pulley?
Answer: B
10
Rotation Quiz Answers ­For class review.notebook
March 17, 2015
20) A uniform solid cylinder of mass 10 kg can rotate about a frictionless axle through its center O, as shown in the cross­sectional view in the figure. A rope wrapped around the outer radius R1 = 1.0 m exerts a force of magnitude F1 = 5.0 N to the right. A second rope wrapped around another section of radius R2 = 0.50 m exerts a force of magnitude F2 = 6.0 N downward. What is the angular acceleration of the cylinder?
Answer: C
11