Cutnell/JohnsonPhysics 8th edition

Classroom Response System Questions

Chapter 9 Rotational Dynamics

Reading Quiz Questions

9.1.1. Which one of the following choices is produced when a net torque is applied to a rigid object?

a) a constant acceleration of the object

b) a constant angular velocity of the object

c) a rotational equilibrium of the object

d) a change in the angular velocity of the object

e) a constant angular displacement of the object

9.1.2. A force is applied to a doorknob. This force will be most effective in causing the door to rotate when which of the following is true?

a) The lever arm is parallel to the direction of the force.

b) The lever arm length is zero meters.

c) The direction of the force is at an angle of 45 with respect to the door.

d) The direction of the force is at an angle of 90 with respect to the door.

e) The lever arm is perpendicular to the door.

9.1.3. The units of torque are which of the following?

a) newtons (N)

b) N m

c) kg/s2

d) kg m2

e) angular newtons (aN)

9.2.1. Which one of the following descriptions indicates that the object is in translational equilibrium?

a) Translational equilibrium occurs only if the object is at rest.

b) Translational equilibrium occurs only if the object is moving with constant acceleration.

c) Translational equilibrium occurs only if the object is at moving with constant velocity.

d) Translational equilibrium occurs if the object is moving with constant velocity or with constant acceleration.

e) Translational equilibrium occurs if the object is at rest or moving with constant velocity.

9.2.2. Consider the drawing. A small disk with a radius r shares an axis with a wheel of radius 4r. An object of mass M1 hangs from a rope that is attached and wrapped around the wheel as shown. Another object of mass M2 hangs from a rope that is attached and wrapped around the disk as shown. Which one of the following conditions must be true if this system is in equilibrium?

a) M1 = M2

b) M1 < M2

c) M1 > M2

9.2.3. Complete the following statement: When determining the net torque on a rigid body, only the torques due to

a) internal forces are considered.

b) external forces are considered.

c) forces that are either parallel or perpendicular to the lever arms are considered.

d) forces that form action-reaction pairs as in applying Newton’s third law of motion are considered.

e) internal and external forces are considered.

9.2.4. An object, which is considered a rigid body, is in equilibrium. Which one of the following statements is false when determining the forces and torques acting on the object?

a) The linear acceleration or the angular acceleration of the object may not be equal to zero.

b) The location of the rotational axis is arbitrary. Therefore, it can be placed at any point on the object that is convenient.

c) In placing an x-y coordinate system on the object, the +x direction is arbitrary and it can be directed toward any direction that is convenient.

d) A free body diagram of the external forces acting on the object is useful in analyzing this situation.

e) The sum of the torques due to external forces must equal zero N m.

9.2.5. A water skier is pulled by a boat traveling with a constant velocity. Which one of the following statements is false concerning this situation?

a) The water skier is in equilibrium.

b) The net acceleration of the skier is zero m/s2.

c) The net force on the skier is zero newtons.

d) There is a net horizontal force on the skier in the direction the boat’s velocity.

e) The net vertical force on the skier is zero newtons.

9.2.6. Which one of the following actions will take place if an object in unstable equilibrium is slightly displaced?

a) The object will move in the direction opposite that of the displacement.

b) The object will stop moving as soon as it reaches a place of stable equilibrium.

c) The kinetic energy will decrease.

d) The potential energy will decrease.

9.3.1. Which one of the following statements most accurately describes the center of gravity of an object?

a) It is the point where gravity acts on the object.

b) It is the point on the object where all the weight is concentrated.

c) It is the point from which the torque produced by the weight of the object can be calculated.

d) It must be experimentally determined for all objects.

e) It is the point where all the mass is concentrated.

9.4.1. A flat disk, a solid sphere, and a hollow sphere each have the same mass m and radius r. The three objects are arranged so that an axis of rotation passes through the center of each object. The rotation axis is perpendicular to the plane of the flat disk. Which of the three objects has the largest moment of inertia?

a) The solid sphere and hollow sphere have the same moment of inertia; and it is the largest.

b) The hollow sphere has the largest moment of inertia.

c) The solid sphere has the largest moment of inertia.

d) The flat disk has the largest moment of inertia.

e) The flat disk and hollow sphere have the same moment of inertia; and it is the largest.

9.4.2. Which one of the following statements concerning the moment of inertia is false?

a) The moment of inertia depends on the angular acceleration of the object as it rotates.

b) The moment of inertia may be expressed in units of kg m2.

c) The moment of inertia depends on the orientation of the rotation axis relative to the particles that make up the object.

d) Of the particles that make up an object, the particle with the smallest mass may contribute the greatest amount to the moment of inertia.

e) The moment of inertia depends on the location of the rotation axis relative to the particles that make up the object.

9.4.3. An object, which is considered a rigid body, is not in equilibrium. Which one of the following statements must be true concerning the angular acceleration and translational acceleration a of the object?

a) = 0 rad/s2 and a = 0 m/s2

b) > 0 rad/s2, but a = 0 m/s2

c) a > 0 m/s2, but = 0 rad/s2

d) > 0 rad/s2 and a > 0 m/s2

e) Either > 0 rad/s2 or a > 0 m/s2

9.4.4. Two solid spheres have the same mass, but one is made from lead and the other from wood. How do the moments of inertia of the two spheres compare?

a) The moment of inertia of the lead sphere is greater than that of the one made of wood.

b) The moment of inertia of the wood sphere is greater than that of the one made of lead.

c) The moment of inertia of the wood sphere is the same as that of the one made of lead.

d) There is no way to compare the spheres without knowing their radii.

9.5.1. An object is rolling, so its motion involves both rotation and translation. Which one of the following statements must be true concerning this situation?

a) The total mechanical energy is equal to the sum of the translational and rotational kinetic energies and the gravitational potential energy of the object.

b) The translational kinetic energy may be equal to zero joules.

c) The gravitational potential energy must be changing as the object rolls.

d) The rotational kinetic energy must be constant as the object rolls.

e) The total mechanical energy is equal to the sum of the translational kinetic energy and the gravitational potential energy of the object.

9.5.2. A solid cylinder is rolling along a flat, horizontal plane. The center of mass of the cylinder is moving toward the south at constant velocity. Which one of the following statements concerning the translational and rotational kinetic energies of the cylinder is true?

a) The translational kinetic energy is greater than the rotational kinetic energy.

b) The translational kinetic energy is less than the rotational kinetic energy.

c) The translational kinetic energy is equal to the rotational kinetic energy.

d) The sum of the translational and rotational kinetic energies equals the gravitational potential energy of the cylinder.

e) The sum of the translational and rotational kinetic energies equals zero joules.

9.6.1. Which one of the following expressions allows one to calculate the angular momentum for a rigid body about a fixed axis?

a) (1/2)I2

b) (1/2)MR2

c) 2I

d) I

e) (1/2)MRv2

9.6.2. A child standing on the edge of a freely spinning merry-go-round moves quickly to the center. Which one of the following statements is necessarily true concerning this event and why?

a) The angular speed of the system decreases because the moment of inertia of the system has decreased.

b) The angular speed of the system increases because the moment of inertia of the system has decreased.

c) The angular speed of the system increases because the moment of inertia of the system has increased.

d) The angular speed of the system decreases because the moment of inertia of the system has increased.

e) The angular speed of the system remains the same because the net torque on the merry-go-round is zero N m.

9.6.3. What happens when a spinning ice skater draws in her outstretched arms?

a) Her moment of inertia decreases causing her to slow down.

b) Her angular momentum decreases.

c) The torque that she exerts increases her moment of inertia.

d) Her angular momentum increases.

e) Her moment of inertia decreases causing her to speed up.

9.6.4. The moment of inertia of a wheel about its axle does not depend on which one of the following properties?

a) the mass of the wheel

b) the diameter of the wheel

c) the angular velocity of the wheel as it rotates

d) the location of the particles that compose the wheel

e) the materials that compose the wheel

9.6.5. A ball moves in a circular path on a horizontal, frictionless surface as shown. It is attached to a light string that passes through a hole in the center of the table. If the string is pulled down, thereby reducing the radius of the path of the ball, the speed of the ball is observed to increase. Which one of the following statements provides an explanation for this increase?

a) When the string is pulled downward, the angular momentum must increase.

b) The total mechanical energy of the ball must remain constant because energy is conserved.

c) The angular momentum of the ball is conserved in this process.

d) The linear momentum of the ball is conserved in this process.

e) This follows from applying Newton’s third law of motion.

9.6.6. Which one of the following choices represents the SI units for angular momentum?

a) kg m/s2

b) kg m2/s

c) kg m2/s

d) kg m rad/s

e) kg rad/s2

9.6.7. A hoop rolls without slipping on a horizontal surface and it moves due east at a constant linear speed. What is the direction of its angular momentum?

a) north

b) east

c) south

d) west

e) downward

9.6.8. There is a restaurant on top of a tall, circular building that is designed to rotate about its center at a constant angular speed. Which one of the following quantities is non-zero and constant for one of the restaurant’s customers seated near a window?

a) linear velocity

b) centripetal acceleration

c) angular momentum

d) angular acceleration