Chapter 8 - Rotational Motion of Solid Objects

A figure skater is spinning with her arms extended. She now pulls her arms close to her body. What happens? A. Her rotational inertia decreases. B. The net torque on her increases. C. Her lever arm gets shorter. D. Her angular acceleration decreases.

A. Her rotational inertia decreases.

An object is moving in a horizontal circle, in a clockwise direction (as seen from above). In which direction does its angular momentum vector point? A. down B. up C. to the left D. toward the center of the circle

A. down

When a planet moves closer the Sun in an elliptical orbit, its rotational inertia A. gets smaller because its distance from the Sun is smaller. B. gets smaller because the lever arm is shorter. C. gets larger because of the angular momentum must be conserved. D. gets larger because the planet has more angular acceleration. E. does not change because the mass of the planet is constant.

A. gets smaller because its distance from the Sun is smaller.

15. A solid disk and a hollow disk with the same mass and diameter are released together (from rest) to roll down an incline. The solid disk reaches the bottom first. From this information we can say that the angular acceleration was A. larger for the solid disk. B. larger for the hollow disk. C. the same for both disks. D. impossible to know without a measurement of time.

A. larger for the solid disk.

Three different bodies - a uniform sphere, a uniform disk, and a hoop - all have same radius and same mass. The body with the largest rotational inertia will be A. the hoop. B. the sphere. C. the disk. D. none, as all have the same value of rotational inertia.

A. the hoop.

Because the rotational inertia is higher, it is hardest in springboard diving to perform a forward somersault in A. the straight position (no bending at knees or waist). B. the pike position (bending at the waist but not the knees). C. the tuck position (bending at both the waist and knees).

A. the straight position (no bending at knees or waist).

If an object has an angular acceleration larger than zero, its ____________ must be changing

Angular Velocity

A sphere and a hoop have the same diameter and the same mass. Which one will have a larger rotational inertia? A. The sphere. B. The hoop. C. Both will require the same time to reach the bottom.

B. The hoop.

When an ice skater twirling on the point of a skate draws her arms in she ends up whirling faster. This is because A. rotational energy is conserved. B. angular momentum is conserved. C. linear momentum is conserved. D. a net torque acts on the skater.

B. angular momentum is conserved.

A flywheel is used to minimize changes in rotational velocity as the torque or the load changes. The rotational inertia of a flywheel should be chosen to be A. as small as possible. B. as large as possible. C. anything -- it doesn't matter

B. as large as possible.

The bigger gears on the rear wheel of a multi-speed bike allow the rider to A. exert more force on the wheel. B. exert more torque on the wheel. C. increase the moment of inertia of the wheel. D. decrease the moment of inertia of the wheel.

B. exert more torque on the wheel.

The figure above shows two yo-yos that have the same mass and rotational inertia. The one on the right has a thicker axle than the one on the left. The torque produced by gravity is A. larger for the yo-yo on the left. B. larger for the yo-yo on the right. C. the same for both yo-yos. D. zero, there is no net torque on the yo-yos.

B. larger for the yo-yo on the right.

A bicycle exhibits greater stability when the angular momentum of the wheels is A. smaller. B. larger. C. any value.

B. larger.

16. In a stable stack of identical boards at the edge of a table, the center of gravity of the top board A. must lie above the table. B. must lie above the next lower board. C. must lie above the bottom board. D. need not lie over anything.

B. must lie above the next lower board.

The angular momentum of a rotating body is conserved when A. no net force acts on the body. B. no net torque acts on the body. C. when the body has a constant angular acceleration. D. when a shape of the body does not change.

B. no net torque acts on the body.

If the net force on a body is zero, the net torque on the body is A. always zero. B. not necessarily zero. C. certainly not zero.

B. not necessarily zero.

If the net torque on a body is zero, the net force on the body is A. always zero. B. not necessarily zero. C. certainly not zero.

B. not necessarily zero.

14. When a bicycle pedal is at the top of its motion, you can exert the greatest torque on the crank by A. pushing straight down on the pedal with your foot. B. pushing straight forward on the pedal with your foot. C. hooking your toes under the pedal and pulling upward.

B. pushing straight forward on the pedal with your foot.

A gyroscope consists of a wheel mounted on an axle aligned along a north-south direction. The south end of the axle is suspended by a rope attached to the ceiling, and the north end is manually held at rest. The wheel is spinning rapidly, so that its angular momentum vector points north. Suddenly the north end is released. Since there is spin, the north end of the axle will begin to move A. north. B. west. C. down. D. east. E. south.

B. west.

A merry-go-round, having a radius of 1.5 m, is set in motion by students applying a force of 500 N tangential to the rim of the wheel. The merry-go-round reaches a rotational velocity of 1.5 rad/s after 5 seconds, starting from rest. The rotational inertia of the merry-go-round is A. 7.5 102 kg m2 B. 1.7 103 kg m2 C. 2.5 103 kg m2 D. 5.6 103 kg m2 E. 1.0 104 kg m2

C. 2.5 103 kg m2

In using a long steel rod as a lever to move a large rock, where should the fulcrum be placed for a person to use the least amount of force to move the rock? A. Close to the person. B. Exactly halfway between the person and the rock. C. Close to the rock. D. Anywhere in between - it makes no difference.

C. Close to the rock.

Three horses are side-by-side on a merry-go-round: one at the edge, one near the axis, and one in between. Each horse has the same angular speed. Which horse has the greatest linear speed? A. The horse nearest the axis. B. The horse in the middle. C. The horse at the edge. D. The linear speed is the same for all three horses.

C. The horse at the edge.

You stand with your heels against a wall and bend ninety degrees forward at the waist. A. You are able to maintain your balance. B. You fall forward because of the momentum you gain when you move. C. You fall forward because your center of gravity moves ahead of your base of support. D. You fall forward because the muscles in your feet aren't strong enough to stop you.

C. You fall forward because your center of gravity moves ahead of your base of support.

Using a longer handled wrench to loosen a tight nut allows you to A. exert more force on the nut. B. increase the moment of inertia of the nut. C. apply more torque to the nut. D. pull harder on the wrench.

C. apply more torque to the nut.

The angular momentum of an isolated object can be changed only by A. changing its rotational inertia. B. applying a force along the axis of rotation. C. applying a torque.

C. applying a torque.

A constant torque is applied to a flywheel. Which of the following mechanical quantities will be constant? A. rotational energy B. angular momentum C. rotational acceleration D. rotational velocity E. linear velocity of a particle on the rim

C. rotational acceleration

A football thrown with a spinning motion is more stable than one thrown without spin because A. there is no net torque on a spinning object. B. air flows more easily around spinning objects. C. the angular momentum of a spinning object can only be changed by applying a torque. D. the torque generated by the spinning action is conserved.

C. the angular momentum of a spinning object can only be changed by applying a torque.

If there is a net torque on a wheel, then A. there must be a net force on the wheel. B. the wheel cannot be spinning. C. the wheel must have a nonzero angular acceleration. D. the angular momentum of the wheel cannot change direction.

C. the wheel must have a nonzero angular acceleration.

The armature of a motor is accelerated uniformly from rest to a rotational velocity of 1800 rev/min in 10 seconds. The rotational acceleration of the motor is A. 180 rad/s2 B. 90 rad/s2 C. 37.7 rad/s2 D. 18.8 rad/s2 E. zero

D. 18.8 rad/s2

A body of rotational inertia 1.0 kg m2 is acted upon by a torque of 2.0 Nm. The angular acceleration of the body will be: A. 0.5 rad/s2 B. 0.5 rev/s2 C. 1.0 rad/s2 D. 2.0 rad/s2 E. 2.0 rev/s2

D. 2.0 rad/s2

The rotational velocity of a 45 rev/min phonograph record in rad/s is approximately: A. 45 rad/s. B. 283 rad/s. C. 0.75 rad/s. D. 4.7 rad/s. E. 15.7 rad/s

D. 4.7 rad/s.

The rotational inertia of an object depends on which of these? A. Its color. B. How fast it is spinning. C. The amount of torque applied to it. D. How its mass is distributed about the spin axis. E. None of these.

D. How its mass is distributed about the spin axis.

An isolated object is initially spinning at a constant speed. Then, although no external forces act upon it, its rotational speed increases. This must be due to A. an increase in the moment of inertia. B. an increase in the mass. C. an increase in the angular momentum. D. a decrease in the moment of inertia. E. impossible, angular momentum conservation is violated

D. a decrease in the moment of inertia.

A torque acting on a body tends to produce A. equilibrium. B. linear velocity. C. a new center of gravity. D. angular rotation.

D. angular rotation.

A dancer begins performing a pirouette with arms extended. (A pirouette is a complete spin about a vertical axis.) This motion is completed most easily when the rotational inertia is decreased by A. ending at the waist at a right angle. B. extending the arms and one leg straight out to the sides. C. holding the arms straight up and extending one foot straight out to the side. D. bringing the arms and legs in a line with the body.

D. bringing the arms and legs in a line with the body.

A wheel is spinning on an axle. The linear velocity of a point on the rim is the product of the radius times the rotational velocity, if the latter is measured in units of: A. deg/s. B. rev/s. C. rev/min. D. rad/s. E. deg/min.

D. rad/s.

A spinning wheel has an angular momentum that points toward the east. If an eastward force is applied to the edge of the wheel, A. the angular momentum will get smaller. B. the angular momentum will get larger. C. the angular momentum of the wheel will change direction but not magnitude. D. the angular momentum of the wheel will not change.

D. the angular momentum of the wheel will not change.

It is possible using the analogies between linear motion and rotational motion to write an expression for rotational kinetic energy. The formula for rotational kinetic energy is A. ½tw2 B. ½ta2 C. ½Ia2 D. ½Iw2

D. ½Iw2

A student plays on a see-saw with her father. The student has a weight of 250 N and is seated 2.4 m from the axis of rotation. Her father balances her when he is seated 0.8 m from the axis. The father's weight is: A. 250 N. B. 500 N. C. 667 N. D. 725 N. E. 750 N.

E. 750 N.

A solid disk and a hoop have different diameters and different masses. Which one has a larger rotational inertia? A. The solid disk. B. The hoop. C. Impossible to know without knowing the diameters. D. Impossible to know without knowing the masses. E. Impossible to know without knowing both the masses and diameters.

E. Impossible to know without knowing both the masses and diameters.

Torque in the metric system is measured in units of: A. N. B. Ns. C. Nrad. D. N/m. E. Nm.

E. Nm.

If you push on a closed door and it does not swing open, then A. its rotational inertia must be much larger than your torque. B. there must not be any torques on the door. C. the lever arm must be very small. D. the angular momentum of the door is larger than your force. E. None of these

E. None of these

Which of the following is an appropriate unit for describing rotational acceleration? A. m/s2 B. rad/min. C. rev/min. D. rev/m/s. E. rev/min2

E. rev/min2

A merry-go-round is set into motion with a child on board. The rotational speed of the merry-go-round _____________ as the child moves inward along a radius.

Increases

When a mechanic uses an extension arm on his wrench to loosen a stubborn nut, he is exerting additional torque because of an increase in the _________________ (two words) of the force.

Lever Arm

Just as torque is the rotational analogue of force in linear motion, rotational inertia is the rotational analogue of _______ in linear motion.

Mass

If the net force on a body is zero the net torque is _______________________(always, not necessarily, cannot be) zero.

Not Necessarily

If a solid disk and a hoop are to have the same mass and rotational inertia, then the hoop must have a _________ radius.

Smaller

The angular momentum of a rotating body is constant when the net _______________ on the body is zero.

Torque