Rotational Dynamics

Chapters 9

Learning Objectives

Objectives for this chapter fall under applications of previous objectives

Table Of Contents

1. The Action of Forces and Torques on Rigid

Objects

2. Rigid Objects in Equilibrium

3. Center of Gravity

4. Newton’s Second Law for rotational Motion

About a Fixed Axis

5. Rotational Work and Energy

6. Angular Momentum

Chapter 9:Rotational Dynamics

Section 1:

The Action of Forces and Torques

on Rigid Objects

In pure translational motion, all points on anobject travel on parallel paths.

The most general motion is a combination oftranslation and rotation.

Translation vs. Rotation

What causes an object to have an angular acceleration?

According to Newton’s second law, a net force causes an object to have an acceleration. TORQUE

The amount of torque depends on where and in what direction the force is applied, as well as the location of the axis of rotation.

Magnitude of Torque = (Magnitude of the force) x (Lever arm)

FDirection: The torque is positive when the force tends to produce a counterclockwise rotation about the axis.

SI Unit of Torque: newton x meter (N·m)

DEFINITION OF TORQUE

Example 2 The Achilles Tendon

The tendon exerts a force of magnitude790 N. Determine the torque (magnitudeand direction) of this force about the ankle joint.

F

m106.355cos

2

mN 15

55cosm106.3N 720 2

9.1.1. You are using a wrench in an attempt to loosen a nut by applying a force as shown. But this fails to loosen the nut. Which one of thefollowing choices is most likely to loosenthis tough nut?

a) Tie a rope of length 2L to the wrench at the same location and apply the sameforce as shown.

b) Place a pipe of length 2L over the handle of the wrench and apply the same forceto the opposite end (farthest from the nut).

c) Double the force.

d) Doubling the length or doubling the force will have the same result, but doublingthe length is easier.

e) Continue applying the same force as in the drawing and eventually the nut will loosen.

9.1.2. A 1.5-kg ball is tied to the end of a string. The ball is then swung at a constant angular velocity of 4 rad/s in a horizontal circle of radius 2.0 m. What is the torque on the stone?

a) 18 Nm

b) 29 Nm

c) 36 Nm

d) 59 Nm

e) zero Nm

9.1.3. A 1.0-m long steel bar is suspended from a rope from the ceiling as shown. The rope is attached to the bar at its mid-point. A force directed at an angle is applied at one end. At the other end, a force is applied perpendicular to the bar. If the magnitudes of the two forces are equal, for which one of the following values of the angle will the net torque on the bar have the smallest magnitude? The net torque is the sum of the torques on the bar.

a) 0

b) 90

c) 135

d) 180

e) 270

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9.1.4. An interesting method for exercising a dog is to have it walk on the rough surface of a circular platform that freely rotates about its center as shown. When the dog begins walking near the outer edge of the platform as shown, how will the platform move, if at all? Assume the bearing on which the platform can rotate is frictionless.

a) When the dog walks, the platform will rotate counterclockwise when viewed from above.

b) When the dog walks, the platform will rotate clockwise when viewed from above.

c) When the dog walks, the platform will not rotate.

9.1.5. When using pruning shears, such as the pair shown, to cut a branch from a tree, it is better to insert the branch closer to the hinge than near the end of the shears. Which one of the following statements best explains the reason this observation is true?

a) The torque acting on the branch is smallest near the hinge.

b) The torque acting on the branch is largest near the hinge.

c) The torque exerted on the shears yields the greatest force on the branch near the hinge.

d) The long handles determine the force exerted on the branch, which is the same no matter where on the shears the branch is placed.

e) The same torque is exerted on the shears and the branch, regardless of the force applied to the handles.

9.1.6. An object with a triangular cross-section is free to rotate about the axis represented by the black dot shown. Four forces with identical magnitudes are exerted on the object. Which one of the forces, if any, exerts the largest torque on the object?

a) 1

b) 2

c) 3

d) 4

e) The same torque is exerted by each force.

Chapter 9:Rotational Dynamics

Section 2:

Rigid Objects in Equilibrium

If a rigid body is in equilibrium, neither its linear motion nor its rotational motion changes.

0 xF 0yF 0

0 yx aa 0

Rigid Objects in Equilibrium

A rigid body is in equilibrium if it has zero translationalacceleration and zero angular acceleration. In equilibrium,the sum of the externally applied forces is zero, and thesum of the externally applied torques is zero.

0 0yF0 xF

EQUILIBRIUM OF A RIGID BODY

Reasoning Strategy

1. Select the object to which the equations for equilibrium are to be applied.

2. Draw a free-body diagram that shows all of the external forces acting on the object.

3. Choose a convenient set of x, y axes and resolve all forces into components that lie along these axes.

4. Apply the equations that specify the balance of forces at equilibrium.

(Set the net force in the x and y directions equal to zero.)

5. Select a convenient axis of rotation. Set the sum of the torques about this axis equal to zero.

6. Solve the equations for the desired unknown quantities.

Example 3 A Diving Board

A woman whose weight is 530 N is poised at the right end of a diving board with length 3.90 m. The board has negligible weight and is supported by a fulcrum 1.40 m away from the left end. Find the forces that the bolt and the fulcrum exert on the board.

022 WWF

m 1.40

m 90.3N 5302 F

22

WWF 021 WFFFy

0N 530N 14801 F

N 9501 FN 1480

The arm is horizontal and weighs 31.0 N. The deltoid muscle can supply 1840 N of force. What is the weight of the heaviest dumbbell he can hold?

Example 5 Bodybuilding

0 Mddaa MWW

0.13sinm 150.0M

d

Maad

MWW

N 1.86

m 620.0

0.13sinm 150.0N 1840m 280.0N 0.31

dW

9.2.1. At the circus, a clown balances a step ladder on his forehead. A few people in the audience notice that he is continually moving to keep the ladder from falling off his forehead. Why is this movement necessary?

a) The clown is trying to apply a torque to the ladder in thedirection opposite to other torques on the ladder.

b) The clown is trying to keep the center of mass of the ladder directly above his head so that the torque due to the gravitational force is zero Nm.

c) By rocking the ladder on his forehead, the ladder will be more stable than if it were stationary. This is similar to riding a bicycle. You can easily balance a bicycle when it’s rolling, but not when it’s stationary.

d) This movement is not necessary. The clown is trying to make this look harder than it really is for entertainment value. The ladder will easily balance on the clown’s forehead.

9.2.2. In the seventeenth century, French mathematician Gilles de Roberval developed a balance, shown in part A in the figure, for commercial weighing; and it is still in use today. A variation of this device, shown part B of the figure, is used for physics demonstrations. In this case, the two triangular objects have equal mass and rest on the two horizontal arms at an equal distance from the vertical bars. When the system is released, there is no movement because the system is in equilibrium. One of the objects is then slid to the right as shown in part C, what will happen when the system is released?

a) The arm on the right will go up.

b) The arm on the left will go up.

c) Neither arm will move.

9.2.3. Consider the three situations shown in the figure. Three forces act on the triangular object in different ways. Two of the forces have magnitude F and one of the forces has a magnitude 2F. In which case(s), if any, will the object be in equilibrium? In each case, the forces may act at the center of gravity or at the center of a corner.

a) A only

b) B only

c) C only

d) A and C

e) A and B

9.2.4. A 4.0-m board is resting directly on top of a 4.0-m long table. The weight of the board is 340 N. An object with a weight of 170 N is placed at the right end of the board. What is the maximum horizontal distance that the board can be moved toward the right such that the board remains in equilibrium?

a) 0.75 m

b) 1.0 m

c) 1.3 m

d) 1.5 m

e) 2.0 m

9.2.5. Jack is moving to a new apartment, so he has loaded a hand truck with four boxes: box A is full of books and weighs 133 N, box B has more books and weighs 111 N, box C contains his music collection on CDs and weighs 65 N, and box D contains clothes and weighs 47 N. The height of each box is 0.30 m. The center of gravity of each of the boxes is located at its center. In preparing to pull the hand truck up the ramp of the moving truck he rotates it to the position shown. What is the magnitude of the force that Jack is applying to the hand truck at a distance of 1.4 m from the axel of the wheel?

a) 360 N

b) 200 N

c) 150 N

d) 96 N

e) 69 N

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9.2.6. A pair of fuzzy dice is hanging from the rearview mirror of a sports car. As the car accelerates smoothly, the strings of the dice are tilted slightly toward the rear of the car. From the perspective of the driver, which one of the following statements is true, if the dice are stationary?

a) The dice are in static equilibrium.

b) The dice are not in equilibrium because the torque on the dice is not zero.

c) The dice are in equilibrium, but not static equilibrium.

d) The dice are not in equilibrium because the linear momentum of the dice is not zero.

e) None of the above statements are true.

9.2.7. Which one of the following pictures best represents the forces that prevent the ladder from slipping while someone is standing on it?

Chapter 9:Rotational Dynamics

Section 3:

Center of Gravity

The center of gravity of a rigid body is the point at whichits weight can be considered to act when the torque dueto the weight is being calculated.

DEFINITION OF CENTER OF GRAVITY

When an object has a symmetrical shape and its weight is distributed uniformly, the center of gravity lies at its geometrical center.

CENTER OF GRAVITY

CENTER OF GRAVITY

21

2211

WW

xWxWxcg

Example 6 The Center of Gravity of an Arm

The horizontal arm is composedof three parts: the upper arm (17 N),the lower arm (11 N), and the hand (4.2 N).

Find the center of gravity of thearm relative to the shoulder joint.

21

2211

WW

xWxWxcg

N 2.4N 11N 17

m 61.0N 2.4m 38.0N 11m 13.0N 17

cgx m 28.0

Conceptual Example 7 Overloading a Cargo Plane

This accident occurred because the plane was overloaded towardthe rear. How did a shift in the center of gravity of the plane cause the accident?

Finding the center of gravity of an irregular shape

9.3.1. Six identical bricks are stacked on top of one another. Note that the vertical dashed line indicates that the left edgeof the top brick is located to the right of the right side of thebottom brick. Is the equilibrium configuration shownpossible, why or why not?

a) Yes, this is possible as long as the combined center of gravity of the blocks above a given brick does not extend beyond the right side of the brick below.

b) Yes, this is possible as long as the left side of each block is directly above the center of gravity of the brick directly below it.

c) Yes, this is possible as long as the center of gravity of the blocks above a given brick remains directly above the center of gravity of the blocks below that brick.

d) No, this is not possible because the center of gravity of the top two blocks extends beyond the right edge of the bottom two blocks.

e) No, because the center of gravity of the top block is to the right of the third block from the top.

9.3.2. Consider the diamond-shaped object shown that is designed to balance on a thin thread like a tight rope walker at a circus. At the bottom of the diamond, there is a narrow notch that is as wide as the thickness of the thread. The mass of each of the metal spheres at the ends of the wires connected to the diamond is equal to the mass of the diamond. Which one of the points indicated is the most likely location of the center of gravity for this object?

a) A

b) B

c) C

d) D

e) E

9.3.3. Consider the object shown. A bottle is inserted into a board that has a hole in it. The bottle and board are then set up on the table and are in equilibrium. Which of the points indicated is the most likely location for the center of mass for the bottle and board system?

a) A

b) B

c) C

d) D

e) E

Chapter 9:Rotational Dynamics

Section 4:

Newton’s Second Law for Rotational Motion About a Fixed Axis

displacement

velocity

elapsed time

acceleration

x

v

t

a

t

inertia m I

Cause “a/” F

Translation vs. Rotation

TT maF

raT rFT

IMoment of Inertia, I=kmr2

k depends on shape and axis

Newton’s Second Law for Rotational Motion About a Fixed Axis

mr

r

ROTATIONAL ANALOG OF NEWTON’S SECOND LAW FORA RIGID BODY ROTATING ABOUT A FIXED AXIS

onaccelerati

Angular

inertia

ofMoment torqueexternalNet

I

2mrIRequirement: Angular acceleration must be expressed in radians/s2.

Example 9 The Moment of Inertial Depends on Wherethe Axis Is.

Two particles each have mass and are fixed at the ends of a thin rigid rod. The length of the rod is L. Find the moment of inertia when this object rotates relative to an axis that is perpendicular to the rod at (a) one end and (b) the center.

(b)

222

211

2 rmrmmrI 22 21 LrLr mmm 21

221 mLI

(a)

222

211

2 rmrmmrI Lrr 21 0mmm 21

2mLI

220 LmmI

22 22 LmLmI

Common values of I

SlenderRod

RectangularPlane

Sphere

Cylinder

axisthroughcenter

axisthroughcenter

thin-walledhollow

hollow

thin-walledhollow

axisthroughend

axisalongedge

solid

solid

Example 12 Hoisting a Crate

The combined moment of inertia of the dual pulley is 50.0 kg·m2. The crate weighs 4420 N. A tension of 2150 N is maintained in the cableattached to the motor. Find the angular acceleration of the dualpulley.

ITT 2211 yy mamgTF 2

massless rope, assume equal

ymamgT 2

2ya

ImamgT y 211

ImmgT 2211

2srad3.6

22

211

mI

mgT

22

2

m 200.0kg 451mkg 46.0

m 200.0sm80.9kg 451m 600.0N 2150

9.4.1. Two solid disks, which are free to rotate independently about the same axis that passes through their centers and perpendicular to their faces, are initially at rest. The two disks have the same mass, but one of has a radius R and the other has a radius 2R. A force of magnitude F is applied to the edge of the larger radius disk and it begins rotating. What force must be applied to the edge of the smaller disk so that the angular acceleration is the same as that for the larger disk? Express your answer in terms of the force F applied to the larger disk.

a) 0.25F

b) 0.50F

c) F

d) 1.5F

e) 2F

9.4.2. The corner of a rectangular piece of wood is attached to a rod that is free to rotate as shown. The length of the longer side of the rectangle is 4.0 m, which is twice the length of the shorter side. Two equal forces with magnitudes of 22 N are applied to two of the corners. What is the magnitude of the net torque on the block and direction of rotation, if any?

a) 44 Nm, clockwise

b) 44 Nm, counterclockwise

c) 88 Nm, clockwise

d) 88 Nm, counterclockwise

e) zero Nm, no rotation

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9.4.3. Consider the following three objects, each of the same mass and radius:

(1) a solid sphere (2) a solid disk (3) a hoop

All three are released from rest at the top of an inclined plane. The three objects proceed down the incline undergoing rolling motion without slipping. In which order do the objects reach the bottom of the incline?

a) 1, 2, 3

b) 2, 3, 1

c) 3, 1, 2

d) 3, 2, 1

e) All three reach the bottom at the same time.

9.4.4. A long board is free to rotate about the pivot shown in each of the four configurations shown. Weights are hung from the board as indicated. In which of the configurations, if any, is the net torque about the pivot axis the largest?

a) 1

b) 2

c) 3

d) 4

e) The net torque is the same for all four situations.

9.4.5. The drawing shows a yo-yo in contact with a tabletop. A string is wrapped around the central axle. How will the yo-yo behave if you pull on the string with the force shown?

a) The yo-yo will roll to the left.

b) The yo-yo will roll to the right.

c) The yo-yo will spin in place, but not roll.

d) The yo-yo will not roll, but it will move to the left.

e) The yo-yo will not roll, but it will move to the right.

Chapter 9:Rotational Dynamics

Section 5:

Rotational Work and Energy

sFW

rs

rF

W

Rotational Work

Fr

The rotational work done by a constant torque in turning an object through an angle is

RW

Requirement: The angle mustbe expressed in radians.

SI Unit of Rotational Work: joule (J)

DEFINITION OF ROTATIONAL WORK

2221 mrKE

221

TmvKE

rvT

Rotational Energy

2221 mr

2221 mr 2

21 I

221 IKER

The rotational kinetic energy of a rigid rotating object is

Requirement: The angular speed mustbe expressed in rad/s.

SI Unit of Rotational Kinetic Energy: joule (J)

DEFINITION OF ROTATIONAL KINETIC ENERGY

Example 13 Rolling Cylinders

A thin-walled hollow cylinder (mass = mh, radius = rh) anda solid cylinder (mass = ms, radius = rs) start from rest atthe top of an incline.

Determine which cylinder has the greatest translationalspeed upon reaching the bottom.

mghImvEtotal 2212

21

iiifff mghImvmghImv 2212

212

212

21

iff mghImv 2212

21

ENERGY CONSERVATION

rv ff

iff mghrvImv 22212

21

2

2

rIm

mghv o

f

The cylinder with the smaller moment of inertia will have a greater final translational speed.

9.5.1. Four objects start from rest and roll without slipping down a ramp. The objects are a solid sphere, a hollow cylinder, a solid cylinder, and a hollow sphere. Each of the objects has the same radius and the same mass, but they are made from different materials. Which object will have the greatest speed at the bottom of the ramp?

a) Since they are all starting from rest, all of the objects will have the same speed at the bottom as a result of the conservation of mechanical energy.

b) solid sphere

c) hollow cylinder

d) solid cylinder

e) hollow sphere

9.5.2. A bowling ball is rolling without slipping at constant speed toward the pins on a lane. What percentage of the ball’s total kinetic energy is translational kinetic energy?

a) 50 %

b) 71 %

c) 46 %

d) 29 %

e) 33 %

9.5.3. A hollow cylinder is rotating about an axis that passes through the center of both ends. The radius of the cylinder is r. At what angular speed must the this cylinder rotate to have the same total kinetic energy that it would have if it were moving horizontally with a speed v without rotation?

a)

b)

c)

d)

e)

2

2

v

r

2v

r

v

r

2

v

r

2

2

v

r

9.5.4. Two solid cylinders are rotating about an axis that passes through the center of both ends of each cylinder. Cylinder A has three times the mass and twice the radius of cylinder B, but they have the same rotational kinetic energy. What is the ratio of the angular velocities, A/B, for these two cylinders?

a) 0.25

b) 0.50

c) 1.0

d) 2.0

e) 4.0

9.5.5. Consider the drawing. A rope is wrapped around one-third of the circumference of a solid disk of radius R = 2.2 m that is free to rotate about an axis that passes through its center. The force applied to the rope has a magnitude of 35 N; and the disk has a mass M of 7.5 kg. Assuming the force is applied horizontally as shown and the disk is initially at rest, determine the amount of rotational work done until the time when the end of the rope reaches the top of the disk?

a) 140 N

b) 160 N

c) 180 N

d) 210 N

e) 250 N

Chapter 9:Rotational Dynamics

Section 6:

Angular Momentum

9.6 Angular Momentum

DEFINITION OF ANGULAR MOMENTUM

The angular momentum L of a body rotating about a fixed axis is the product of the body’s moment of inertia and its angular velocity with respect to thataxis:

IL

Requirement: The angular speed mustbe expressed in rad/s.

SI Unit of Angular Momentum: kg·m2/s

Translation vs Rotation

displacement

velocity

elapsed time

acceleration

x

v

t

a

t

inertia m I

Cause “a/” F

momentum p L

PRINCIPLE OF CONSERVATION OFANGULAR MOMENTUM

The angular momentum of a system remains constant (is conserved) if the net external torque acting on the system is zero.

Conceptual Example 14 A Spinning Skater

An ice skater is spinning with botharms and a leg outstretched. Shepulls her arms and leg inward andher spinning motion changesdramatically.

Use the principle of conservationof angular momentum to explainhow and why her spinning motionchanges.

Example 15 A Satellite in an Elliptical Orbit

An artificial satellite is placed in an elliptical orbit about the earth. Its pointof closest approach is 8.37x106mfrom the center of the earth, andits point of greatest distance is 25.1x106m from the center ofthe earth.

The speed of the satellite at the perigee is 8450 m/s. Find the speedat the apogee.

IL

angular momentum conservation

PPAA II

rv

P

PP

A

AA r

vmr

r

vmr 22

PPAA vrvr

A

PPA r

vrv

m 1025.1

sm8450m 1037.86

6

sm2820

2mrI

9.6.1. A star is rotating about an axis that passes through its center. When the star “dies,” the balance between the inward pressure due to the force of gravity and the outward pressure from nuclear processes is no longer present and the star collapses inward; and its radius decreases with time. Which one of the following choices best describes what happens as the star collapses?

a) The angular velocity of the star remains constant.

b) The angular momentum of the star remains constant.

c) The angular velocity of the star decreases.

d) The angular momentum of the star decreases.

e) Both angular momentum and angular velocity increase.

9.6.2. A solid sphere of radius R rotates about an axis that is tangent to the sphere with an angular speed . Under the action of internal forces, the radius of the sphere increases to 2R. What is the final angular speed of the sphere?

a) /4

b) /2

c)

d) 2

e) 4

9.6.3. While excavating the tomb of Tutankhamen (d. 1325 BC), archeologists found a sling made of linen. The sling could hold a stone in a pouch, which could then be whirled in a horizontal circle. The stone could then be thrown for hunting or used in battle. Imagine the sling held a 0.050-kg stone; and it was whirled at a radius of 1.2 m with an angular speed of 2.0 rev/s. What was the angular momentum of the stone under these circumstances?

a) 0.14 kg m2/s

b) 0.90 kg m2/s

c) 1.2 kg m2/s

d) 2.4 kg m2/s

e) 3.6 kg m2/s

9.6.4. Joe has volunteered to help out in his physics class by sitting on a stool that easily rotates. As Joe holds the dumbbells out as shown, the professor temporarily applies a sufficient torque that causes him to rotate slowly. Then, Joe brings the dumbbells close to his body and he rotates faster. Why does his speed increase?

a) By bringing the dumbbells inward, Joe exerts a torque on the stool.

b) By bringing the dumbbells inward, Joe decreases the moment of inertia.

c) By bringing the dumbbells inward, Joe increases the angular momentum.

d) By bringing the dumbbells inward, Joe increases the moment of inertia.

e) By bringing the dumbbells inward, Joe decreases the angular momentum.

9.6.5. Joe has volunteered to help out in his physics class by sitting on a stool that easily rotates. Joe holds the dumbbells out as shown as the stool rotates. Then, Joe drops both dumbbells. How does the rotational speed of stool change, if at all?

a) The rotational speed increases.

b) The rotational speed decreases, but Joe continues to rotate.

c) The rotational speed remains the same.

d) The rotational speed quickly decreases to zero rad/s.

9.6.6. Joe has volunteered to help out in his physics class by sitting on a stool that easily rotates. Joe holds the dumbbells out as shown as the stool rotates. Then, Joe drops both dumbbells. Then, the angular momentum of Joe and the stool changes, but the angular velocity does not change. Which of the following choice offers the best explanation?

a) The force exerted by the dumbbells acts in opposite direction to the torque.

b) Angular momentum is conserved, when no external forces are acting.

c) Even though the angular momentum decreases, the moment of inertia also decreases.

d) The decrease in the angular momentum is balanced by an increase in the moment of inertia.

e) The angular velocity must increase when the dumbbells are dropped.

9.6.7. Sarah has volunteered to help out in her physics class by sitting on a stool that easily rotates. The drawing below shows the view from above her head. She holds the dumbbells out as shown as the stool rotates. Then, she drops both dumbbells. Which one of the four trajectories illustrated best represents the motion of the dumbbells after they are dropped?

9.6.8. Two ice skaters are holding hands and spinning around their combined center of mass, represented by the small black dot in Frame 1, with an angular momentum L. When the skaters are at the position shown in Frame 2, theyrelease hands and movein opposite directions asshown in Frame 3. What is the angularmomentum of theskaters in Frame 3?

a) zero kg m2/s

b) a value that is greater than zero kg m2/s, but less than L

c) a value less than L and decreasing as they move further apart

d) a value that is greater than L

e) L