chapter vector mechanics for engineers: …web.boun.edu.tr/ozupek/me242/chapt15_lecture.pdf ·...

1

VECTOR MECHANICS FOR ENGINEERS:

DYNAMICS

Seventh Edition

Ferdinand P. BeerE. Russell Johnston, Jr.

Lecture Notes:J. Walt OlerTexas Tech University

CHAPTER

© 2003 The McGraw-Hill Companies, Inc. All rights reserved.

15Kinematics of

Rigid Bodies


Vector Mechanics for Engineers: Dynamics

SeventhEdition

15 - 2

ContentsIntroductionTranslationRotation About a Fixed Axis: VelocityRotation About a Fixed Axis: AccelerationRotation About a Fixed Axis:

Representative SlabEquations Defining the Rotation of a Rigid

Body About a Fixed AxisSample Problem 5.1General Plane MotionAbsolute and Relative Velocity in Plane

MotionSample Problem 15.2Sample Problem 15.3Instantaneous Center of Rotation in Plane

MotionSample Problem 15.4Sample Problem 15.5

Absolute and Relative Acceleration in Plane Motion

Analysis of Plane Motion in Terms of a Parameter

Sample Problem 15.6Sample Problem 15.7Sample Problem 15.8Rate of Change With Respect to a Rotating

FrameCoriolis AccelerationSample Problem 15.9Sample Problem 15.10Motion About a Fixed PointGeneral MotionSample Problem 15.11Three Dimensional Motion. Coriolis

AccelerationFrame of Reference in General MotionSample Problem 15.15

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Introduction• Kinematics of rigid bodies: relations between

time and the positions, velocities, and accelerations of the particles forming a rigid body.

• Classification of rigid body motions:

- general motion- motion about a fixed pointgeneral plane motion- rotation about a fixed axis

• curvilinear translation• rectilinear translation

- translation:



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Translation• Consider rigid body in translation:

- direction of any straight line inside the body is constant,

- all particles forming the body move in parallel lines.

• For any two particles in the body,ABAB rrr rrr +=

• Differentiating with respect to time,

AB

AABAB

vv

rrrrrr

&r&r&r&r

=

=+=

All particles have the same velocity.

AB

AABAB

aa

rrrrrr

&&r&&r&&r&&r

=

=+=• Differentiating with respect to time again,

All particles have the same acceleration.

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Rotation About a Fixed Axis. Velocity

• Consider rotation of rigid body about a fixed axis AA’

• Velocity vector of the particle P is tangent to the path with magnitude

dtrdv rr =dtdsv =

( ) ( )

( ) φθθφ

θφθ

sinsinlim

sin

0&r

tr

dtdsv

rBPs

t=

∆∆

==

∆=∆=∆

→∆

locityangular vekk

rdtrdv

===

×==r&

rr

rrr

r

θωω

ω

• The same result is obtained from



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Rotation About a Fixed Axis. Acceleration• Differentiating to determine the acceleration,

( )

vrdtd

dtrdr

dtd

rdtd

dtvda

rrrr

rrr

r

rvr

r

×+×=

×+×=

×==

ωω

ωω

ω

•

kkk

celerationangular acdtd

r&&

r&

r

rr

θωα

αω

===

==

componenton accelerati radial )(componenton accelerati l tangentia

)(

=××=×

××+×=

rr

rra

rrr

rr

rrrrrr

ωωα

ωωα

• Acceleration of P is combination of two vectors,

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Rotation About a Fixed Axis. Representative Slab• Consider the motion of a representative slab in

a plane perpendicular to the axis of rotation.

• Velocity of any point P of the slab,

ωωω

rvrkrv

=×=×= rrrrr

• Acceleration of any point P of the slab,

rrk

rrarrr

rrrrrr

2

)(

ωα

ωωα

−×=

××+×=

• Resolving the acceleration into tangential and normal components,

22 ωωααrara

rarka

nn

tt=−==×=

rr

rrr



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Equations Defining the Rotation of a Rigid Body About a Fixed Axis

• Motion of a rigid body rotating around a fixed axis is often specified by the type of angular acceleration.

θωωθωα

ωθθω

dd

dtd

dtd

ddtdtd

===

==

2

2

or• Recall

• Uniform Rotation, α = 0:tωθθ += 0

• Uniformly Accelerated Rotation, α = constant:

( )020

2

221

00

0

2 θθαωω

αωθθ

αωω

−+=

++=

+=

tt

t

5



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Sample Problem 5.1

Cable C has a constant acceleration of 9 in/s2 and an initial velocity of 12 in/s, both directed to the right.

Determine (a) the number of revolutions of the pulley in 2 s, (b) the velocity and change in position of the load B after 2 s, and (c) the acceleration of the point D on the rim of the inner pulley at t = 0.

SOLUTION:

• Due to the action of the cable, the tangential velocity and acceleration of D are equal to the velocity and acceleration of C. Calculate the initial angular velocity and acceleration.

• Apply the relations for uniformly accelerated rotation to determine the velocity and angular position of the pulley after 2 s.

• Evaluate the initial tangential and normal acceleration components of D.



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Sample Problem 5.1SOLUTION:• The tangential velocity and acceleration of D are equal to the

velocity and acceleration of C. ( ) ( )( )

( )srad4

312

sin.12

00

00

00

===

=

→==

rv

rvvv

D

D

CD

ω

ω

rr ( )( )

( ) 2srad339

sin.9

===

=

→==

ra

raaa

tD

tD

CtD

α

α

rr

• Apply the relations for uniformly accelerated rotation to determine velocity and angular position of pulley after 2 s.

( )( ) srad10s 2srad3srad4 20 =+=+= tαωω

( )( ) ( )( )rad14

s 2srad3s 2srad4 22212

21

0

=

+=+= tt αωθ

( ) revs ofnumber rad 2

rev 1rad 14 =⎟⎠⎞

⎜⎝⎛=π

N rev23.2=N

( )( )( )( )rad 14in. 5

srad10in. 5==∆==θ

ωry

rv

B

B

in. 70sin.50

=∆↑=

B

By

vr

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Sample Problem 5.1• Evaluate the initial tangential and normal acceleration

components of D.( ) →== sin.9CtD aa rr

( ) ( )( ) 2220 sin48srad4in. 3 === ωDnD ra

( ) ( ) ↓=→= 22 sin.48sin.9 nDtD aa rr

Magnitude and direction of the total acceleration,

( ) ( )22

22

489 +=

+= nDtDD aaa2sin.8.48=Da

( )( )

948

tan

=

=tD

nDaa

φ

°= 4.79φ



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General Plane Motion

• General plane motion is neither a translation nor a rotation.

• General plane motion can be considered as the sum of a translation and rotation.

• Displacement of particles A and B to A2 and B2can be divided into two parts: - translation to A2 and- rotation of about A2 to B2

1B′1B′

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Absolute and Relative Velocity in Plane Motion

• Any plane motion can be replaced by a translation of an arbitrary reference point A and a simultaneous rotation about A.

ABAB vvv rrr +=

ωω rvrkv ABABAB =×= rrr

ABAB rkvv rrrr ×+= ω



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• Assuming that the velocity vA of end A is known, wish to determine the velocity vB of end B and the angular velocity ω in terms of vA, l, and θ.

• The direction of vB and vB/A are known. Complete the velocity diagram.

θ

θ

tan

tan

AB

A

B

vvvv

=

=

θω

θω

cos

cos

lv

lv

vv

A

A

AB

A

=

==

8



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• Selecting point B as the reference point and solving for the velocity vA of end Aand the angular velocity ω leads to an equivalent velocity triangle.

• vA/B has the same magnitude but opposite sense of vB/A. The sense of the relative velocity is dependent on the choice of reference point.

• Angular velocity ω of the rod in its rotation about B is the same as its rotation about A. Angular velocity is not dependent on the choice of reference point.



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Sample Problem 15.2

The double gear rolls on the stationary lower rack: the velocity of its center is 1.2 m/s.

Determine (a) the angular velocity of the gear, and (b) the velocities of the upper rack R and point D of the gear.

SOLUTION:

• The displacement of the gear center in one revolution is equal to the outer circumference. Relate the translational and angular displacements. Differentiate to relate the translational and angular velocities.

• The velocity for any point P on the gear may be written as

Evaluate the velocities of points B and D.

APAAPAP rkvvvv rrrrrr ×+=+= ω

9



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Sample Problem 15.2

x

y

SOLUTION:

• The displacement of the gear center in one revolution is equal to the outer circumference.

For xA > 0 (moves to right), ω < 0 (rotates clockwise).

θπθ

π 122rx

rx

AA −=−=

Differentiate to relate the translational and angular velocities.

m0.150sm2.1

1

1

−=−=

−=

rvrv

A

A

ω

ω

( )kkrrr srad8−==ωω



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Sample Problem 15.2• For any point P on the gear, APAAPAP rkvvvv rrrrrr ×+=+= ω

Velocity of the upper rack is equal to velocity of point B:

( ) ( ) ( )( ) ( )ii

jki

rkvvv ABABR

rr

rrr

rrrrr

sm8.0sm2.1m 10.0srad8sm2.1

+=

×+=

×+== ω

( )ivRrr sm2=

Velocity of the point D:

( ) ( ) ( )iki

rkvv ADADrrr

rrrr

m 150.0srad8sm2.1 −×+=

×+= ω

( ) ( )sm697.1

sm2.1sm2.1=

+=

D

Dv

jivrrr

10



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• Absolute acceleration of a particle of the slab,

ABAB aaa rrr +=

• Relative acceleration associated with rotation about A includes tangential and normal components,

ABar

( )( ) ABnAB

ABtAB

ra

rkarr

rrr

2ω

α

−=

×= ( )( ) 2ω

α

ra

ra

nAB

tAB

=

=



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• Givendetermine

, and AA va rr

. and αrrBa

( ) ( )tABnABA

ABAB

aaa

aaarrr

rrr

++=

+=

• Vector result depends on sense of and the relative magnitudes of ( )

nABA aa and Aar

• Must also know angular velocity ω.

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+→ x components: θαθω cossin0 2 llaA −+=

↑+ y components: θαθω sincos2 llaB −−=−

• Solve for aB and α.

• Write in terms of the two component equations,ABAB aaa rrr +=



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Analysis of Plane Motion in Terms of a Parameter• In some cases, it is advantageous to determine the

absolute velocity and acceleration of a mechanism directly.

θsinlxA = θcoslyB =

θωθθ

coscos

llxv AA

==

=&

&

θωθθ

sinsin

ll

yv BB

−=−=

=&

&

θαθω

θθθθ

cossin

cossin2

2

ll

ll

xa AA

+−=

+−=

=

&&&

&&

θαθω

θθθθ

sincos

sincos2

2

ll

ll

ya BB

−−=

−−=

=

&&&

&&

12



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Instantaneous Center of Rotation in Plane Motion

• Plane motion of all particles in a slab can always be replaced by the translation of an arbitrary point A and a rotation about A with an angular velocity that is independent of the choice of A.

• The same translational and rotational velocities at A are obtained by allowing the slab to rotate with the same angular velocity about the point C on a perpendicular to the velocity at A.

• The velocity of all other particles in the slab are the same as originally defined since the angular velocity and translational velocity at A are equivalent.

• As far as the velocities are concerned, the slab seems to rotate about the instantaneous center of rotation C.



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Instantaneous Center of Rotation in Plane Motion• If the velocity at two points A and B are known, the

instantaneous center of rotation lies at the intersection of the perpendiculars to the velocity vectors through Aand B .

• If the velocity vectors at A and B are perpendicular to the line AB, the instantaneous center of rotation lies at the intersection of the line AB with the line joining the extremities of the velocity vectors at A and B.

• If the velocity vectors are parallel, the instantaneous center of rotation is at infinity and the angular velocity is zero.

• If the velocity magnitudes are equal, the instantaneous center of rotation is at infinity and the angular velocity is zero.

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Instantaneous Center of Rotation in Plane Motion• The instantaneous center of rotation lies at the intersection of

the perpendiculars to the velocity vectors through A and B .

θω

coslv

ACv AA == ( ) ( )

θθ

θω

tancos

sin

A

AB

vl

vlBCv

=

==

• The velocities of all particles on the rod are as if they were rotated about C.

• The particle at the center of rotation has zero velocity.

• The particle coinciding with the center of rotation changes with time and the acceleration of the particle at the instantaneous center of rotation is not zero.

• The acceleration of the particles in the slab cannot be determined as if the slab were simply rotating about C.

• The trace of the locus of the center of rotation on the body is the body centrode and in space is the space centrode.



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Sample Problem 15.4

The double gear rolls on the stationary lower rack: the velocity of its center is 1.2 m/s.

Determine (a) the angular velocity of the gear, and (b) the velocities of the upper rack R and point D of the gear.

SOLUTION:

• The point C is in contact with the stationary lower rack and, instantaneously, has zero velocity. It must be the location of the instantaneous center of rotation.

• Determine the angular velocity about C based on the given velocity at A.

• Evaluate the velocities at B and D based on their rotation about C.

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Sample Problem 15.4SOLUTION:• The point C is in contact with the stationary lower rack

and, instantaneously, has zero velocity. It must be the location of the instantaneous center of rotation.

• Determine the angular velocity about C based on the given velocity at A.

srad8m 0.15sm2.1====

A

AAA r

vrv ωω

• Evaluate the velocities at B and D based on their rotation about C.

( )( )srad8m 25.0=== ωBBR rvv( )ivR

rr sm2=

( )( )( )srad8m 2121.0

m 2121.02m 15.0==

==ωDD

Drv

r

( )( )sm2.12.1sm697.1

jivv

D

Drrr +=

=



SeventhEdition

15 - 28

Sample Problem 15.5

The crank AB has a constant clockwise angular velocity of 2000 rpm.

For the crank position indicated, determine (a) the angular velocity of the connecting rod BD, and (b) the velocity of the piston P.

SOLUTION:

• Determine the velocity at B from the given crank rotation data.

• The direction of the velocity vectors at B and D are known. The instantaneous center of rotation is at the intersection of the perpendiculars to the velocities through B and D.

• Determine the angular velocity about the center of rotation based on the velocity at B.

• Calculate the velocity at D based on its rotation about the instantaneous center of rotation.

15



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Sample Problem 15.5SOLUTION:• From Sample Problem 15.3,

( )( )°=

=−=95.13

sin.3.628sin.3.4819.403β

BB vjivrrr

• The instantaneous center of rotation is at the intersection of the perpendiculars to the velocities through B and D.

°=−°=°=+°=

05.769095.5340

βγβγ

D

B

°=

°=

° sin50in. 8

95.53sin05.76sinCDBC

in. 44.8in. 14.10 == CDBC

• Determine the angular velocity about the center of rotation based on the velocity at B.

( )

in.10.14sin.3.628

==

=

BCv

BCv

BBD

BDB

ω

ω

• Calculate the velocity at D based on its rotation about the instantaneous center of rotation.

( ) ( )( )srad0.62in. 44.8== BDD CDv ω

sft6.43sin.523 === DP vv

srad0.62=BDω



SeventhEdition

15 - 30

Sample Problem 15.6

The center of the double gear has a velocity and acceleration to the right of 1.2 m/s and 3 m/s2, respectively. The lower rack is stationary.

Determine (a) the angular acceleration of the gear, and (b) the acceleration of points B, C, and D.

SOLUTION:

• The expression of the gear position as a function of θ is differentiated twice to define the relationship between the translational and angular accelerations.

• The acceleration of each point on the gear is obtained by adding the acceleration of the gear center and the relative accelerations with respect to the center. The latter includes normal and tangential acceleration components.

16



SeventhEdition

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Sample Problem 15.6SOLUTION:• The expression of the gear position as a function of θ

is differentiated twice to define the relationship between the translational and angular accelerations.

ωθθ

11

1

rrvrx

A

A

−=−=

−=&

srad 8m 0.150sm2.1

1−=−=−=

rvAω

αθ 11 rraA −=−= &&

m 150.0sm3 2

1−=−=

raAα

( )kkrrr 2srad20−==αα



SeventhEdition

15 - 32

Sample Problem 15.6

( ) ( )

( ) ( ) ( ) ( ) ( )( ) ( ) ( )jii

jjki

rrka

aaaaaa

ABABA

nABtABAABAB

rrr

rrrr

rrrr

rrrrrr

222

222

2

sm40.6sm2sm3

m100.0srad8m100.0srad20sm3

−+=

−−×−=

−×+=

++=+=

ωα

( ) ( ) 222 sm12.8sm40.6m5 =−= BB ajisarrr

• The acceleration of each point is obtained by adding the acceleration of the gear center and the relative accelerations with respect to the center.

The latter includes normal and tangential acceleration components.

17



SeventhEdition

15 - 33

Sample Problem 15.6

( ) ( ) ( ) ( ) ( )( ) ( ) ( )jii

jjki

rrkaaaa ACACAACAC

rrr

rrrr

rrrrrrr

222

222

2

sm60.9sm3sm3


+−=

−−−×−=

−×+=+= ωα

( )jacrr 2sm60.9=

( ) ( ) ( ) ( ) ( )( ) ( ) ( )iji

iiki

rrkaaaa ADADAADAD

rrr

rrrr

rrrrrrr

222

222

2

sm60.9sm3sm3


++=

−−−×−=

−×+=+= ωα

( ) ( ) 222 sm95.12sm3m6.12 =+= DD ajisarrr



SeventhEdition

15 - 34

Sample Problem 15.3

The crank AB has a constant clockwise angular velocity of 2000 rpm.

For the crank position indicated, determine (a) the angular velocity of the connecting rod BD, and (b) the velocity of the piston P.

SOLUTION:

• Will determine the absolute velocity of point D with

BDBD vvv rrr +=

• The velocity is obtained from the given crank rotation data.

Bvr

• The directions of the absolute velocity and the relative velocity are determined from the problem geometry.

Dvr

BDvr

• The unknowns in the vector expression are the velocity magnitudeswhich may be determined from the corresponding vector triangle.

BDD vv and

• The angular velocity of the connecting rod is calculated from .BDv

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SeventhEdition

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Sample Problem 15.3SOLUTION:• Will determine the absolute velocity of point D with

BDBD vvv rrr +=

• The velocity is obtained from the crank rotation data. Bvr

( ) ( )( )srad 4.209in.3

srad 4.209rev

rad2s60

minminrev2000

==

=⎟⎠⎞

⎜⎝⎛⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛=

ABB

AB

ABv ω

πω

The velocity direction is as shown.

• The direction of the absolute velocity is horizontal. The direction of the relative velocity is perpendicular to BD. Compute the angle between the horizontal and the connecting rod from the law of sines.

Dvr

BDvr

°==° 95.13

in.3sin

in.840sin ββ



SeventhEdition

15 - 36

Sample Problem 15.3

• Determine the velocity magnitudes from the vector triangle.

BDD vv and

BDBD vvv rrr +=

°=

°=

° sin76.05sin.3.628

50sin95.53sinBDD vv

sin.9.495sft6.43sin.4.523

===

BD

Dvv

srad 0.62in. 8

sin.9.495

=

==

=

lv

lv

BDBD

BDBD

ω

ω

sft6.43== DP vv

( )kBDrr srad 0.62=ω

19



SeventhEdition

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Sample Problem 15.7

Crank AG of the engine system has a constant clockwise angular velocity of 2000 rpm.

For the crank position shown, determine the angular acceleration of the connecting rod BD and the acceleration of point D.

SOLUTION:

• The angular acceleration of the connecting rod BD and the acceleration of point D will be determined from

( ) ( )nBDtBDBBDBD aaaaaa rrrrrr ++=+=

• The acceleration of B is determined from the given rotation speed of AB.

• The directions of the accelerationsare

determined from the geometry. ( ) ( )

nBDtBDD aaa rrr and,,

• Component equations for acceleration of point D are solved simultaneously for acceleration of D and angular acceleration of the connecting rod.



SeventhEdition

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Sample Problem 15.7

• The acceleration of B is determined from the given rotation speed of AB.

SOLUTION:• The angular acceleration of the connecting rod BD and

the acceleration of point D will be determined from ( ) ( )

nBDtBDBBDBD aaaaaa rrrrrr ++=+=

( )( ) 221232

AB

sft962,10srad4.209ft

0constantsrad209.4rpm2000

===

====

ABB

AB

ra ω

αω

( )( )jiaBrrr °−°−= 40sin40cossft962,10 2

20



SeventhEdition

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Sample Problem 15.7

• The directions of the accelerations are determined from the geometry.

( ) ( )nBDtBDD aaa rrr and,,

From Sample Problem 15.3, ωBD = 62.0 rad/s, β = 13.95o.

( ) ( ) ( )( ) 221282 sft2563srad0.62ft === BDnBD BDa ω

( ) ( )( )jianBD

rrr °+°−= 95.13sin95.13cossft2563 2

( ) ( ) ( ) BDBDBDtBD BDa ααα 667.0ft128 ===

The direction of (aD/B)t is known but the sense is not known,

( ) ( )( )jia BDtBDrrr °±°±= 05.76cos05.76sin667.0 α

iaa DDr

mr =



SeventhEdition

15 - 40

Sample Problem 15.7

( ) ( )nBDtBDBBDBD aaaaaa rrrrrr ++=+=

• Component equations for acceleration of point D are solved simultaneously.

x components:°+°−°−=− 95.13sin667.095.13cos256340cos962,10 BDDa α

°+°+°−= 95.13cos667.095.13sin256340sin962,100 BDα

y components:

( )( )ia

k

D

BDrr

rr

2

2

sft9290

srad9940

−=

=α

21



SeventhEdition

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Sample Problem 15.8

In the position shown, crank AB has a constant angular velocity ω1 = 20 rad/scounterclockwise.

Determine the angular velocities and angular accelerations of the connecting rod BD and crank DE.

SOLUTION:

• The angular velocities are determined by simultaneously solving the component equations for

BDBD vvv rrr +=

• The angular accelerations are determined by simultaneously solving the component equations for

BDBD aaa rrr +=



SeventhEdition

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Sample Problem 15.8SOLUTION:• The angular velocities are determined by simultaneously

solving the component equations forBDBD vvv rrr +=

( )ji

jikrv

DEDE

DEDDEDrr

rrrrrr

ωωωω1717

1717−−=

+−×=×=

( )ji

jikrv BABBrr

rrrrrr

16028014820

+−=

+×=×=ω

( )ji

jikrv

BDBD

BDBDBDBDrr

rrrrrr

ωω

ωω

123

312

+−=

+×=×=

BDDE ωω 328017 −−=−x components:

BDDE ωω 1216017 ++=−y components:

( ) ( )kk DEBDrrrr srad29.11srad33.29 =−= ωω

22



SeventhEdition

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Sample Problem 15.8• The angular accelerations are determined by

simultaneously solving the component equations forBDBD aaa rrr +=

( ) ( ) ( )jiji

jijik

rra

DEDE

DE

DDEDDED

rrrr

rrrrr

rrrr

217021701717171729.111717 2

2

−+−−=

+−−+−×=

−×=

ααα

ωα

( ) ( )ji

jirra BABBABBrr

rrrrrr

56003200148200 22

+−=

+−=−×= ωα

( ) ( ) ( )jiji

jijik

rra

DBDB

DB

DBBDDBBDBD

rrrr

rrrrr

rrrr

2580320,10123

31233.29312 2

2

−−+−=

+−+×=

−×=

αα

α

ωα

x components: 690,15317 −=+− BDDE αα

y components: 60101217 −=−− BDDE αα

( ) ( )kk DEBDrrrr 22 srad809srad645 =−= αα



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Motion About a Fixed Point• The most general displacement of a rigid body with a

fixed point O is equivalent to a rotation of the body about an axis through O.

• With the instantaneous axis of rotation and angular velocity the velocity of a particle P of the body is,ωr

rdtrdv rrr

r ×== ω

and the acceleration of the particle P is

( ) .dtdrra ωαωωαr

rrrrrrr =××+×=

• Angular velocities have magnitude and direction and obey parallelogram law of addition. They are vectors.

• As the vector moves within the body and in space, it generates a body cone and space cone which are tangent along the instantaneous axis of rotation.

ωr

• The angular acceleration represents the velocity of the tip of .ωr

αr

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General Motion• For particles A and B of a rigid body,

ABAB vvv rrr +=

• Particle A is fixed within the body and motion of the body relative to AX’Y’Z’ is the motion of a body with a fixed point

ABAB rvv rrrr ×+= ω

• Similarly, the acceleration of the particle P is

( )ABABA

ABAB

rra

aaarrrrrr

rrr

××+×+=

+=

ωωα

• Most general motion of a rigid body is equivalent to: - a translation in which all particles have the same

velocity and acceleration of a reference particle A, and - of a motion in which particle A is assumed fixed.

chapter vector mechanics for engineers: …web.boun.edu.tr/ozupek/me242/chapt15_lecture.pdf ·...

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