dynamics-ch1
-
Upload
miguel-a-chavez -
Category
Documents
-
view
5 -
download
2
Transcript of dynamics-ch1
DynaDEn
DynaDEn
DynaDEn
X(t) ∫∑∑=
=
∞=
=→∆
=
=
=∆=∆≈2
0101)()()( lim
tt
t
i
it
ni
idttFttFttF
t∆
F(t)
t
F(t)
DynaDEn
DynaDEn
DynaDEn
DynaDEn
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
7 - 49
Rectilinear Motion: Position, Velocity & Acceleration
• Consider particle with motion given by326 ttx −=
2312 ttdtdxv −==
tdt
xddtdva 6122
2−===
• at t = 0, x = 0, v = 0, a = 12 m/s2
• at t = 2 s, x = 16 m, v = vmax = 12 m/s, a = 0
• at t = 4 s, x = xmax = 32 m, v = 0, a = -12 m/s2
• at t = 6 s, x = 0, v = -36 m/s, a = 24 m/s2
DynaDEn
DynaDEn
DynaDEn
A=const
V=at+v0
X=1/2at2+v0t+x0
V2- v02=2as, s=x - x0
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
11 - 49
Sample Problem 11.2
Determine:• velocity and elevation above ground at
time t, • highest elevation reached by ball and
corresponding time, and • time when ball will hit the ground and
corresponding velocity.
Ball tossed with 10 m/s vertical velocity from window 20 m above ground.
SOLUTION:
• Integrate twice to find v(t) and y(t).
• Solve for t at which velocity equals zero (time for maximum elevation) and evaluate corresponding altitude.
• Solve for t at which altitude equals zero (time for ground impact) and evaluate corresponding velocity.
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
12 - 49
Sample Problem 11.2
( )( ) tvtvdtdv
adtdv
ttv
v81.981.9
sm81.9
00
2
0
−=−−=
−==
∫∫
( ) ttv
−= 2s
m81.9sm10
( )( ) ( ) 2
21
00
81.91081.910
81.910
0
ttytydttdy
tvdtdy
tty
y−=−−=
−==
∫∫
( ) 22s
m905.4sm10m20 ttty
−
+=
SOLUTION:• Integrate twice to find v(t) and y(t).
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
13 - 49
Sample Problem 11.2• Solve for t at which velocity equals zero and evaluate
corresponding altitude.
( ) 0sm81.9
sm10 2 =
−= ttv
s019.1=t
• Solve for t at which altitude equals zero and evaluate corresponding velocity.
( )
( ) ( )22
22
s019.1sm905.4s019.1
sm10m20
sm905.4
sm10m20
−
+=
−
+=
y
ttty
m1.25=y
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
14 - 49
Sample Problem 11.2• Solve for t at which altitude equals zero and
evaluate corresponding velocity.
( ) 0sm905.4
sm10m20 2
2 =
−
+= ttty
( )s28.3
smeaningles s243.1=−=
tt
( )
( ) ( )s28.3sm81.9
sm10s28.3
sm81.9
sm10
2
2
−=
−=
v
ttv
sm2.22−=v
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
15 - 49
Sample Problem 11.3
Brake mechanism used to reduce gun recoil consists of piston attached to barrel moving in fixed cylinder filled with oil. As barrel recoils with initial velocity v0, piston moves and oil is forced through orifices in piston, causing piston and cylinder to decelerate at rate proportional to their velocity.
Determine v(t), x(t), and v(x).
kva −=
SOLUTION:
• Integrate a = dv/dt = -kv to find v(t).
• Integrate v(t) = dx/dt to find x(t).
• Integrate a = v dv/dx = -kv to find v(x).
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
16 - 49
Sample Problem 11.3SOLUTION:
• Integrate a = dv/dt = -kv to find v(t).( ) ( ) kt
vtvdtk
vdvkv
dtdva
ttv
v−=−=−== ∫∫
00ln
0
( ) ktevtv −= 0
• Integrate v(t) = dx/dt to find x(t).
( )
( )( )
tkt
tkt
tx
kt
ek
vtxdtevdx
evdtdxtv
00
00
0
0
1
−==
==
−−
−
∫∫
( ) ( )ktekvtx −−= 10
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
17 - 49
Sample Problem 11.3• Integrate a = v dv/dx = -kv to find v(x).
kxvv
dxkdvdxkdvkvdxdvva
xv
v
−=−
−=−=−== ∫∫
0
00
kxvv −= 0
• Alternatively,
( ) ( )
−=
0
0 1v
tvkvtx
kxvv −= 0
( ) ( )0
0 or v
tveevtv ktkt == −−
( ) ( )ktekvtx −−= 10with
and
then
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
18 - 49
Uniform Rectilinear Motion
For particle in uniform rectilinear motion, the acceleration is zero and the velocity is constant.
vtxxvtxx
dtvdx
vdtdx
tx
x
+==−
=
==
∫∫
0
0
00
constant
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
19 - 49
Uniformly Accelerated Rectilinear MotionFor particle in uniformly accelerated rectilinear motion, the acceleration of
the particle is constant.
atvv
atvvdtadvadtdv tv
v
+=
=−=== ∫∫
0
000
constant
( )
221
00
221
000
000
attvxx
attvxxdtatvdxatvdtdx tx
x
++=
+=−+=+= ∫∫
( ) ( )
( )020
2
020
221
2
constant00
xxavv
xxavvdxadvvadxdvv
x
x
v
v
−+=
−=−=== ∫∫
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
20 - 49
Motion of Several Particles: Relative Motion• For particles moving along the same line, time
should be recorded from the same starting instant and displacements should be measured from the same origin in the same direction.
=−= ABAB xxx relative position of Bwith respect to A
ABAB xxx +=
=−= ABAB vvv relative velocity of Bwith respect to A
ABAB vvv +=
=−= ABAB aaa relative acceleration of Bwith respect to A
ABAB aaa +=
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
21 - 49
Sample Problem 11.4
Ball thrown vertically from 12 m level in elevator shaft with initial velocity of 18 m/s. At same instant, open-platform elevator passes 5 m level moving upward at 2 m/s.
Determine (a) when and where ball hits elevator and (b) relative velocity of ball and elevator at contact.
SOLUTION:
• Substitute initial position and velocity and constant acceleration of ball into general equations for uniformly accelerated rectilinear motion.
• Substitute initial position and constant velocity of elevator into equation for uniform rectilinear motion.
• Write equation for relative position of ball with respect to elevator and solve for zero relative position, i.e., impact.
• Substitute impact time into equation for position of elevator and relative velocity of ball with respect to elevator.
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
22 - 49
Sample Problem 11.4SOLUTION:• Substitute initial position and velocity and constant
acceleration of ball into general equations for uniformly accelerated rectilinear motion.
22
221
00
20
sm905.4
sm18m12
sm81.9
sm18
ttattvyy
tatvv
B
B
−
+=++=
−=+=
• Substitute initial position and constant velocity of elevator into equation for uniform rectilinear motion.
ttvyy
v
EE
E
+=+=
=
sm2m5
sm2
0
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
23 - 49
Sample Problem 11.4• Write equation for relative position of ball with respect to
elevator and solve for zero relative position, i.e., impact.
( ) ( ) 025905.41812 2 =+−−+= ttty EB
( )s65.3
smeaningles s39.0=−=
tt
• Substitute impact time into equations for position of elevator and relative velocity of ball with respect to elevator.
( )65.325+=Eym3.12=Ey
( )( )65.381.916
281.918
−=
−−= tv EB
sm81.19−=EBv
DynaDEn
PROBLEM 11.14 The acceleration of a particle is defined by the relation 20.15 m/s .a = Knowing that 10x = − m when 0t = and 0.15= −υ m/s when 2t = s, determine the velocity, the position, and the total distance traveled when
5t = s.
SOLUTION
Determine velocity. 0.15 2 2 0.15v t tdv a dt dt− = =∫ ∫ ∫
( ) ( )( )15 0.15 0.15 2v t− − = −
0.15 0.45 m/sv t= −
At 5 s,t = ( )( )5 0.15 5 0.45v = − 5 0.300 m/s v =
When 0,v = 0.15 0.45 0 3.00 st t− = = For 0 3.00 s,t≤ ≤ 0, is decreasing.v x≤ For 3.00 5 s,t≤ ≤ 0, is increasing.v x≥
Determine position. ( )10 0 0 0.15 0.45x t tdx v dt t dt− = = −∫ ∫ ∫
( ) ( )2 2
010 0.075 0.45 0.075 0.45
tx t t t t− − = − = −
20.075 0.45 10 mx t t= − −
At 5 s,t = ( )( ) ( )( )25 0.075 5 0.45 5 10 10.375 mx = − − = −
5 10.38 m x = −
At 0,t = 0 10 m (given)x = −
At 3.00 s,t = ( )( ) ( )( )23 min 0.075 3.00 0.45 3.00 10 10.675 mmx x= = − − = −
Distances traveled: Over 0 3.00 s,t≤ ≤ 1 0 min 0.675 md x x= − =
Over 3.00 s 5 s,t< < 2 5 min 0.300 md x x= − =
Total distance traveled: 1 2 0.975 m d d d= + =
DynaDEn
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
20 - 49
Motion of Several Particles: Relative Motion• For particles moving along the same line, time
should be recorded from the same starting instant and displacements should be measured from the same origin in the same direction.
=−= ABAB xxx relative position of Bwith respect to A
ABAB xxx +=
=−= ABAB vvv relative velocity of Bwith respect to A
ABAB vvv +=
=−= ABAB aaa relative acceleration of Bwith respect to A
ABAB aaa +=
DynaDEn
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
21 - 49
Sample Problem 11.4
Ball thrown vertically from 12 m level in elevator shaft with initial velocity of 18 m/s. At same instant, open-platform elevator passes 5 m level moving upward at 2 m/s.
Determine (a) when and where ball hits elevator and (b) relative velocity of ball and elevator at contact.
SOLUTION:
• Substitute initial position and velocity and constant acceleration of ball into general equations for uniformly accelerated rectilinear motion.
• Substitute initial position and constant velocity of elevator into equation for uniform rectilinear motion.
• Write equation for relative position of ball with respect to elevator and solve for zero relative position, i.e., impact.
• Substitute impact time into equation for position of elevator and relative velocity of ball with respect to elevator.
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
22 - 49
Sample Problem 11.4SOLUTION:• Substitute initial position and velocity and constant
acceleration of ball into general equations for uniformly accelerated rectilinear motion.
22
221
00
20
sm905.4
sm18m12
sm81.9
sm18
ttattvyy
tatvv
B
B
−
+=++=
−=+=
• Substitute initial position and constant velocity of elevator into equation for uniform rectilinear motion.
ttvyy
v
EE
E
+=+=
=
sm2m5
sm2
0
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
23 - 49
Sample Problem 11.4• Write equation for relative position of ball with respect to
elevator and solve for zero relative position, i.e., impact.
( ) ( ) 025905.41812 2 =+−−+= ttty EB
( )s65.3
smeaningles s39.0=−=
tt
• Substitute impact time into equations for position of elevator and relative velocity of ball with respect to elevator.
( )65.325+=Eym3.12=Ey
( )( )65.381.916
281.918
−=
−−= tv EB
sm81.19−=EBv
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
11 - 25
Sample Problem 11.5
Pulley D is attached to a collar which is pulled down at 75 mm/s. At t = 0, collar A starts moving down from Kwith constant acceleration and zero initial velocity. Knowing that velocity of collar A is 300 mm/s as it passes L, determine the change in elevation, velocity, and acceleration of block B when block A is at L.
SOLUTION:
• Define origin at upper horizontal surface with positive displacement downward.
• Collar A has uniformly accelerated rectilinear motion. Solve for acceleration and time t to reach L.
• Pulley D has uniform rectilinear motion. Calculate change of position at time t.
• Block B motion is dependent on motions of collar A and pulley D. Write motion relationship and solve for change of block B position at time t.
• Differentiate motion relation twice to develop equations for velocity and acceleration of block B.
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
11 - 26
Sample Problem 11.5SOLUTION:• Define origin at upper horizontal surface with
positive displacement downward.
• Collar A has uniformly accelerated rectilinear motion. Solve for acceleration and time t to reach L.
( ) ( )[ ]( ) ( ) 22
020
2
s/mm225mm2002mm300
2
==
−+=
AA
AAAAA
aa
xxavv
( )s 333.1s/mm 225mm300 2
0
==
+=
t
tavv AAA
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
11 - 27
Sample Problem 11.5• Pulley D has uniform rectilinear motion. Calculate
change of position at time t.
( )( ) ( )( ) mm 100s333.1s/mm 750
0
==−
+=
DD
DDD
xxtvxx
• Block B motion is dependent on motions of collar A and pulley D. Write motion relationship and solve for change of block B position at time t.
Total length of cable remains constant,
( ) ( ) ( )( )[ ] ( )[ ] ( )[ ]
( ) ( ) ( )[ ] 0mm1002mm20002
22
0
000
000
=−++
=−+−+−
++=++
BB
BBDDAA
BDABDA
xxxxxxxx
xxxxxx
( ) mm4000 −=− BB xx
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
11 - 28
Sample Problem 11.5• Differentiate motion relation twice to develop
equations for velocity and acceleration of block B.
( ) ( ) 0mm752mm30002constant2
=++=++=++
B
BDA
BDA
vvvvxxx
s/mm 450=Bv
( ) 0s/mm 22502=+
=++
B
BDA
vaaa
2s/mm 225−=Ba
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.47 and P11.48
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.49 and P11.50
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.51 and P11.52
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.53 and P11.54
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.55 and P11.56
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.57 and P11.58
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.59 and P11.60
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.127
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.128
DynaDEn
PROBLEM 11.49
Block C starts from rest and moves downward with a constant acceleration. Knowing that after 12 s the velocity of block A is 456 mm/s, determine (a) the accelerations of A, B, and C, (b) the velocity and the change in position of block B after 8 s.
SOLUTION
Let x be position relative to the fixed supports, taken positive if downward.
Constraint of cable on left: 2 3 constantA Bx x+ =
2 22 3 0, or , and 3 3A B B A B Av v v v a a+ = = − = −
Constraint of cable on right: 2 constantB Cx x+ =
1 1 12 0, or , and 2 3 3B C C B A C Av v v v v a a+ = = − = =
Block C moves downward; hence, block A also moves downward.
(a) Accelerations.
( ) ( ) 200
456 0 or 38.0 mm/s12
A AA A A A
v vv v a T a
t− −
= + = = =
238.0 mm/sA =a
( ) 22 2 38.0 25.3 mm/s3 3B Aa a = − = − = −
225.3 mm/sB =a
( ) 21 1 38.0 12.67 mm/s3 3C Aa a = = =
212.67 mm/sC =a
(b) Velocity and change in position of B after 8 s.
( ) ( )( )0 0 25.3 8 203 mm/sB B Bv v a t= + = + − = −
203 mm/sB =v
( ) ( ) ( )( )220 0
1 10 25.3 8 811 mm2 2B B B Bx x v t a t− = + = + − = −
811 mmBx∆ =
DynaDEn
DynaDEn
DynaDEn
DynaDEn
DynaDEn
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
37 - 49
Rectangular Components of Velocity & Acceleration• Rectangular components particularly effective
when component accelerations can be integrated independently, e.g., motion of a projectile,
00 ==−==== zagyaxa zyx &&&&&&
with initial conditions,( ) ( ) ( ) 0,,0 000000 ==== zyx vvvzyx
Integrating twice yields
( ) ( )( ) ( ) 0
02
21
00
00
=−==
=−==
zgtyvytvx
vgtvvvv
yx
zyyxx
• Motion in horizontal direction is uniform.
• Motion in vertical direction is uniformly accelerated.
• Motion of projectile could be replaced by two independent rectilinear motions.
DynaDEn
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sample Problem 11.7a
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sample Problem 11.7b
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sample Problem 11.7c
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sample Problem 11.8a
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sample Problem 11.8b
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sample Problem 11.8c
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Sample Problem 11.8d
DynaDEn
DynaDEn
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.97
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.98
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.99
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.101
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.104
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.105
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.106
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.109
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.111
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.112
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.114
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.115
DynaDEn
PROBLEM 11.97
A baseball pitching machine “throws” baseballs with a horizontal velocity 0v . Knowing that height h varies between 788 mm and 1068 mm, determine (a) the range of values of 0,v (b) the values of α corresponding to 788h = mm and 1068h = mm.
SOLUTION
(a) Vertical motion: 0 1.5 m,y = ( )0 0yv =
( ) ( )020 0
21 or 2y
y yy y v t gt t
g−
= + − =
At point B, ( )02 or B
y hy h t
g−
= =
When 788 mm 0.788 m,h = = ( )( )2 1.5 0.7880.3810 s
9.81Bt−
= =
When 1068 mm 1.068 m,h = = ( )( )2 1.5 1.0680.2968 s
9.81Bt−
= =
Horizontal motion: ( )0 000, ,xx v v= =
0 0 or B
B
x xx v t vt t
= = =
With 12.2 m,Bx = 012.2we get 32.02 m/s
0.3810v = =
012.2and 41.11 m/s
0.2968v = =
032.02 m/s 41.11 m/sv≤ ≤ or 0115.3 km/h 148.0 km/h v ≤≤
(b) Vertical motion: ( )0y yv v gt gt= − = −
Horizontal motion: 0xv v=
( )( ) 0
tany BB
x B
vdy gtdx v v
α = − = − =
For 0.788 m,h = ( )( )9.81 0.3810tan 0.11673,
32.02α = = 6.66 α = °
For 1.068 m,h = ( )( )9.81 0.2968tan 0.07082,
41.11α = = 4.05 α = °
12.2 m
1.5 mA v0
h
a
B
DynaDEn
PROBLEM 11.98
While delivering newspapers, a girl throws a paper with a horizontal velocity 0.v Determine the range of values of 0v if the newspaper is to land between points B and C.
SOLUTION
Sketch the limiting trajectories.
0
0
Point : 00
A x xy y= =
= =
Point : 2.2 m1.0 m
B
B
B xy
=
= −
Point : 4.0 m0.6 m
c
c
C xy
=
= −
Vertical motion with ( ) ( ) 200 0
1 20: or 2y y
yv y y v t gt tg−
= = + − = (1)
Horizontal motion with ( ) 0 0 00 : or xxv v x v t vt
= = = (2)
Substituting equation (1) into equation (2) gives 0 2gv x
y=
− (3)
Applying equation (3):
at point B, ( ) ( )0
9.812.2 4.87 m/s2 1.0
v = =− −
at point C, ( ) ( )0
9.814.0 11.44 m/s2 0.6
v = =−
Range of 0 :v 04.87 m/s 11.44 m/s v≤ ≤
0.4 m
1.2 m
1 m
0.2 m0.2 m0.2 m
2.2 m
B
C
A v0
DynaDEn
PROBLEM 11.99
A ski jumper starts with a horizontal take-off velocity of 25 m/s and lands on a straight landing hill inclined at o30 . Determine (a) the time between take-off and landing, (b) the length d of the jump, (c) the maximum vertical distance between the jumper and the landing hill.
SOLUTION
(a) At the landing point, tan 30y x= − °
Horizontal motion: ( )0 00xx x v t v t= + =
Vertical motion: ( ) 2 20 0
1 12 2yy y v t gt gt= + − = −
from which 2 02 2 tan 30 2 tan 30y x v ttg g g
° °= − = =
Rejecting the 0t = solution gives ( )( )0 2 25 tan 302 tan 309.81
vtg
°°= = 2.94 s t =
(b) Landing distance: ( )( )0 25 2.94cos30 cos30 cos30
x v td = = =° ° °
84.9 m d =
(c) Vertical distance: tan 30h x y= ° +
or 20
1tan 302
h v t gt= ° −
Differentiating and setting equal to zero,
0tan 30tan 30 0 or odh vv gt t
dt g°
= ° − = =
Then, ( ) ( ) 20 0 0
maxtan 30 tan 30 1 tan 30
2v v vh g
g g° ° °
= −
( ) ( )( )( )
2 22 20 25 tan 30tan 30
2 2 9.81v
g°°
= = max 10.6 m h =
25 m/s
DynaDEn
PROBLEM 11.100
A golfer aims his shot to clear the top of a tree by a distance h at the peak of the trajectory and to miss the pond on the opposite side. Knowing that the magnitude of 0v is 25 m/s, determine the range of values of h which must be avoided.
SOLUTION
Horizontal motion: ( )0 000
, or xxx x v t v t tv
= + = =
Vertical motion: ( )2
2 20 0 0 20
0
1 1 or 2 2 2y
gxy y v t gt y gt y yv
= + − = − = −
At ground level, 0,y = so that 2
0 202
gxyv
=
At 45 m,x = ( ) ( )( )( )
2
0 29.81 45
15.89 m2 25
y = =
0 12 3.89 mh y= − =
At 48 m,x = ( )( )( )( )
2
0 29.81 48
18.08 m2 25
y = =
0 12 6.08 mh y= − =
Range to avoid: 3.89 m 6.08 m h< <
h
12 m
45 m3 m
Pond
v0
DynaDEn
PROBLEM 11.101
A golfer hits a golf ball with an initial velocity of 48 m/s at an angle of 25°with the horizontal. Knowing that the fairway slopes downward at an average angle of 5 ,° determine the distance d between the golfer and point B where the ball first lands.
SOLUTION
Horizontal motion: 0 0cos 25 cos 25Ax x v t v t= + ° = °
At point B, 0cos5 cos 25B Bx d v t= ° = °
Vertical motion: 2 20 0 0
1 1sin 25 sin 252 2
y y v t gt v t gt= + ° − = ° −
01sin 5 sin 252B B By d v gt t = − ° = ° −
Ratio: ( )1
0 20 0
0
sin 25sin 5 1 or cos 25 tan 5 sin 25cos 5 cos 25 2
B BBB
B B
v gt ty v v gtx v t
° −°= − = − ° ° = ° −
° °
or ( ) ( )( ) ( )0 2 482 cos 25 tan 5 sin 25 cos 25 tan 5 sin 25 4.912 s9.81B
vtg
= ° ° + ° = ° ° + ° =
Then, ( )( )( )0 48 cos 25 4.912cos 25cos5 cos5 cos5
B Bx v td°°
= = =° ° °
215 m d =
25°
d
AB 5°
v0
DynaDEn
PROBLEM 11.102
Water flows from a drain spout with an initial velocity of 0.76 m/s at an angle of 15° with the horizontal. Determine the range of values of the distance d for which the water will enter the trough BC.
SOLUTION
Vertical motion: ( ) 20 0
12yy y v t gt= + −
( ) ( )20 0
1 02 ygt v t y y− − − =
With 0 3 m and 0.36 m at level ,y y BC= = 0 2.64 my y− =
( )0 0.76sin15 0.19670 m/syv = − ° = −
Then, 24.905 0.19670 2.64 0t t+ − =
Solving the quadratic equation,
( ) ( )( )( )( )( )
20.19670 0.19670 4 4.905 2.640.7139 s
2 4.905t
− + − −= =
Horizontal motion: ( ) ( )( )0 0.76cos15 0.7139 0.524 mxx v t= = ° =
For water impact at C, 0.524 ft with 0.62 0.096 mc cx d x= = − = −
For water impact at B, 0.524 mBx d= =
So, 0.096 m 0.524 md− < <
Owing to the foot of the wall, d cannot be negative; hence, the allowable range of d is 0 0.524 m d< <
15°
3 m
v0
A
B C0.36 m
0.62 md
DynaDEn
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 11.14
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 11.15a
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 11.15b
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 11.15c
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 11.15d
DynaDEn
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Photo 11.4
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 11.23
DynaDEn
DynaDEn
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. P11.177
DynaDEn
DynaDEn
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
44 - 49
Sample Problem 11.10
A motorist is traveling on curved section of highway at 60 mph. The motorist applies brakes causing a constant deceleration rate.
Knowing that after 8 s the speed has been reduced to 45 mph, determine the acceleration of the automobile immediately after the brakes are applied.
SOLUTION:
• Calculate tangential and normal components of acceleration.
• Determine acceleration magnitude and direction with respect to tangent to curve.DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
45 - 49
Sample Problem 11.10
ft/s66mph45ft/s88mph60
==
SOLUTION:• Calculate tangential and normal components of
acceleration.( )
( )2
22
2
sft10.3
ft2500sft88
sft75.2
s 8sft8866
===
−=−
=∆∆
=
ρva
tva
n
t
• Determine acceleration magnitude and direction with respect to tangent to curve.
( ) 2222 10.375.2 +−=+= nt aaa 2sft14.4=a
75.210.3tantan 11 −− ==
t
naaα °= 4.48α
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
46 - 49
Sample Problem 11.12
Rotation of the arm about O is defined by θ = 0.15t2 where θ is in radians and tin seconds. Collar B slides along the arm such that r = 0.9 - 0.12t2 where r is in meters.
After the arm has rotated through 30o, determine (a) the total velocity of the collar, (b) the total acceleration of the collar, and (c) the relative acceleration of the collar with respect to the arm.
SOLUTION:
• Evaluate time t for θ = 30o.
• Evaluate radial and angular positions, and first and second derivatives at time t.
• Calculate velocity and acceleration in cylindrical coordinates.
• Evaluate acceleration with respect to arm.
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
47 - 49
Sample Problem 11.12SOLUTION:• Evaluate time t for θ = 30o.
s 869.1rad524.0300.15 2
==°==
ttθ
• Evaluate radial and angular positions, and first and second derivatives at time t.
2
2
sm24.0
sm449.024.0m 481.012.09.0
−=
−=−==−=
r
trtr
&&
&
2
2
srad30.0
srad561.030.0rad524.015.0
=
==
==
θ
θθ
&&
& tt
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
48 - 49
Sample Problem 11.12• Calculate velocity and acceleration.
( )( )
rr
r
vvvvv
rvsrv
θθ
θ
β
θ
122 tan
sm270.0srad561.0m481.0m449.0
−=+=
===
−==&
&
°== 0.31sm524.0 βv
( )( )
( )( ) ( )( )
rr
r
aaaaa
rra
rra
θθ
θ
γ
θθ
θ
122
2
2
2
22
2
tan
sm359.0
srad561.0sm449.02srad3.0m481.0
2sm391.0
srad561.0m481.0sm240.0
−=+=
−=
−+=
+=
−=
−−=
−=
&&&&
&&&
°== 6.42sm531.0 γa
DynaDEn
© 2004 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: Dynamics
SeventhEdition
49 - 49
Sample Problem 11.12• Evaluate acceleration with respect to arm.
Motion of collar with respect to arm is rectilinear and defined by coordinate r.
2sm240.0−== ra OAB &&
DynaDEn
PROBLEM 11.145 From a photograph of a homeowner using a snowblower, it is determined that the radius of curvature of the trajectory of the snow was 9 m as the snow left the discharge chute at A. Determine (a) the discharge velocity
Av of the snow, (b) the radius of curvature of the trajectory at its maximum height.
SOLUTION
(a) The acceleration vector is 9.81 m/s2 .
At point A. Tangential and normal components of a are as shown in the sketch.
2cos 40 9.81cos 40 7.51 m/sna a= ° = ° =
( ) ( )( )2 2 29 7.51 67.59 m /sA A A nv aρ= = =
8.22 m/sA =v 40°
8.22 cos 40 6.30 m/sxv = ° =
(b) At maximum height, 6.30 m/sxv v= =
29.81 m/s ,na g= =
( )22 6.309.81n
va
ρ = = 4.05 mρ =
vA
A 40°
DynaDEn
PROBLEM 11.184 Assuming a uniform acceleration of 23.5 m/s and knowing that the speed of a car as it passes A is 50 km/h, determine (a) the time required for the car to reach B, (b) the speed of the car as it passes B.
SOLUTION
Let the origin lie at point A. 0, 50 mA Bx x= =
250 km/h 13.889 m/s, 3.5 m/sAv a= = =
(a) 212B A Ax x v t at= + +
2.69 st =
(b)
83.9 km/hBv =
352. t 2 + 13.889t – 50 = 0 t = –10.63s, 2.69s
vB = vA + at = 13.889 + (3.5) (2.69) = 23.3 m/s
vA = 50 km/h
50 m
A B
DynaDEn
PROBLEM 11.185 Block B starts from rest and moves downward with a constant acceleration. Knowing that after slider block A has moved 400 mm its velocity is 4 m/s, determine (a) the accelerations of A and B, (b) the velocity and the change in position of B after 2 s.
SOLUTION
Constraint of cable with Ax increasing to the left and By increasing downward.
3 constant, 3 0A B A Bx y x y+ = ∆ + ∆ =
3 0, 3 0A B A Bv v a a+ = + =
Block B moves downward so that Bv and Ba are positive.
Since, 3 , A B Av v v= − is negative, i.e. to the right.
For block A, 2 2A A Av a x= ∆
(a) ( ) ( )2 2
2( 4) 20 m/s2 2 0.4
AA
A
vax
−= = = −
∆ − or 20 m/s2
A =a
21 6.67 m/s3B Aa a= − = 6.67 m/s2
B =a
(b) At 2 s,t =
( )( )6.67 2B Bv a t= = 213.33 m/sBv =
( )( )221 1 6.67 22 2B By a t∆ = = 13.33 m2
By∆ =
DynaDEn