dynamics_ solved exercises
TRANSCRIPT
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G en er al P h y s i c s
1
A P H
Y 1 1 1
D y n ami c s
2 . 3
N ew t on’ s F i r s t
L aw
• E v er y o b j e c t c on t i n u
e s i ni t s s t a t e of r e s t ,
or of uni f or mv el o c i t y i n a s t r ai g
h t l i n e a s
l on g a s n on e t f or c e
a c t s oni t
–
’
2
a b s en c e o
f af or c e.D o e s n o t d e s c r i b e
z er on e t f or c e.
–A l s o t el l s u s t h a t wh enn of or c e a c t s on
an o b j e c t , t h e
a c c el er a t i on of t h e o b j e c
t i s z er o.
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2 .4 N
ew t on’ s S e c on
d L aw
• T h e a c c e
l er a t i on of an o b j e c
t i s d i r e c t l y
pr o p or t i on al t o t h e
n e t f or c e
a c t i n g on
i t
an d i nv er s el y pr o p or t i on al t oi t s
m a s s
3
–
,
m e a s ur e
d b y t h e a c c
el er a t i on
• A l g e b r ai c
al l y ,
a m
F
v
v
=
Σ
•
i s t h en e t f or c e
–T h i s i s t h e
v e c t or s um
of al l t h ef or
c e s a c t i n g
on t h e o b j e c t
•
N ew t on’ s S
e c on d L aw
c an b e ex pr e s s e d
i n t er m s of
c om p on en
t s ( s e el a t e
r ) :
Σ F x =m ax
=
∑ F r
4
y
y
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5
E x am pl e: A c ar of m
a s s 1 0 0 0 K gi s m ov i n g t o t h er i gh t wi t h av el o c i t y of 1
0 0
K m
/ h .Wh en t h e b r e ak s ar e u s e d t h ef or c
e of f r i c t i on s t o p s t h e c ar ov er a d i s t an
c e
of 5 0 m.E s t i m a t e t h ef or c e of f r i c t i on ( d i r e
c t i on an d m a gni t u d
e ) .
6
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Gr av i t a t i on al F or c
e
•
T h e gr av i t a t i on al f or c e ,
,i s t h
ef or c e t h a t
t h e e ar t h
ex er t s on a
n o b j e c t
•
T h i s f or c e
i s d i r e c t e d t ow ar d t h e c en t er of
t h e e ar t h
•
’
gF r
7
–
•
I t s m a gni t u d ei s c al l e d t h ew ei g
h t of t h e
o b j e c t
–
W ei gh t =F
g=m g
g
m=
F
g
r
r
2 . 5
T h eF or c e of F r
i c t i on
• F r i c t i oni s ar e s
ul t of s ur f a c e
r o u gh n e s s .
• T h e d i r e c t i on of f r i c t i oni s al w a y s
o p p o s i t e t o t h ef or c e t en d i n g t o gi v e
m o t i on.
• B ef or em o t i on t
ak e s
l a c ef r i c t i oni s
8
c al l e d s t a t i c f r i c t i on.
• A f t er m o t i on t ak
e s pl a c ef r i c t i oni s
c al l e d k i n e t i c f r i c
t i on
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N
F
µ
=
Di r e
c t i on: o p p o s i t e t o t h
a t
of m
o t i on or i m p en d i n g
m o t i on
9
K i n e t i c F r i c t i on ,f k : c on s t an t
( n om or e c
on t a c t p oi n t s
av ai l a b l e ) .
D ev i a t i on onl y a t
v er y h i gh v
el o c i t i e s
S t a t i c F r i c t i on ,
f s
m a x
F
f s ≤
µ s = C o ef f i c i en t of s t a t i c f r i c t i on
µk = C o ef f i c i en t of k
i n e t i c f r i c t i on
Di f f e r e n t c om b i n a t i on s of
m a t e r i a l s h a
v e a d i f f e r e n t
c o e f f i c i e n t s o
f s t a t i c f r i c t i on
1 0
T h e c o ef f i c i en t of
f r i c t i on , µ , d e p en d s
en t i r el y on t h e t y p e of m a t er i al s t h a t
ar ei n c on t a c t .
T y p e o
f m a t er i al s i n c on
t a c t
µ s
µκ
A l umi n um
A l umi n um
1 . 3 5 -1 .4
1 . 0 5
A l umi n um
S t e el
0 . 6 1
0 .4 7
Gl a s s
Gl a s s
0 . 9 4
0 .4 9
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E x am pl e1 : A 1 0 0 0 k
g c ar i s m ov i n gh o
r i z on t al l y wi t h av e
l o c i t y of 2 5 m / s .
T h
e d r i v er s t e p s on t h e b r ak e.A s s umi n
g t h a t t h er o a d h a s a c o ef f i c i en t of
f r i c t i on e q u al t o 0 .7
c al c ul a t e t h ev el o c i t y of t h e c ar 3 s
af t er t h e d r i v er
a p
pl i e d t h e b r ak e s .
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2 . 6
N ew t on
’ s T h i r d
L aw
•
I f t w o o b j e c t s i n t er a c t , t h ef or c e
ex er t e d b
y
o b j e c t 1 o
n o b j e c t 2
i s e q u al i n
m a gni t u d e
an d
o p p o s i t e
i n
d
i r e c t i on
t o
t h e
f or c e1
3
•
( N o t e onn o t a t i on:
i s t h ef or c e ex
er t e d onA b y B )
1 2
2 1
=−
F
F
r
r
A B
F r
F or c e s al w a y s o c c ur
i n p ai r s
2 .7 S ol v i n gP r o b l em s
F
r e e- b o d y Di a gr am
s
I n or d er t o s ol v eD y n ami c s P r o b l em s w
en e e d a
F
r e e-B o d y d i a gr am
of o ur o b j e c t :
A s s um e t h a t t h e b o d y i s i s ol a t e d
A s s um e t h a t t h e b o d y i s a p oi n t p ar t i c l e
1 4
Dr aw al l t h ef or c e s
a c t i n g on t h e b o d y
al on e.
W
e c an t h e a p pl y N ew t on’ s 3 r d l awf or e a c h b o d y
i n d e p en d en t of t h e o t h er s
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T w o b ox e s an
d a p ul l e y : Dr awF r
e e-B o d y d i a gr am s
f or e a c h b ox , t ak i n gi n t o a c c o un t
t h ef or c e of f r i c t i on.
C al c ul a t e t h e c omm on a c c el er a t i on of t h e b o d i e s i f
t h e c o ef f i c i en t
of f r i c t i oni s 0 .1 .
F r e e- b o d y d i a gr amf or
c onn e c t e d o b j e c t s
1 5
F r e e- b o d y d i a gr am on
ani n c l i n e
T h e
n or m al f or c ei s al w a y s a t r i gh t
A n gl e t o t h e s ur f a c e.T h
ew ei gh t i s
al w a
y s d ownw ar d s an d
f r i c t i on t o
t h e o
p p o s i t e d i r e c t i on of m o t i on.
1 6
E x am pl e: D
r awf r e e- b o d y d i a g
r am s f or m a s s e s
m1 an d m2 .T ak ef r i c t i oni n t o a c c o un t .
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E x a
m pl e2 : T h r e em a s s e s ,m1 = 5 k g ,m2 =7 k g an d m
3 = 8 k g a
r e c onn e c t e d wi t h
l i gh t i n ex t en s i b l e s t r i n g
s a s s h owni n t h ef ol l owi n g d i a gr am an d ar e pl a c e d on a
s m o
o t h f l o or . Onm a s s
m1 a c t s ah or i z on t al f or c e of 4 0 N.
F =4 0 N
m1
m2
m 3
1 7
(
a ) Dr aw t h ef r e e b o
d y f or c e d i a gr amf or e a c h m a s s .
(
b ) C al c ul a t e t h en o
r m al r e a c t i on of t h e
f l o or on e a c h m a s s .
(
c ) A p pl y t h eN ew t on’ s s e c on d l awf or e a c h m a s s i n t h eh or i z on t al
d
i r e c t i on.
(
d ) F i n d t h e a c c el er a t i on a of t h e s y s t em an d t h e t en s i on s
T 1 an d T 2
i n
t h e s t r i n g s .
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2 0
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A v e c t or m a y b
er e s ol v e d
i n t o
c om p on en
t s b y t h e
a p p
r o pr i a t e c h
oi c e of
2 . 8 S ol v i n gP r o b l em s
V
e c t or C o
m p on en
t s
2 1
.
•
F x =F C o s Θ
•
F y =F
S i n Θ
E x am pl e: A b l o c k of m a s s
m= 5 k gi s r el e a s e d
f r omr e s t f r om t h e t o p of a
ni n c l i n e of h ei gh t
1 m an d θ = 3 0 ο.
( a ) C al c ul a t e t h e a c c el er a t i on of t h e b l o c k wh en
t h er ei s n of r i c t i on
( b ) C al c ul a t e t h e a c c el er a t i on of t h e b l o c k wh en
t h e c o ef f i c i en t of f r i c t i oni s
0 .1
( c ) Wh a t i s t h ev el o c i t y of t h e b l o c k a t t h e
b o t t om
of t h ei n c l i n e?
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2 3
2 4
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E x am pl e: I f m1 =1 k g an d m2 =2 k gwr i t e d ownN ew t on’ s
s e c
on d l awi n t er m s of c om p on en t s f or e a c h b
o d y :
Σ F x =m ax
Σ F
y =m a
y
N
e gl e c t f r i c t i on.
U s e y o ur e q u a t i on s i n or d er
t o c al c ul a t e
t h
e c omm on a c c el er a t i on of t h
e b o d i e s
2 5
2 6
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2 . 9 G en er al P r o b l em s
E l ev a t or an d c o un t er w ei gh t ( A t w o o d ' s
m a c h i n e ) :
T w o b o d i e s , an el ev a t or wi t h m a s s mE =1 1 5 0 K g an d a
c o un t er w ei gh t of m
a s s m
c =1 0 0 0 k g ar e c onn e c t e d wi t h a
l i gh t
s t r i n g a s s h owni n
t h e d i a gr am.T h e
s t r i n g p a s s e s ov er t h e
n e c k of a s m o o t h p ul l e y ,wh i c h i s n o t
al l ow e d t or o t a t e.
T h e
s y s t emi s l ef t t om ov e.E v al u a t e
( a ) T h e c omm on a c c el er a t i on of t h e t w
o b o d i e s .
2 7
( b ) T h e t en s i oni n t h e s t r i n g.
( c ) T h e s p e e d e a c h b o d y a c q ui r e s af t er t h e y t r av el a d i s t an c e
of 1 m , s t ar t i n gf r om
r e s t .
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3 0
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3 1
3 2
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3 4
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3 6
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3 7
A m a
nwh o s em a s s i s 8
0 k g s t an d s i n an el ev a t or .T h e el ev a t o
r h a s a s p e e d of
3 m / s .F i n d t h er e a c t i on of t h ef l o or of t h e el ev a t or on t h e s t an d
i n gm ani n t h e
f ol l ow
i n g c a s e s :
( a ) T h e el ev a t or i s m ov i n g u pw ar d wi t h an a c c el er a t i on of 2 m / s 2 .
( b ) T h e el ev a t or i s m ov i n g d ownw ar d wi t h a
n a c c el er a t i on of 2 m / s 2 .
( c ) T h e el ev a t or i s m ov i n g u pw ar d wi t h a d e
c el er a t i on of 2 m / s 2
.
( d ) T h e el ev a t or i s m ov i n g d ownw ar d wi t h a
d e c el er a t i on of 2 m
/ s 2 .
( e ) T h e el ev a t or i s m ov i n g u pw ar d wi t h c on
s t an t s p e e d .
( f ) T h
e el ev a t or i s m ov i n
g d ownw ar d wi t h c on s t an t s p e e d .
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3 9