Ti liu bi dng hc sinh gii THCS mụn VT Lí
A- áp suất của chất lỏng và chất khí
I - Tóm tắt lý thuyết.
1/ Định nghĩa áp suất:

S
F
P =
! "#!$%&'(
")!*+
,
-
".!+/0
,
-
2/ Định luật Paxcan
á%1+234- 5461
+234-37389 :'
3/ Máy dùng chất lỏng.
s
S
f
F
=
");!*<2.&';.&1+
,
-
"=!>%.&1+/-
"#!>%.&'+/-
?5@13@A.&$32.&42$2 !
?BS.H = s.h

! 1M32
.!D@P
5/ Bình thông nhau.
"Q5&2R2S1R3;1T2
&&2
"Q5&2R27142R3;(
4&2@S(2+ S1-
2+5-





=
+=
+=
BA
B
A
PP
hdPP
hdPP
220
110
.
.
6/ Lực đẩy Acsimet.
F = d.V - d: Trọng lợng riêng của chất lỏng hoặc chất khí (N/m
3
)

B
N
)c#

?'$2 <2'4M3R:6<2cc'4&23
Y#
L
B#
,
23
N
)CB
N
)c#


?5:6#

XNBX)U)CBXUCBX'F
Bµi 2!  C25K2D5
%+5a-K&3;R2'T
EJ4 JT;'T,
22/E24 J$'T
5Q?5V3' 5Q6M2 
LdC6[F32G$ G24
'T5Q5T4 Je
f G@5%9 &R?
B
3
1

L
?
L
 B?BX
1
1
V
V
d
d
=
+?;?
L
$@' 5Q'$24-
A32!
h
h
SsSsh
SsSsh
h
h
V
V
P
P
1
111
1
1
1

  P  hR    1     ,
H
g Q
/i'
*P
Tài liệu bồi dưỡng học sinh giỏi THCS môn VẬT LÝ
V22C1' K
F34&;GF35F3
9 '$ e
Gi¶i :b:.
N
$ 4O3@;
L
$
,
P6$:6<2
'$P;$72 1A( GKZD@g
+Kd '-
.
g
B.
N
c
L

DQ+Kd P.
Q
B.
N
c

G :  6   <2 P  $ <2 '  P 6  $!

L
BnNNN/0
H
I
,
BLNNNN/0
H
I

Giải:
b:
L
;
,
$2 'T5g$5Q4oM
)
g

L
c)
Q

,
B?
,
⇒
LNN
L


L
c
L

H
B
,

,
LNNNN
L
cnNNNHNBLNNNN
,
⇒

,
B
L
c,m +,- 
A+L-$+,-232!

L
c,+
L
c,m-Blm
⇒

L
B,


L

,
Tài liệu bồi dưỡng học sinh giỏi THCS môn VẬT LÝ
b:
L
;?
L
;*
L
;$4K6;@$4K6<2$
b:
,
;?
,
;*
,
;$4K6;@$4K6<2D
JM $4&4
.
N
B+
L
c


,
-LN +L- 
JM '

D
m
D
m
mm
B
B














−+









N









−
2
1
D
D
$
LN
,
*








−
21
11

Tài liệu bồi dưỡng học sinh giỏi THCS môn VẬT LÝ
Gi¶i :f:@T('<2&,
J2(OFM5!
)1(
2
2
0
1
1
S
m
hD
S
m
=+
+*
N
$4K6<2'-
J(V(&'5!
2
2
11
1
2
2
1
1
S
m
S

m
+=+
+H-
AG'G<2+L- +H-26!
*
N
s*
N
CB"
2
0
2
)(
S
m
DhH
S
m
=−⇒

h
S
S
H
S
hSD
DhH )1()(
2
1
2

BnNNN/0
H
+:6<2'$
L
BLNNNN/0
H
;1
4& $t$ 2-hT4 J@25&2Co3!
2-`72 <2( 1T,5
Bµi 1: Q5&2u,
5%GP6$)
L
;)
,
$R2
'('(&1;4K
6
L 
$
,
h',2L
 D
2-54K6<2OFM(
&'@'T,22
-/G(OFM2&1
5'dM3Ea2L D
2 
p



2,1
10
04,0.10.906,0.10.8
4
33
1
2233
=

=

?V3( 1H Q2
( 1, g$!
cmxhhh 8,0)2,14(6)(
23
=+=+=
b) ?5
,
BN;l
L
)
,
B
2
2
1
3
4
12
2

15
4,14216
=

?V3@'?
Q
F3O24 J$!
?
Q
BHCBHLH;mmBmN;H,
H
(III) .Bài tập về lực đẩy Asimet:
Ph ơng pháp giải:
"*2$ 74M!wJVM 15.B#
g
x
.!>$:6<2V;#
g
$y329%V+#
g
B?-
Bài 1:h4K]5vVG)BmN
,
2 BLNf
4K6BLpN
q


,


,
BLLHNN40
H
4F$  '"
E23''(<24K]5M∆<2]
Gi¶i:
2-J4K]M '5:6<24K]M'
y3g9b:[$P4K]Y(';2
.B#
g
⇒LNBLN*
N
)+"[-
cm
SD
m
6
.
-h x
0
==⇒
-JK]244 Y4K6$
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L
B"∆B*
L
+)"∆)∆-
?'*
L
$4K6<2]!

N
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cm
S
hS
m
D
mhSD
5,5
)
.
(
.
=h ==>
2
0
=
∆−
−
Bµi 2:C2OFP(@]OF$?BLNN
H
6K'2
6M384& oF '+5a-
JK6OFP'mP4K6
OFP4M5L0,@
n
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g
c#z
g
BX
(2)
2
3
D D
021
D=+
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L
BH0LN*
N
BHNN40
H
*
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Bm*
L
BL,NN40
H
Q-ZAOFP!
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g
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L
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g
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5
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BN;,/
Bµi 3: 55%G)
N
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2 CB,N/E2F$ 52u;G72  
YjR 55'M D ∆Bm
2-/G52 ' $ $5'aM2 2 
 '3ef 4K<22$'P6$*BN;n0
H
;
*
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 $ $ 'f @2$lN
H

N
∆
* 2M.B#
g
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h
S
S
D
D
∆
00
+L-
J25 $ $ ';'ML6@<22
b:∆C$P'Md$32!)lB)
N
∆C+,-
A+L-$+,-32 ∆CB
h
D
D
∆.
0
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cm. 25 . H H'

N
BL0
H

- /K4K]'LV(4K6$*
L
Bn0
H
LM3F
O2M<2('4K]/E23PY<24K]$zB,
54K6<2V($k<2M3
LN
)
N
C
∆C
)
.
#z
g
#
Cz
Tài liệu bồi dưỡng học sinh giỏi THCS môn VẬT LÝ
Bµi 3:  55%G)
L
 BHN
H
 R24K6*
L
 BL

54000054,0
27000
458,1
cm
d
P
n
===
b:@P(<2OFP244 ]$?z`@OFP
W '5:6.z<2OFPFM'y3!.z
B#
g)
 
 
?zB
'
?
⇒
?zB
3
hom
20
27000
54.10000.
cm
d
Vd
n
nuoc
==

– P = 10D’V – 10DV
Công của lực này là: A
2
= (10D’V – 10DV)h’
Theo định luật bảo toàn công: A
1
= A
2
⇒ 10DVh = (10D’V – 10DV)h’
LL
Ti liu bi dng hc sinh gii THCS mụn VT Lí
D =
'
'
'
D
hh
h
+
Thay s, tớnh c D = 812,5 Kg/m
3
B - Các máy cơ đơn giản.
I - Tóm tắt lý thuyết
1/ Ròng rọc cố định:
"}_:K^%$23Y'<2;4&
%23Y'<2
2/ Ròng rọc động
"*S_:2662P72P7E
4&6657&
3/ Đòn bẩy.

100.
A
A
H =
g
L
$&
g$& $P
gBg
L
cg
,
+g
,
$&2 -
II- Bài tập về máy cơ đơn giản
L,
l
#
.

Tài liệu bồi dưỡng học sinh giỏi THCS môn VẬT LÝ
Bµi 1:4 # E62M3QGV(:6
.BL,N/+Q1O22;4K6<2_:$M3-
Gi¶i:9 uM5a!JKM2
"T52- p#B.BX#B.0pBL,N0pB,N/
"T5- n#B.BX#B.0nBL,N0nBLl/
"T5- l#B.BX#B.0lBL,N0lB,m/
Bµi 2!hE 6.BpNN/R
69 $ ,_:5

#
# # # # #
#
.
•
•
•
• • •
•
•
Tài liệu bồi dưỡng học sinh giỏi THCS môn VẬT LÝ
2! zB,I#B,zBm
 B#0mBq,N0mBLnN/
b:•$E;2!
•B.sBpNN/sLnN/Bm,N/
-b:.z$:6; K$
V3;$4KM2 z  c
B.zc•
BXHB.zc•BX.zBHs•
BX.zBHLnNsm,NBL,N/
?V3E$m,N/$
:6$L,N/
Bµi 3:f K5a!?VL:"
6$.
L
;
?V,:6$.
,
h]_::"
6$L/Q1O22;4K6<22

Tài liệu bồi dưỡng học sinh giỏi THCS môn VẬT LÝ
Gi¶i:b:.$:6<2_:
 E6R42gQ
M2!
3
1
2
==
AB
CB
P
F
h(4;_:M
2_! ,#B.c.
L

BX#B
( )
2
1
PP +
23$ 26!
( )
3
1
2
2
1
=
+

31
PPP ++
23$ 2!
2
1
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Tài liệu bồi dưỡng học sinh giỏi THCS môn VẬT LÝ
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