bài tập điểm 9-10 chương dao động cơ hoc vật lý 12 (buổi hoc số 3)

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m 9  u hc
( bui hc s 1 )

1 Mt vt nh ng M = 0,9 (kg), g th cng
i c nh. Mt vt nh ng m=0,1 (kg) chuyng theo
ng vi t 0,2
2
n va chm mm vi M. Sau va chm hai v
i trc cy gia
tc trng g=10m/s
2
 
A 4,5 cm B 4 cm C 4
2
cm D 4
3
cm
Gii: Vn tc ca hai vt sau va chm: (M + m)V = mv
> V = 0,02
2
(m/s)
Sau va chm: x
0
=
mg
k
=
k

mg
= 0,04m = 4cm
A
2
= x
0
2
+
2
2

V
= x
0
2
+
k
mMV )(
2

= 0,0016 > A = 0,04m = 4cm



2 : Con lm vt n cng k = 200N/m, lt trc
th v bt m = 200g t  cao h = 3,75cm so
v do, va chm mm v, ly g = 10m/s
2
Sau va chm
hai vn gc t ti v ng ca h, chi

vi gian t = ng ca h hai v
A. x = 1,08cos(20t + 0,387)cm. B. x = 2cos(20t +
4

)cm.
C. x = 1,57cos(20t + 0,155)cm. D. x = 2cos(20t +
3

)cm
Gii: Vn tc ca vt m khi va ch
v =
gh2

Vn tc v
0
ca h hai vt sau va chm:
(M+m)v
0
= mv > v
0
=
mM
ghm

2

 a h hai v ng ca h
x
0
- 

0
=
m
g
k
= 0,01m = 1cm
 ng ca h: A
2
= x
0
2
+
2
2
0

v

Vi  =
mM
k

=
5,0
200
= 20 (rad/s)
A =
2
2
0

2
0
x

v

=
2
2
20
12,0
0,01 
= 0,02 m = 2cm
m
M

O
m
h
M
+
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ng ca h hai vt x = Acos(20t +)
khi t = 0 x = x
0
= A/2 > cos = 0,5 >  =
3


> x = 2cos(20t +
3













3: Con li vt n
.Mt vt nh khn va ch
vt M sau va chm 2 v a h 

A.Wo/5 B.Wo C.4Wo/5 D.W0/4


Gii: Vn tc v
0
ca vc khi va ch
W
0
=
2

2
0
mv
>
0
v
=
m
W
0
2
= 2
M
W
0
2

Vn tc v ca h hai vt sau va chm:
(M + m) v = mv
0
>
4
5
Mv =
4
1
Mv
0

> v =

5
0
v
= 0,4
M
W
0
2

ng ca h 
W =
2
)(
2
vmM 
=
4
5
2
M
v
2
=
4
5
2
M
.0,16.
M
W

0
2
= 0,2W
0
= W
0



4. 


2


A. 15 cm
B. 20 cm
C. 10 cm
D. 12 cm

Gii: G
O
M

x
m

O

h

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Vn tc cc khi chm M: v
0
=
gh2
=
18
= 3
2
m/s
Gn tc cm
MV + mv = mv
0
(1) vi v
0
= - 3
2
m/s
2
2
MV
+
2
2
mv
=
2

2
0
mv
(2)
T 
3
2
v
0
= - 2
2
m/s > V
max
= 2
2
m/s
Tn s ng :
 =
M
k
=
2,0
20
= 10
2
rad/s
 ng : A =

max
V

=
210
22

5.

2


m


A. 2,5 cm
B. 2 cm
C. 5,5 cm
D. 7 cm

Gii:G ng
 t  VTCB

k
mg


k
gmm )( 

- 
k
gmm )( 

-
k
mg
=
k
mg
=
100
10.15,0
0,015m = 1,5cm





6 : Mt con lt nm ngang gm v s
c ng v 5cm. Khi M
qua v i ta th nh v
 ng v
A.
25cm
B. 4,25cm C.
32cm
D.
22cm

Gii:
Tn s a con lc:  =
M
k

=
4,0
40
= 10 rad/s.
T ca M khi qua VTCB v = A = 50 cm/s
 M

 O



m

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T c
mM
Mv

= 40 cm/s
Tn s a h con lc: 
mM
k

=
5,0
40
=
5

20
rad/s.

 ng ca h
'
'

v
= 2
5
