Cdu 8: M o t chat diem dao dong dieu hoa vdri chu k i T Trong khoang
thcfi gian ngan n h a t k h i d i tCr v i t r i bien c6 l i do x = A den v i t r i
- A
X = , chat diem c6 toe do trung b i n h la:
Ạ 3A
6A
C 4A
D 9A 2T
Giai
Gia sur phi/cfng t r i n h dao dong la x = A cos cot
Tai thcfi diem to = 0 t h i x = Ạ T a i thori diem t t a c6:
A
' 2
3'
A ^ 231^
X = Acoscot = <=> A c o s — t =
2 T
2n 1 2n
<=>cos—1 = — <^
T 2 T
Mac khac S = A
-= — -=>t -=
3
A ^ _ 3 A
2j 2 •
3A
Van toe trung b i n h = ^ = j = ^
Vdy chgn dap an D
Cdu 9: T a i ncfi c6 gia tdc t r o n g trifdng g, m o t con l^c dorn dao dong
dieu hoa vdri bien do goc nhọ Lay mdc the nSng d v i trx can bkng Khi con l^c chuyen dong nhanh dan theo chieu dUcfng den v i t r i c6 dong nSng bang the tiSng t h i l i do goc a cua con l^c bang:
Ạ
GiSi
Tai thdri diem t , k h i goc lech la a , thé nang cua con He l a :
W, = mg/ (1 - cos a) « mg/.2 sin^ ^ « 2mg/ ^ = mg/ậ
K h i dong nhng bkng the nang t a c6:
W = 2W, = 2 ^ m g / a ' = m g / a ^ (1)