谁可以帮帮我！！！（物理问题）

zipp_882000

As shown in the figure below ,uniform rod OA (length:2a ;mass: M)is attached to a vertical surface at end 0,which acts as a fulcrum that allows the rod to rotate in a vertical plane .A string is attached to the rod at a point distance L from O (L<a).The string is pulled vertically upward with the force F until the rod is horizontal,and then the rod is kept at that position


Q1 Under this conditional ,what are the magnitude of F and the magnitude of the for f acting on O ?
from 1-4 below choose the correct combination



[ 本帖最后由 zipp_882000 于 4-11-2009 10:30 PM 编辑 ]

zipp_882000

As shown in Figure 1 below ,three identical springs(spring constant:k;natural length L)are suspended from a ceiling ,and weight of mass m is attached to the lower end of each spring.The spring's mass and the weights' size are negligible .





Q2,As shown in Figure 2 ,one spring is kept suspended from the ceiling ,and  from its lower end the other two springs are suspended in parallel using a rod of negligible mass,under this condition,the distance from the ceiling to the bottom end is 2L+a.Next,two of the springs are suspended in parallel from the ceiling as shown in Figure 3,and the third spring is suspended from their lower ends,Under this condition ,the distance from the ceiling to the bottom end is 2L+b .from the below 1-7 below choose the answer that correctly indicates the value of a:b



[ 本帖最后由 zipp_882000 于 31-10-2009 02:39 PM 编辑 ]

zipp_882000

a certain rocket (mass:m),which has an initial speed of zero ,is launched vertically upward from the earth's surface with uniform thrust of F.

Q3.The thrust is terminated at time （t0) following launch,Afterward ,what is the highest altitude above the earth attained by the rocket?from 1-6 below choose the correct answer .assume that the change in m and the change in g due to height  are negligible

Log

my answers are
1. ③
2.  ④
3. ②
第三题的是不是 （F-mg) t^2 / 2m ?
如有错误，请告诉我

[ 本帖最后由 Log 于 31-10-2009 09:31 PM 编辑 ]

littlepenguin

1。 take momentum around O
static:
momentum around O = 0
Fl=Ma
F=(a/l)M

take momentum around centre=0
fa=(a-l)F
f=((a-l)/a)F

answer=4

Log

我的是take moment  about around at F
so 才得③

littlepenguin

Q2:

mg=kx

for both configuration, lowest spring, no more attachment, so are (l+x)

for 2,
upper part spring:
mass = m+2m=3m
3mg=3kx
so elongation=3x

lower string:
mg=kx
so elongation=x

total elongation, a=4x


for 3,
upper spring:
mg=kx
so elongation by own weight=x

+mass from lower spring
mg=kx/2  (shared by both spring)
so total upper elongation=(1+1/2)x=3/2x

lower spring:
elongation=x

total elongation，b=3/2+1=5/2 x

a:b= 4: 5/2
=8:5

answer :7

littlepenguin

oops...sorry...

answer wrongly: i answer f=(a-l)/a F

haha, the question want M not F, u correct

answer is 3

littlepenguin

Q3:

From t=0-t=t0

s1=ut+1/2at^2

a=(F-mg)/m, u=0

s1=(F-mg)/2m x  to^2

v=u+at,  u=0

v=at
=(F-mg)/m X to

after to, highest height:

v^2=u^2-2gs

at highest height v=0

s=u^2/2g

s2= [(F-mg)/m X to]^2 / 2g

total height s=s1+s2
                    =(F-mg)/2m x  to^2 + [(F-mg)/m X to]^2 / 2g
                    =[ (F-mg)/2m x to^2 ]  [1+(F-mg)/mg ]
                    =[ (F-mg)/2m x to^2 ] [ F/mg ]
                    =[F(F-mg)/ (2g m^2)] x to^2

answer: 6

so long didnt do physics question liao....correct me if i am wrong...

Log

yaa,i think your solution is correct
just now i thought the rocket will reach at highest point at t&#186; but actually it said the THRUST  stop at t&#186;
thanks aaa

zipp_882000

第二题的 2l+a 还有 2l+b 还是不明白 ？？？可以解释一下吗？

zipp_882000

A train traveling at 30 km/h is approached from behind by an ambulance sounding its siren.The ambulance ,which is traveling at a constant speed along a road parallel to the railroad,passes the train and subsequently stops at a distant point ahead of the train .The change in the frequency of the siren'sound was measure on the train .It was observed that frequency f' just after the ambulance stopped was the same as the frequency when the ambulance approached from far behind the train

Q2 What is the relationships between the magnitudes of f' and f ,the frequency of the siren 's sound source ?also ,what was the approximate speed at the ambulance traveled?from 1-6 below choose the correct combination .<Assume that the speed of the sound is sufficiently greater than the speed of the train and the ambulance .>这个是什么意思？？？



[ 本帖最后由 zipp_882000 于 1-11-2009 12:07 AM 编辑 ]

zipp_882000

As show in the figure below ,circuit A is formed by connecting two direct power sources(electromotive force :15v and 5 v ) and two capacitor (capacitor 1uF and C),and circuit B is formed by connecting the same two power sources and two resistors (resistance:1kohm and R) .The connection point between the two power sources is grounded ,and the electric potential at that point is zero.The internal resistance of the power sources is negligible .



Q4 The electirc potential needs to be kept at zero at point a (connection between the two capacitor)in circuit A and at point b(connection between the two resistors) in circuit B.What values of C and R necessary to satisfy this condition ?from 1-6 below choose the best condition .

可以解释下吗？

[ 本帖最后由 zipp_882000 于 1-11-2009 12:11 PM 编辑 ]

zipp_882000

A charge Q is placed at the origin (0,0) of an x-y plane.The proportionality constant of Coulomb 's law is donated as Ko .

Q1 ,From 1-5 below choose the answer that correctly represents field E at point (x.y) ,where E=(Ex,Ey).here Ex is the x-component of E and Ey is the y-component of E .



[ 本帖最后由 zipp_882000 于 1-11-2009 12:37 PM 编辑 ]

idontwant2b

这些真的是stpm题目吗？
很新颖耶