al phy mc c

(1)
no difference
roughly draw a v-t graph
when v changes its sign(say, positive to negative)
v must be zero somewhere between

the answer is A??


(2)
by conservation of energy
KE at lowest point=PE when the ball is at rest
(1/2)mv^2=mgr
mv^2/r=2mg

at the lowest point
centripetal force
=mv^2/r
(same expression tho not in circular motion which is uniform, the proof is slightly different)

so
T-mg=mv62/r=2mg
so T=3mg~


(3)

(A)
the oscillation will die out eventually
frequency=0
(if damping forces are present, the period is longer. you can show it by differential equations... only in AL applied maths)

(B)
driving frequency is NOT close to natural frequency....
never infinite amplitude
infinite amplitude will onlly happen when
1. driving frequency = natural frequency
2. no damping forces

(C)
No...
http://www.youtube.com/watch?v=-PHqCm0wNi4
http://www.youtube.com/watch?v=PSjchBpKK9U




2010-02-20 14:36:42 補充：
correction:
for(3)
A is correct as the force is continuously driving the system

2010-02-20 23:17:39 補充：
i missed Q4
you can regard it as a parallel plate capacitor
and the charge is conserved
so capacitance increases -> V decreases

2010-02-20 23:18:15 補充：
use E=Q/(epsilon*A) you can get the ans

Just supplemention some information to the question you asked.

1. There is not much difference, though the process described in (C) takes place later than the process in (B).

2. The tension only depends on the weight and position of the bob.

3. The follwoing web-site gives a good animation for forced oscillation. It can be displayed in slow motion, with graphical presentation. Try the animation and you could understand the forced oscillation mechanism.

http://www.walter-fendt.de/ph14e/resonance.htm


4. No. Since plate B is isolated, the induced charges on plate A must be equal in magnitude to that on plate B (only opposite in sign). This is the Law of Conservation of Charges.