有關物理問題!! (10點)

I am not quite sure if I have misunderstood your question.

1) As friction is compensated [or do you mean gravity is compensated?], there is no friction, so when you push it, you are applying a horizontal net force on it, thus it will accelerate. Simply, F=ma.

2) It is indeed an interesting issue. At first scientists believe that currect flows from ve terminal to the -ve one, but then later they find out that electricity is carried by electrons flowing from -ve terminal to ve one. So, for the term of current, we are talking about the --conventional current-- which flows from ve to -ve, though electricity is actually transmitted from -ve to ve.

3) Usually P=V^2/R is the most accurate method since current may vary due to resistance of wire or other accessories.

4) I assume what you are talking about is a stationary wave, which is being captured on a screen and ... it is moving, but then since the projector (?) has the same frequency with the wave, it appears to be not moving on the screen.

5) Theoretically when destructive interference occurs, i.e. trough meets crest.

Hope it helps :)

第(3)都唔係好明! 可唔可以詳細d呢? thx