物理高手請進!!!!

(1)c---pq向右移...感生電流向上...
既然係1 o岩2就錯=.=.....3佢有emf先有current..所以都係o岩o既..
(2)b---t=4s時o既速度：2*4=8ms^-1
p=f*v
p=900*8=7200w
(3)b---撞擊中momentum改變(幅圖下o既面積)：240*0.1/2=12kgms^-1
設撞擊後速度=xms^-1
(x+40)0.2=12
x=20ms^-1
(4)b---average force=momentum改變/時間
=12/0.1=120N

i'm not sure whether it is correct, but i hope it helps you ^^'

37. D
when the rod moves in the magnetic field, the electrons inside the copper rod experiences an upward magnetic force. The electrons are pushed up and P becomes negative terminal, just like a cell. an e.m.f. is set up, P is -ve while Q is +ve
Current flow from negative to positive inside the cell and from positve to negative in external circuit.
So 2,3 are correct.

17. B
Net force acted on the car= 900 * 2 = 1800N
By s=ut+1/2at^2
s=displacement travelled
u = initial velocity = 0ms^-1
a = acceleration

s = 0.5 * 2*(4^2)
= 16m
Work done by the force = 16 *1800 = 28800J
Power = work done/time
=28800/4
=7200W
=7.2kW

15. B
area under the graph=change of momentum
=0.5 * 0.1 * 240
=12 kg ms^-1
change of momentum = 0.2 (v-u)
12 = 0.2 (v-40)
v = -20ms^-1(in opposite direction)

16. A
Force = rate of change of momentum
= 12/0.2
=60N