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[Phy] efficiency and mechanic

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efficiency and mechanic

please help me calculate part b
the answer is 99.7%

仲有第二條關於 mechanic
A helicopter is descending vertically at a constant speed of 3ms^-1 . A sandbag is released from the helicopter . The sandbag hits the ground 5 s later What was the height of the helicopter above the ground at the time the andbag was released?

the answer is 137.5m
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I think the number of turns' ratio
is 16 :1

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b efficiency=((48-powerloss)/48)(100%)
               (48-2(0.125)*2(2))/48)(100%)
          =99.87%
i agree with no. 2

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1.Turn ratio =16:1
Voltage in transmission wires=384V
I=0.125A
Voltage taken over by transmmision wires=0.125x4=0.5V
Voltage after step down=(384-0.5)/16=23.96875V
Real power gained by bulb=23.96875^2/12=47.875W
Efficiency=47.875/(47.875+0.125^2 x4)=99.9%?
如果係47.875/48就係99.7%~~e度我唔明  ...
一開始個voltage就係24V  中間會少左  去到後面就唔會係24V  我諗條題目係想陰e個位

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引用:
原帖由 tommo 於 2010-4-17 02:55 PM 發表
b efficiency=((48-powerloss)/48)(100%)
               (48-2(0.125)*2(2))/48)(100%)
          =99.87%
i agree with no. 2
我計唔到呢個-,-
佢話 current through transmisson wires 係 0 .125
所以唔知點解 係 0.125^2 x 2 x 2

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>>A helicopter is descending vertically at a constant speed of 3ms^-1 . A sandbag is released from the helicopter . The sandbag hits the ground 5 s later What was the height of the helicopter above the ground at the time the sandbag was released?

s=ut+(1/2)at^2
=3*5 + (1/2)(10)(5^2)

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引用:
原帖由 smallfatfat0506 於 2010-4-17 03:03 PM 發表
1.Turn ratio =16:1
Voltage in transmission wires=384V
I=0.125A
Voltage taken over by transmmision wires=0.125x4=0.5V
Voltage after step down=(384-0.5)/16=23.96875V
Real power gained by bulb= ...
個 voltage taken over by trans. wire 係 0.125  x 4?
唔係 parallel 咩? 定真係 in series?

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DELECTED

[ 本帖最後由 roviury 於 2010-4-17 03:22 PM 編輯 ]

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係 in series

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回覆 #8 roviury 的帖子

powerloss係I^2R=.125^2*4=0.0625
??

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