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※楓』 2012-12-4 08:47 PM

phy 急問

以下題目以弟百思不得其解,還望有人能夠解答萬分感激

題目一:
A rocket standing on its laughing platform points straight upward.Its engine are activated and eject gas     
at a rate of 500kg/s.The molecules are expelled with a speed of 5km/s.How much mass can the rocket initially have if it is slowly to rise because of the engines?
題目二:
A light inelastic string passes around two small and smooth pulleys A and B in the same horizontal line at a distance 2a apart,and carries masses m1 at each end and a mass m2 (<2m1)at its middle point.The system is then released from rest with m2 at the middle point of AB.The system comes to instantaneous rest when m2 has fallen a distance.By applying the principle of conservation of mechanical energy,find the distance that m2 travlled?

alvinw8 2012-12-7 11:59 PM

[quote]原帖由 [i]※楓』[/i] 於 2012-12-4 08:47 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=348921452&ptid=21212584][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
以下題目以弟百思不得其解,還望有人能夠解答萬分感激

題目一:
A rocket standing on its laughing platform points straight upward.Its engine are activated and eject gas     
at a rate of 500kg/s.The mo ... [/quote]
1. At the short time internal t after time =0, calculate the momentum of the gas which is then equal to the momentum of the rocket. Then calculate the force acting on the rocket.

2. The potential energy lost of m2 is equal to the potential energy gain of the 2 m1.
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