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Momentum and Energy

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↗亢龍無悔↙ 2013-4-25 07:35 PM

M g 4 = 2M g h
h = 2cm
This answer seems incorrect~

[[i] 本帖最後由 ↗亢龍無悔↙ 於 2013-4-25 07:36 PM 編輯 [/i]]

dustbus 2013-4-25 07:35 PM

1/2v^2=gh

m1u1=(m1+m2)v1,where m1=m2 and they stuck together,so they have same velocity

WA龍哥都秒殺左:smile_30:

Here is the solution:
When the two masses stuck together,the total mass doubled.As momentum is conserved,the[color=#ff0000] kinetic energy E=p^2/2m is halved.[/color]
By conservation of energy E=mgh,the maximum height h was decreased by a factor of 4 to 1 cm.The answer is D.

4mg/2=2mgh
h=1cm，as the solution stated

↗亢龍無悔↙ 2013-4-25 08:02 PM

dustbus 2013-4-25 08:09 PM

btw題外話

Zzlaz 2013-4-25 11:44 PM

energy is not conserved in this case la la...

[quote]原帖由 [i]Zzlaz[/i] 於 2013-4-25 23:44 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=359565694&ptid=21816364][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
energy is not conserved in this case la la... [/quote]yes as it is inelastic collision.

Zzlaz 2013-4-26 06:19 PM

dustbus 2013-4-26 09:15 PM

Zzlaz 2013-4-27 09:11 PM