# 查看完整版本 : 唔識點做落去- Problem on Momentum Conservation

halo18 2013-10-19 11:12 AM

## 唔識點做落去- Problem on Momentum Conservation

A 20.0kg  wood ball hangs from a 2.10m  -long wire. The maximum tension the wire can withstand without breaking is 500N  . A 0.800kg  projectile traveling horizontally hits and embeds itself in the wood ball.

Q. What is the largest speed this projectile can have without causing the cable to break?

Work by tension=Fd=(500)(2.10)=1050NmKE=(1/2)mv^2
1050=(1/2)(20.0+0.800)v2^2
v2=10.0m/sP before collision= P after collision
(0.800)v1=(20.0+0.800)(10.0)
v1=260m/s

[[i] 本帖最後由 halo18 於 2013-10-20 06:38 AM 編輯 [/i]]

↗亢龍無悔↙ 2013-10-19 11:38 AM

T - (M + m) g = (M + m) v*2 / R

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halo18 2013-10-19 12:38 PM

sorry我真係get唔到步驟係點做落去, 請解釋清楚少少

↗亢龍無悔↙ 2013-10-19 03:07 PM

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[quote]原帖由 [i]halo18[/i] 於 2013-10-19 11:12 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=373981602&ptid=22574238][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
A 20.0kg  wood ball hangs from a 2.10m  -long wire. The maximum tension the wire can withstand without breaking is 500N  . A 0.800kg  projectile traveling horizontally hits and embeds itself in the wo ... [/quote]

Your concept of momentum conservation is correct. However, there is no conversion of PE and KE in breaking the wire. Furthermore the ball can only swing, there is no "work done" by the tension in vertical displacement. It is difficult to guess how you think in coming up your first "equation". It only showed that you have not touch the right topic which is not included in the title of this post.

To answer the question, you have to understand the mechanism to break the wire. When the ball is at rest, the wire tension is only 200N to hang the ball. Why the tension increases when the ball begins to swing? (remove reference to other posts)

[[i] 本帖最後由 善良之安 於 2013-10-20 12:45 AM 編輯 [/i]]

↗亢龍無悔↙ 2013-10-20 12:16 AM

## 回覆5 #的帖子

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halo18 2013-10-20 06:42 AM

## 回覆 6# 的帖子

Thank you both of you, and I hope that neither of you gets offended. I know... My physics, in this level, is not good so I need the explanation step by step.