# 查看完整版本 : 物理一問

lht021044 2013-8-10 08:45 PM

## 物理一問

mgh=0.5mv^2

dustbus 2013-8-10 10:20 PM

[quote]原帖由 [i]lht021044[/i] 於 2013-8-10 08:45 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=369094040&ptid=22293214][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

mgh=0.5mv^2

G係地心加速率都係10咋喎

velocity就變:lDD_001:

37182456 2013-8-11 01:44 AM

[quote]原帖由 [i]dustbus[/i] 於 10-8-2013 22:20 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=369100863&ptid=22293214][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

G係地心加速率都係10咋喎

velocity就變:lDD_001: [/quote]係地球半徑2倍,g係2.45咋

lht021044 2013-8-11 06:52 AM

## 回覆 3# 的帖子

mgh=0.5mv^2
(GMm/9R^2)(2R)=0.5mv^2

0.5mv^2=2GMm/9R

↗亢龍無悔↙ 2013-8-11 07:40 AM

#3 makes a very serious mistake

#4 in this case , you need to do integration for the newton law of gravitation with the limit from 3R to R in order to find the workdone by gravitational force to move the mass from 3R to R.

bear in mind that the g changes significantly when the mass travel from 3R to R, direct usage for m g h = 1/2 m v* 2 no longer hold except you do the integration

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[[i] 本帖最後由 ↗亢龍無悔↙ 於 2013-8-11 07:48 AM 編輯 [/i]]

↗亢龍無悔↙ 2013-8-11 07:54 AM

In case you can't understand

F = ( G M m ) / R*2
Integrate the above equation with respect to R with the upper limit R and lower limit 3R

[ - ( G M m ) / R ] with the upper limit R and lower limit 3R

Decrease in gravitational potential energy = [ - ( G M m ) / R - - ( G M m ) / 3R ]
= ( 2 G M m ) / 3R ] = 1/2 m v*2
Sub. G M R into the equation, you can find v

lht021044 2013-8-11 08:32 AM

## 回覆 5# 的帖子

[[i] 本帖最後由 lht021044 於 2013-8-11 08:33 AM 編輯 [/i]]

dustbus 2013-8-11 02:44 PM

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jmlo 2013-8-12 12:06 AM

## 回覆 8# 的帖子

It's implied from the Newton's law of gravitation, which is inversely proportional to r^2. The situation is analogous to the Coulomb's law in electrostatics.

thomas1126 2013-8-15 12:36 PM

[quote]原帖由 [i]lht021044[/i] 於 2013-8-10 08:45 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=369094040&ptid=22293214][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

mgh=0.5mv^2

mgh=0.5mv^2　＜＜－－－　呢條式成立既前提係個g不變所以只可以適用於大約離地面幾十公里內既距離太遠 遠到去月球咁遠就要用第2條式 -->  -GMm/r

↗亢龍無悔↙ 2013-8-15 12:49 PM

[quote]原帖由 [i]thomas1126[/i] 於 15-8-2013 12:36 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=369468649&ptid=22293214][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

mgh=0.5mv^2　＜＜－－－　呢條式成立既前提係個g不變所以只可以適用於大約離地面幾十公里內既距離太遠 遠到去月球咁遠就要用第2條式 -->  -GMm/r [/quote]

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thomas1126 2013-8-15 08:36 PM