查看完整版本 : projectile motion

startanew 2013-11-2 04:09 PM

projectile motion

[img]http://i1163.photobucket.com/albums/q554/startanewww/projectilemotion1.png[/img]

[img]http://i1163.photobucket.com/albums/q554/startanewww/projectilemotion2-1.png[/img]

點解 2b 3b 我唔可以搵最後既 v, (1/2 u^2 + gh = 1/2 v^2), 再用 u/v = cos θ 搵 θ?

Thanks!

[[i] 本帖最後由 startanew 於 2013-11-2 05:07 PM 編輯 [/i]]

↗亢龍無悔↙ 2013-11-2 04:24 PM

你sure係cos?



[url=http://www.discuss.com.hk/android][img=100,23]http://i.discuss.com.hk/d/images/r10/androidD.jpg[/img][/url]

startanew 2013-11-2 04:29 PM

7cos15/v = cos θ
wrong?

↗亢龍無悔↙ 2013-11-2 05:28 PM

[quote]原帖由 [i]startanew[/i] 於 2-11-2013 04:29 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=374908134&ptid=22627106][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
7cos15/v = cos θ
wrong? [/quote] 仲覺得無問題?

dustbus 2013-11-2 06:12 PM

用少少trigo諗

cos係計vertical定horizontal既speed

startanew 2013-11-2 07:26 PM

我都係唔明我點解錯
我係用 resultant v, 唔係 Vy/Vx = tan θ

2b) vertical distance: 1/2 (9.81)(0.8)^2 = 3.14 m
(3.14)(9.81) + 1/2 (7)^2 = 1/2 v^2
v = 10.5 ms^(-1)
Ux / v = cos θ
θ = 50.0

startanew 2013-11-2 07:26 PM

ans: θ = 41.8
答案係用 Vy/Vx = tan θ

但點解我咁做就唔岩?

[[i] 本帖最後由 startanew 於 2013-11-4 05:31 PM 編輯 [/i]]

↗亢龍無悔↙ 2013-11-2 08:27 PM

[quote]原帖由 [i]startanew[/i] 於 2-11-2013 07:26 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=374917155&ptid=22627106][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]ans: θ = 41.8答案係用 Vy/Vx = tan θ但點解我咁做就唔岩? [/quote]返去form 2重學trigonometry 啦:smile_04:



[url=http://www.discuss.com.hk/android][img=100,23]http://i.discuss.com.hk/d/images/r10/androidD.jpg[/img][/url]

dustbus 2013-11-2 09:11 PM

[quote]原帖由 [i]startanew[/i] 於 2013-11-2 07:26 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=374917117&ptid=22627106][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
我都係唔明我點解錯
我係用 resultant v, 唔係 Vy/Vx = tan θ

2b) vertical distance: 1/2 (9.81)(0.8)^2 = 3.14 m
(3.14)(9.81) + 1/2 (7)^2 = 1/2 v^2
v = 10.5 ms^(-1)
Ux / v = cos θ
θ = 50.0 [/quote]

你究竟點得出3.14m...
人地都比左個height你:smile_27:

startanew 2013-11-2 09:31 PM

我唔明你地咩事要咁講我.......
我自己睇多次, 發現我只係vertical distance計錯, cos冇用錯囉!!!!!!!!!!!!!!!!!!!!!!!!! :smile_o03:  :')

vertical distance: s = 7sin15 (0.8) - 1/2 g (0.8)^2 = -1.6898 m
(1.6898)(9.81) + 1/2 7^2 = 1/2 v^2
v = 9.06 ms^(-1)
7cos15/9.06 = cos θ
θ = 41.8

dustbus 2013-11-3 12:20 PM

LOL:smile_30: :smile_30: :smile_30: :smile_30:

startanew 2013-11-3 12:56 PM

回覆 11# 的帖子

笑咩
:smile_o08:

dustbus 2013-11-3 08:48 PM

:smile_30: :smile_30: 冇野了你做得很好

↗亢龍無悔↙ 2013-11-4 08:34 AM

[quote]原帖由 [i]startanew[/i] 於 2-11-2013 09:31 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=374923838&ptid=22627106][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]我唔明你地咩事要咁講我.......我自己睇多次, 發現我只係vertical distance計錯, cos冇用錯囉!!!!!!!!!!!!!!!!!!!!!!!!! :smile_o03:  :') vertical distance: s = 7sin15 (0.8) - 1/2 g (0.8)^2 = -1.6898 m ... [/quote]最尾個cos angle 岩咩?



[url=http://www.discuss.com.hk/android][img=100,23]http://i.discuss.com.hk/d/images/r10/androidD.jpg[/img][/url]

startanew 2013-11-4 01:12 PM

#7
ans: 41.8

You still think I'm wrong? = =
:smile_o08: :smile_o02:

startanew 2013-11-4 01:14 PM

Although the method I use is slow, it is correct!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
:Q

↗亢龍無悔↙ 2013-11-4 02:52 PM

[quote]原帖由 [i]startanew[/i] 於 4-11-2013 01:12 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=375035360&ptid=22627106][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]#7ans: 41.8You still think I'm wrong? = =:smile_o08: :smile_o02: [/quote]你之前用紅字果到明明係用cos ,幾時變左tan ?
我就係見你vertical , horizontal component combine一齊用cos 我先直接話你錯
“7cos15/9.06 = cos θ”

仲有#6果到,點唔一開始就用v=u+at呢?
[url=http://www.discuss.com.hk/android][img=100,23]http://i.discuss.com.hk/d/images/r10/androidD.jpg[/img][/url]

[[i] 本帖最後由 ↗亢龍無悔↙ 於 2013-11-4 02:57 PM 使用[url=http://www.discuss.com.hk/android][img=100,23]http://i.discuss.com.hk/d/images/r10/androidD.jpg [/img][/url] 編輯 [/i]]

startanew 2013-11-4 05:29 PM

#7 我本來就係打 Vy/Vx = tan θ
"[b][color=#ff0000]vertical distance / horizontal distance[/color][/b] = tan θ" <-- 我搞錯左, 呢個係做第3題既方法

[quote]我就係見你vertical , horizontal component combine一齊用cos 我先直接話你錯[/quote]
#6 我已經好清楚咁話你知我用緊 Ux / [b]resultant v[/b] = cos θ
我唔明你點解要叫我"返去form 2重學trigonometry ".........
你答人問題唔代表你可以係唔係都話人錯!!!

[quote]仲有#6果到,點唔一開始就用v=u+at呢?[/quote]
我係 #7 已經講左我只係想知我咁做點解錯...
用 v=u+at 係搵 Vy, 同我既方法唔一樣。
我知用 v=u+at 係會快 d 搵到答案, 但我咁做唔叫錯囉!

startanew 2013-11-4 05:32 PM

同埋 #6 (3.14)(9.81) + 1/2 (7)^2 = 1/2 v^2 呢步都話你知我係搵 resultant v 啦............................

#1 1/2 u^2 + gh = 1/2 v^2 都係.............

你點可以因為我用 cos 就叫我返 f2 學 trigo!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[[i] 本帖最後由 startanew 於 2013-11-4 05:33 PM 編輯 [/i]]

↗亢龍無悔↙ 2013-11-4 06:14 PM

妖,你睇下#10 尾二果行啦
7cos15 係horizontal, 9.06係vertical , 有咩理由個angle 係arc cos去搵[url=http://www.discuss.com.hk/android][img=100,23]http://i.discuss.com.hk/d/images/r10/androidD.jpg[/img][/url]

[[i] 本帖最後由 ↗亢龍無悔↙ 於 2013-11-4 06:16 PM 使用[url=http://www.discuss.com.hk/android][img=100,23]http://i.discuss.com.hk/d/images/r10/androidD.jpg [/img][/url] 編輯 [/i]]

startanew 2013-11-4 06:20 PM

回覆 20# 的帖子

你冇野呀?? 9.06 係 RESULTANT V

startanew 2013-11-4 06:22 PM

你吹我唔漲喎, 我計到答案係事實。

而我都係唔明D人死都唔肯認自己講錯野 :smile_42: :smile_42: :smile_42:

[[i] 本帖最後由 startanew 於 2013-11-4 06:24 PM 編輯 [/i]]

startanew 2013-11-4 06:22 PM

一係你返F4學返D mechanics啦 :smile_42: :smile_42: :smile_42:

startanew 2013-11-4 06:39 PM

你自己禁機都知啦
用 v=u+at, tanθ = Vy/Vx, θ = 41.75667019...
用 7cos15/resultant V = cos θ,  θ = 41.75667019...
(全部用最準既數)

冇用 tan 唔代表錯囉 :smile_42: :smile_42: :smile_42:

↗亢龍無悔↙ 2013-11-4 06:45 PM

我從來都拎過計數機禁過,點會知9.06係resultant. 上一個step 計緊vertical displacement, 下一個step 就計velocity, 好容易令人覺得你計緊vertical speed.
我錯在無拎計數禁過條數
至於返f4學下mechanics ,就無咁嘅需要喇, 當年考評局比我呢啲咁嘅人都過到AL關,我都無計



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startanew 2013-11-4 06:53 PM

回覆 25# 的帖子

如果我要搵vertical speed, 我點會寫 conservation of energy....
一見到 1/2 u^2 + gh = 1/2 v^2 就知 v 係 resultant v....... 唔駛用cal機都會知.......

↗亢龍無悔↙ 2013-11-4 07:30 PM

回覆 26# 的帖子

又 a.s 又 1/2 , 我一望好自然以為你用緊 equation of motion, 始終第 1 個step, 計 displacement, 先入為主就會你會繼續用 equation of motion去計 speed, 當然兼以為你拆埋 component
#21唔講9.06 係 resultant, 我都仲會以為係 vertical component
如果寫嘅野唔係咁容易令人誤會的話, #5果位都唔會咁講

startanew 2013-11-4 07:46 PM

#1 1/2 u^2 + gh = 1/2 v^2 如果你有認真睇既話就唔會誤會......

ok 下次我 show steps 會加多幾隻字...
:smile_o08:

dustbus 2013-11-4 07:51 PM

[quote]原帖由 [i]startanew[/i] 於 2013-11-4 06:22 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=375052354&ptid=22627106][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
一係你返F4學返D mechanics啦 :smile_42: :smile_42: :smile_42: [/quote]

龍哥係香討數一數二既phy人:smile_44::smile_44:

如果你叫佢學番D mechanics

你應該要讀番小學:smile_30:

其實phy點勁都人都可能會錯

但希望你既語句可以溫和番少少

雖然係龍哥今次可能錯左,同埋一開始佢用既語句都唔好

但畢竟係你既師兄,指出師兄有錯既時候可以一開始先解釋下自己做乜,同埋對方XXX錯係邊

咁樣可能比較好:loveliness::loveliness:

唔係話你錯:loveliness::loveliness:但可能語句好少少比較好:smile_40::smile_40:

startanew 2013-11-4 08:47 PM

係佢 #8 "返去form 2重學trigonometry 啦" 挑起我既不滿先, 本人D數唔算超好, 但係唔叫差囉!
佢之後又唔認自己錯, 仲妖我, 我好嬲先會咁講...
我自問自己語氣已經算好好...

我冇解釋係因為我覺得太明顯...
重申 #1: 1/2 u^2 + gh = 1/2 v^2
同埋我 #6 一早都已經講左我用 resultant v, 會唔會係你地冇認真睇過先誤會我做錯呢?

"如果你叫佢學番D mechanics

你應該要讀番小學"
你冇野呀???? 你呢D係咩邏輯??
我叫佢返去溫返D mechanics係因為佢好似真係唔熟d equations囉

而我唔明點解我問條問題, 要比人係咁話, 最後仲係我自己答返自己..............

:smile_o16:
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