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亡命天唉 2013-9-20 12:40 PM

Force (Newton's Laws)

[font=Tahoma][size=3][color=#333333]A 4.75[/color][color=#333333]kg[/color][color=#333333] box is suspended from the end of a short vertical rope of negligible mass. An upward force F(t) is applied to the end of the rope, and the height of the box above its initial position is y(t) = 2.8 t + 0.61 t^3[/color][/size][/font]
[font=Tahoma][size=3][color=#333333][font=Tahoma][size=3]What is the magnitude of the force F when t=4.3 s?

dustbus 2013-9-20 02:37 PM

y(t) = 2.8 t + 0.61 t^3

sub t=4.3,then find height

and then use s=ut+1/2at^2 find a

finally,use Fnet=ma

upward force-weight=ma

Zzlaz 2013-9-21 12:09 AM

use differentiation la...

then total force = mg+ma....

亡命天唉 2013-9-21 12:20 AM

here's my steps :
0.5a(4.3)^2 = (2.8)(4.3)+(0.61)(4.3)^3
a = 6.548 ms^-2

F net = (4.75)(6.548) - (4.75)(9.8)

but the answer i got is incorrect...what's wrong?

Zzlaz 2013-9-21 12:24 AM

[quote]原帖由 [i]亡命天唉[/i] 於 2013-9-21 12:20 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=372040611&ptid=22459631][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
here's my steps :
0.5a(4.3)^2 = (2.8)(4.3)+(0.61)(4.3)^3
a = 6.548 ms^-2

F net = (4.75)(6.548) - (4.75)(9.8)

but the answer i got is incorrect...what's wrong? [/quote]
向上加速梗, F-mg= ma => F= mg[color=#ff0000] +[/color] ma

亡命天唉 2013-9-21 12:44 AM

[quote]原帖由 [i]Zzlaz[/i] 於 2013-9-21 12:24 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=372040938&ptid=22459631][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

向上加速梗, F-mg= ma => F= mg + ma [/quote]
i changed la, but still wrong..

dustbus 2013-9-21 01:06 AM

[quote]原帖由 [i]Zzlaz[/i] 於 2013-9-21 12:09 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=372039753&ptid=22459631][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
use differentiation la...

then total force = mg+ma.... [/quote]
=.= I am stupid:smile_39::smile_39:

↗亢龍無悔↙ 2013-9-21 10:09 AM

The acceleration in this question is not constant. The answer in #5 should be smaller than the correct answer.



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Zzlaz 2013-9-21 05:24 PM

回覆8 #的帖子

Not my ans



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