# 查看完整版本 : mechanics

vinclavv 2015-3-27 10:53 PM

## mechanics

A 2.00-kg block hangs from a rubber cord, being supported so that the cord is not stretched. The unstretched length of the cord is 0.500 m, and its mass is 5.00 g. The “spring constant” for the cord is 100 N/m. The block is released and stops momentarily at the lowest point. (a) Determine the tension in the cord when the block is at this lowest point. (b) What is the length of the cord in this “stretched” position?

thx a lot

vinclavv 2015-3-27 10:58 PM

[quote]原帖由 [i]vinclavv[/i] 於 2015-3-27 10:53 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=412763687&ptid=24492633][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
A 2.00-kg block hangs from a rubber cord, being supported so that the cord is not stretched. The unstretched length of the cord is 0.500 m, and its mass is 5.00 g. The “spring constant” for the cord ... [/quote]

I am very confused why the ans for (a) is 2*mg instead of mg.

[7] 2015-3-28 01:29 AM

[quote]原帖由 [i]vinclavv[/i] 於 2015-3-27 10:58 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=412764046&ptid=24492633][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

I am very confused why the ans for (a) is 2*mg instead of mg. [/quote]because in the logic of your model answer, they consider it as an S.H.M., so the accel. of the the two end would be equal in mag. but opposite in nature, which is g, so at the lowest pt., we have
T-mg=mg

vinclavv 2015-3-28 09:26 AM

[quote]原帖由 [i][7][/i] 於 2015-3-28 01:29 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=412773537&ptid=24492633][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
because in the logic of your model answer, they consider it as an S.H.M., so the accel. of the the two end would be equal in mag. but opposite in nature, which is g, so at the lowest pt., we have
T- ... [/quote]

[7] 2015-3-28 12:51 PM

[quote]原帖由 [i]vinclavv[/i] 於 2015-3-28 09:26 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=412786831&ptid=24492633][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]