| Author | Message | ||
     Lau Kai Kwong Username: lkk Registered: 04-2003 |
A uniform rod 1m long has weights 20N and 30N attached to its ends. If the weight of the rod is 50N, find the point on the rod where a support should be placed so that the rod will rest horizontally. 黃筠賢, please post your solution. | ||
| 971142 Username: 971142 Registered: 04-2003 |
Left side which lifted a 20N object, 0.5(20) = 10 Right side which lifted a 30N object, 0.5(30) = 15 S (50) = 5 S = 0.1 ∴the 50N should put at 0.5-0.1 = 0.4m from left. | ||
| 971142 Username: 971142 Registered: 04-2003 |
wrong tim~~~ let s be the pt. on the rod where a support should be placed, take moment at 20N, 0 + 50(0.5) + 30(1) = s(20+50+30) 55 = s(100) s = 0.55m from the end with 20N | ||
| Lau Kai Kwong Username: lkk Registered: 04-2003 |
Okay. But the presentation can be improved. For example, Let s be the distance between the support and the end with 20N weight. By taking moment at the end with 20N weight, ... | ||
| Lau Kai Kwong Username: lkk Registered: 04-2003 |
Please notice that you cannot take moment about a force! | ||
| Ming Username: 971004 Registered: 04-2003 |
Let s be the distance between the support and the end with 20N weight. By taking moment at the end with 20N weight, ... but the next post.... Please notice that you cannot take moment about a force! 係文字既錯(20N,20N weight)定係真係唔可以take moment about a force? i think the steps is correct but not the presentation, right? | ||
| Lau Kai Kwong Username: lkk Registered: 04-2003 |
係筠在文字上用錯，唔應該用 "take moment at 20N"，而是 "take moment at the end with 20N weight". | ||
| Lau Kai Kwong Username: lkk Registered: 04-2003 |
而事實上，只可以 take moment about a point, 或者 take moment about a line. 唔可以 take moment "at" a "force". |