Physics Help Forum Weight question

 General Physics General Physics Help Forum

 Mar 23rd 2017, 02:16 AM #1 Junior Member   Join Date: Mar 2017 Posts: 3 Weight question Need some help with question 12 and 13. I got 12 right and I just used my intuition. It seemed obvious that if the left side had 8/15 of the weight then it the center of mass was 7/15 away from it(maybe that's faulty reasoning?) . The verbal explanation given to me explained it in terms of torque and I didn't understand it. Also it may sound stupid but why is the answer to 13 A and not B? Last edited by sphys2017; Mar 23rd 2017 at 02:22 AM.
 Mar 23rd 2017, 07:28 AM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,322 The center of mass is defined to be the point where the torques due to the various components of weight of an object = 0. Remember torque = force (in this case weight) times moment arm distance. Let's call the location of the CM as being distance D as measured from point Y. There are two moment armsacting about the CM to consider: the one acting at point Y has a force of 800N and has a moment arm length from the CM of D, so the torque this force presents at point D is Dx800N in the clockwise direction. The torque due to the 700N load at point X has a moment arm length of (L-D), so presents a torque to point D of 700(L-X) in the counter-clockwise direction. By convention counter-clockwise torques are considered positive and clockwise is negative. The sum of these two torques acting about point D must equal zero (that's the definition of the center of mass): $\displaystyle \sum T = 700(L-X)-800X = 0$ Solve for X. As for question 13 - it doesn't matter whether you weigh both ends of the bed at the same time or not - it's static, and so its weight doesn't change, and neither does the center of mass. Hence you get the same readings whether you use two scales on one.

 Tags question, weight

 Thread Tools Display Modes Linear Mode

 Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post burgess General Physics 2 Jul 3rd 2014 03:15 AM Jg596 Kinematics and Dynamics 1 Mar 22nd 2009 08:35 PM mitnord Thermodynamics and Fluid Mechanics 1 Mar 10th 2009 11:41 AM mrpedantic Kinematics and Dynamics 2 Jan 24th 2009 06:11 PM rachana sisodia Kinematics and Dynamics 5 Nov 23rd 2008 05:34 AM