Physics Help Forum Force variation with increasing/decreasing distance and angle?
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 Jul 31st 2011, 03:02 AM #1 Junior Member   Join Date: Jul 2011 Posts: 11 Force variation with increasing/decreasing distance and angle? Practicle/Real Setup: I have a glass screen of length 35" and width 46.5". I have mounted one force sensor at the left top of glass screen and another sensor on the right top of the glass screen. Now, if I press my glass screen from any point on the screen I shall get some force at the left and some force at the right sensor output accordingly. Please refer to the attached figure. I have hard and tought time to solve this problem. I pressed at point A with (x,y) = (3,32). At the left sensor I am getting 7.015 Newtons. How much force I shall expect/get at right sensor?? Please refer the below info as well. Point A with left sensor: Angle = 45 degrees Distance from sensor = 4.24" Point B with right sensor: Angle = 86.06 degrees Distance from sensor = 43.6" You can calculate angle and distance with your own calculation if I am wrong. Thanks alot in advance, Best Regards, Naveed Attached Thumbnails
 Aug 3rd 2011, 09:51 AM #2 Physics Team     Join Date: Jun 2010 Location: Mauritius Posts: 609 I would have used the principle of moments... but I'm not sure if that would be applicable to 100%, especially if the glass sheet is not uniform. And I think that the mass of the glass sheet is important. Anyway, you press at A with a force F, and as the weight of the glass is W, the total force at A becomes: Sl = W/4 + (A)(32/35)([46.5-3]/46.5) Sl is the share of force at the left sensor. For the right sensor... Sr = W/4 + (A)(32/35)(3/46.5) Two equations with 3 variables, that's not possible to estimate the force registered by the right sensor unless the weight of the glass is known. Also, a shortcoming in this 'experiment', when you press somewhere on the glass, does the glass remain dead horizontal? Because on the two other sides, the height of the corner above the ground might remain the same while the height of the corners at the sensors might decrease, meaning that the glass sheet is no more horizontal, and some of the weight of the glass sheet gets onto the two sensors, causing them to show a little more force than you expect. __________________ Jerry (Got my results!) It is easier to protect your feet with slippers than to cover the earth with carpet. No one can go back and change a bad beginning; but anyone can start now and create a successful ending. If a problem can be solved, no need to worry about it. If it cannot be solved what is the use of worrying?
 Aug 4th 2011, 06:28 AM #3 Junior Member   Join Date: Jul 2011 Posts: 11 Thx alot dear, I have uploaded the rough picture of screen for you to get some idea, sensors are in red color in picture , sensors are mounted between aluminum and glass screen. When you press glass it will press sensors. Thx for the calculations, I have a question. We already have the value of left sensor which is 7.01N I got from sensor. So we only need to find the expected value at right sensor. I dont require exact/accurate force calculation. I want to find expected right sensor force(F2 or Fr) to do calibration for the right sensor by taking left sensor as a reference, As I am getting improper values as a result. Thx alot for guidence, Best Regards, Naveed Attached Thumbnails
 Aug 4th 2011, 06:36 AM #4 Junior Member   Join Date: Jul 2011 Posts: 11 Dear, Just to update you, With my sensor's reading at when I pressed at point A. I got 7.01N at left sensor and 0.22N at right sensor. I know both values are wrong but left value looks bit ok! So I want to calibrate right sensor. The application is not to measure the accurate pressure/force, the application is in touch sensing. Your comments plz, Regards, Naveed
 Aug 4th 2011, 06:42 AM #5 Junior Member   Join Date: Jul 2011 Posts: 11 Dear one query, For the left sensor... Sl = W/4 + (A)(32/35)([46.5-3]/46.5) For the right sensor... Sr = W/4 + (A)(32/35)(3/46.5) If Sl is 7.01N , we can find "W" accordingly. Then A can also be found and finally Sr. But what is A?? Is it representing the point where we pressed(x,y)? Naveed
 Aug 4th 2011, 06:44 AM #6 Junior Member   Join Date: Jul 2011 Posts: 11 I m sorry, If A is known then W can be found. What is A?
 Aug 4th 2011, 07:53 AM #7 Physics Team     Join Date: Jun 2010 Location: Mauritius Posts: 609 Well, if you have Sr and are sure about that, that should settle it, but your initial question was to find Sr (to then be able to calibrate it) Oh and A is the force that you apply at A. If you know Sl, you cannot find W, because substituting Sl, gives you: 7.01 = W/4 + (A)(32/35)([46.5-3]/46.5) Simplifying... 7.01 = W/4 + (A)(32/35)([46.5-3]/46.5) 28.04 = W + 3.42A And we are left with one equation and two unknowns, and another equation, Sr = W/4 + (A)(32/35)(3/46.5) with three unknowns. (or simplified...) 4Sr = W + 0.236A W is the one that I think is the easiest to find, that is the weight of the glass sheet, but since I don't know the circumstances of the experiment, I can't just say it like that Ideally, I would like to know the absolute constants if I was in your place. The weight of the glass sheet won't change whatever happens, so I would be finding that first. Otherwise, you can get A if you are using weights of specific and known weight. __________________ Jerry (Got my results!) It is easier to protect your feet with slippers than to cover the earth with carpet. No one can go back and change a bad beginning; but anyone can start now and create a successful ending. If a problem can be solved, no need to worry about it. If it cannot be solved what is the use of worrying?
 Aug 5th 2011, 10:11 PM #8 Junior Member   Join Date: Jul 2011 Posts: 11 Dear , Thank you so much , the result looks ok! but I have query and doubt For left sensor Sl = W/4 + (A)(32/35)([46.5-3]/46.5) For right sensor... Sr = W/4 + (A)(32/35)(3/46.5) Can you explain plz that how did you drive this? I have question that left sensor's X-axis is 3" far from A so Sl shouldnt be Sl = W/4 + (A)(32/35)(3/46.5)? Sr = W/4 + (A)(32/35)([46.5-3]/46.5)? Also explain that why did you add W/4 ?
 Aug 5th 2011, 10:13 PM #9 Junior Member   Join Date: Jul 2011 Posts: 11 I mean the relation shall be Sl = W/4 + (A)(32/35)(3/46.5)? Sr = W/4 + (A)(32/35)([46.5-3]/46.5)? Is it?