Physics Help Forum Newtons second law

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 Feb 25th 2011, 01:46 AM #1 Junior Member   Join Date: Feb 2011 Posts: 18 Newtons second law A person stands on a set of bathroom scales in an elevator. The scale reads only 0.75 (i.e, 75%) of the person weight. Calculate the acceleration of the elevator and find the direction of the acceleration. Please help
 Feb 25th 2011, 10:04 AM #2 Physics Team     Join Date: Jun 2010 Location: Mauritius Posts: 609 The resultant force on the person, the mass of the person and his acceleration is related by F = ma, or more explicitly: Fnet = ma What is Fnet? It's the Weight of the person, minus the force of the elevator on the man's feet. Felevator - Weight = ma Weight of person = mg a of elevator = a of person Now, the bathroom scale will record the force that the elevator exerts on the person. Felevator - mg = ma Felevator = ma + mg Felevator = m(a+g) Felevator/m = 75% You know g, find a. __________________ 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?
 Feb 27th 2011, 06:34 AM #3 Junior Member   Join Date: Feb 2011 Posts: 18 I get the problem to 75= a + 9.78 but whatever answer i get is wrong according to the one in the back of the book. The correct answer is 2.45...
 Feb 27th 2011, 07:07 AM #4 Physics Team     Join Date: Jun 2010 Location: Mauritius Posts: 609 Oops, I shouldn't have put the last line. I actually forgot to delete it, sorry for that. =S Felevator = 0.75 mg __________________ 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?
 Feb 27th 2011, 02:46 PM #5 Junior Member   Join Date: Feb 2011 Posts: 18 so it should be 0.75 mg = a + 9.78 ?
 Feb 28th 2011, 07:30 AM #6 Physics Team     Join Date: Jun 2010 Location: Mauritius Posts: 609 0.75 mg = m(a + g) 0.75 g = a + g __________________ 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?
 Feb 28th 2011, 02:44 PM #7 Junior Member   Join Date: Feb 2011 Posts: 18 Thank you
 Mar 1st 2011, 01:44 AM #8 Junior Member   Join Date: Feb 2011 Posts: 18 0.75*9.78=9.78 + a is what i get. But i cant solve to match the answer in the back of the book 2.45m/s^2
 Mar 1st 2011, 07:40 AM #9 Physics Team     Join Date: Jun 2010 Location: Mauritius Posts: 609 Hm... this is a calculation mistake... Okay: 0.75 * 9.78 = 7.335 Hence: 7.335 = 9.78 + a So, a = 7.335 - 9.78 = -2.445 = -2.45 m/s^2 This means that the acceleration has magnitude 2.45 m/s^2 and acting downwards (the answer to the second part), since I first assumed that the force of the elevator was acting upwards, from here: Felevator - Weight = ma Obviously, weight is downwards and I took the up direction as positive. __________________ 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?
 Mar 1st 2011, 02:39 PM #10 Junior Member   Join Date: Feb 2011 Posts: 18 Thank you

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