Physics Help Forum potential and kinetic energy at different points

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 Oct 20th 2015, 05:39 AM #1 Junior Member   Join Date: Jan 2012 Posts: 16 potential and kinetic energy at different points Sir / madam, I have to find the potential energy and the kinetic energy and its total of an object of mass 20 kg falls freely from 4m high from the ground level and the gravity due to earth is being taken as 10 m/s. height Potential Energy Kinetic Energy Total (1) (2) (3) (4) 4m 3m 2m 1m just above the ground. Here to calculate the Kinetic Energy at 3m, I took s = distance = 1m; u = initial velocity = 0; and used the formula s = ut + 1/2 at^2 and found the time t; Then I found v = s/t; Then I found Kinetic Energy using the formula 1/2mv^2; I did not get the answer. My friend used the formula v^2 - u^2 = 2as; He got the answer. But why my formula failed? I do not know. When I have to use this formula? Kindly guide me. with warm regards, Aranga
Oct 20th 2015, 07:08 AM   #2
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 Originally Posted by arangu1508 used the formula s = ut + 1/2 at^2 and found the time t;
Right - you should have at this point that t = sqrt(2s/g)

 Originally Posted by arangu1508 Then I found v = s/t;
No! this formula applies only if acceleration is 0, but acceleration = g = 10 m/s^2, not 0. You should use v=gt instead, which gives v = sqrt(2gs)

 Originally Posted by arangu1508 Then I found Kinetic Energy using the formula 1/2mv^2; I did not get the answer.
If you use the correct formula for 'v' you should get KE= (1/2) mv^2 = (1/2)m (2gs) = mgs.

Last edited by ChipB; Oct 20th 2015 at 10:16 AM.

 Oct 20th 2015, 11:28 AM #3 Junior Member   Join Date: Jan 2012 Posts: 16 thank you very much.

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