Energy and Work Energy and Work Physics Help Forum Oct 20th 2015, 04:39 AM #1 Junior Member   Join Date: Jan 2012 Posts: 19 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, 06:08 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 09:16 AM.   Oct 20th 2015, 10:28 AM #3 Junior Member   Join Date: Jan 2012 Posts: 19 thank you very much.  Tags energy, kinetic, points, potential Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post noddy Energy and Work 5 Jan 3rd 2014 12:21 PM man_in_motion Periodic and Circular Motion 2 Mar 19th 2010 12:43 AM werehk Electricity and Magnetism 1 Jul 8th 2009 10:43 PM s3a Energy and Work 4 Jun 16th 2009 11:08 PM s3a Energy and Work 1 May 28th 2009 06:02 PM 