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 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum Oct 10th 2009, 04:50 AM #21 Banned   Join Date: Aug 2009 Location: UK Posts: 240 OOps !!! I made a mistake with the decimal fraction, it should be 0.404694. So 1 sec / ( 1-(sq-rt ( 1-0.404694)) = 4.3775 secs total time = (1/2)at^2 = 0.5 x 9.8 x 4.3775^2 = 93.89 m my mistake, your answer is correct. Whenever this sort of question shows it's face again i'll memorize it using the following: 38m = 38 m/s + (1/2)g = v (final velocity) = 38 + 4.9 v = gt t= v/g = 42.9 m/s / 9.8 = 4.3775 secs (1/2)gt^2 = 93.89 m Last edited by Paul46; Oct 10th 2009 at 06:47 AM.   Oct 10th 2009, 10:49 PM #22 Physics Team   Join Date: Feb 2009 Posts: 1,425 I am kind of relieved the answer is correct, but i dont understand this bit So 1 sec / (1-(sq-rt (1-0.35681)) = 5.05 secs total time falling I know the 0.35 bit is wrong from your later post, but how do you come to this ratio at all ? 1 sec / (1-(sq-rt (1-0.35681)) Last edited by physicsquest; Oct 10th 2009 at 11:24 PM.   Oct 11th 2009, 02:44 AM #23 Banned   Join Date: Aug 2009 Location: UK Posts: 240 physicsquest, It's just a ratio to confirm the total time. A stone is dropped from a cliff in the final second the stone travels 67m, find the the total displacement. 67m = v= 67 m/s + (1/2)g = 71.9 m/s v=gt t=v/g = 7.3367 secs (1/2)gt^2 = 263.755m total displacement To confirm is the answer was correct i would use: 67m = 0.254023 decimal fraction of total displacement. final 1 sec/(1-sq-rt(1-0.254023)) = 7.3367 secs But since now i know that the final velocity (v) will be displacement (67m) + (1/2g) i no longer need to confirm the answer using that formula. It may seem abit odd, but it's the way i like to drum it into myself. Once i found your answer was correct i started breaking the calculations down to find a simple calculation of the answer, because i couldn't work the answer out, when i broke the calculations down i found the easy way to find (v) was displacement + (1/2)g then do the above calculations. I hope this answers your question.   Oct 11th 2009, 09:06 AM #24 Physics Team   Join Date: Apr 2009 Location: Boston's North Shore Posts: 1,576 Do I understand both of you right now, i.e. all three of us agree on the value of H? If so then I'm quite happy!   Oct 11th 2009, 12:25 PM #25 Senior Member   Join Date: Jul 2009 Location: Kathu Posts: 132 Falling Object Using the last second S = 38 U is unknown, A is Gravity and T is one. S = ut + 0.5At^2 U = 33.1 M / S at the beginning of the final second V = U + AT U = 0, A is Gravity T is unknown to find the time up to point U = 33.1 T = 3.37 Seconds T for the total journey is 4.37 seconds. Substituting back into S = Ut + 0.5AT^2 with U = 0 A Gravity T = 4.37 gives H = 93.89. Give or take a few millimeters depending on the flavour of G (9.8 used here) G of 9.81 gives H as 93.84 This seems to confirm the number already calculated before.   Oct 12th 2009, 12:09 PM #26 Banned   Join Date: Aug 2009 Location: UK Posts: 240 Just out of interest guys, I've been thinking is there a way to solve the above sort of question using longer time durations. Tapping away on my calculator for a few hours i found how to do it. A stone is dropped from a cliff, in the last 3 secs the stone travels 99m, find the total displacement. u = initial velocity v=final velocity a=acceleration (9.8 m/s^2) t=time t1=given time (3) s=displacement s1= given displacement (99) This is what i did to find it: v = s1/t1 + 3a/2 = 33 + 14.7 = 47.7 m/s t= v/a = 4.867 s (3 d.p) s= ut + (1/2)at^2 = 116.07 (2 d.p) Just to confirm: 4.867 s - 3 s = 1.867 s (1/2)at^2 = 17.07 17.07 + 99 = 116.07 Last edited by Paul46; Oct 12th 2009 at 12:29 PM.  Tags falling, object 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 jlyu002 Kinematics and Dynamics 5 Jul 15th 2014 10:28 AM mryeh Electricity and Magnetism 5 Apr 4th 2014 05:54 AM Deadstar Kinematics and Dynamics 2 Apr 5th 2010 12:44 PM tweedydaf Kinematics and Dynamics 2 Sep 18th 2008 10:33 PM colombo Kinematics and Dynamics 1 Sep 6th 2008 09:41 PM