Physics Help Forum I seriously cannot do any Special Relativity Q's, humph =(

 Special and General Relativity Special and General Relativity Physics Help Forum

 Apr 21st 2010, 09:11 AM #1 Junior Member   Join Date: Mar 2010 Posts: 10 I seriously cannot do any Special Relativity Q's, humph =( Hi everyone. So my problem lies in that I understand the material covered, but when I attempt any questions it results in an epic fail. Could someone please lend me a hand? For example, could I post a question and have someone walk me through it please? Here goes: Q. Two spaceships A and B travel along the x axis in opposite directions towards one another. They have velocities (Beta)A= 0.8 and (Beta)B= -0.6, as measured in the Earth frame, respectively. At time t=0, ship A is at position xA=0 and ship B at xB= L = 1 light-second. (a) At what time tc and at which position xc, do the ships cross as seen in the earth frame? (b)Use Lorentz transformations to express at what time (tc)' and at which position (xc)' the two ships cross as seen in the frame of ship A. Thank you very much
 Apr 27th 2010, 02:41 PM #2 Senior Member   Join Date: Mar 2010 Location: Lithuania Posts: 105 Can you find a) ?
 Oct 12th 2010, 08:37 AM #3 Senior Member   Join Date: Aug 2010 Posts: 427 You say that distances are measured in light seconds but don't give any units for the speeds. Are we to assume they are measured in "light seconds per second"? Assuming that: As seen from the earth, the two ships are closing with a net speed of 1.4 ls/s. They will cover the distance between them, 1 ls, in tc= 1/1.4 seconds. In that time, ship A will have moved a distance .8/1.4= 4/7 ls so xc= 4/7 ls. As seen from A, ship B is closing on it at (.6c+ .8c){1+ (.6)(.8)/c^2}= 1.4c/1.48= 7/74 (of course, c is 1 ls per second). Also, as seen from A, the initial distance beween A and B is sqrt(- (.8c)^2/c^2)= sqrt(.36)= .6= 3/5 ls so they will pass, according to A, in (tc)'= (3/5)/(7/74)= 224/35 seconds. In that time, A will have moved .8(224/35)= 896/175 and (xc)' = 896/175 ls. Last edited by HallsofIvy; Mar 27th 2011 at 08:57 AM.

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