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jem777
Regular poster

Post Number: 74
Posted on Sunday, 31 July, 2011 - 11:26 pm:   

a plane is flying horizontally at 1200m above the ground. a parachutist of mass 80kg steps out and falls freely under gravity for x metres befroe opening the parachute where he is subjected to a vertical resistive force of 1120N. when he reaches the ground his vertical component of velocity is 0. show x=360 and the time taken for the fall is 200/7

im not sure if im missing something here. I can get an answer for x of just under 360 and a real mess for the time. It may come out to be something like 200/7 but im getting nothing that clean, just lots of roots etc.

what i have said is,with down as positive:

first part of motion: using v^2=u^2+2as
v^2=2gx

for second part of motion:
F=ma => -1120+80g=-80a => a=14-g

then i have put this value for a into v^2=u^2+2as when landing so

0=2gx-2([14 - g)(1200-x))
0=gx-16800-gx+14x+1200g
16800-1200g=(14)x
which using g=9.81 gives x=359.1.

maybe im going about this in the wrong way and im just lucky to get so close to 360?
Simba (Chris Bryant)
Veteran poster

Post Number: 2451
Posted on Sunday, 31 July, 2011 - 11:33 pm:   

Looks good to me; you get the correct answer exactly if you take g = 9.8, not g = 9.81. Try that for the time too, might help :-) .
jem777
Regular poster

Post Number: 75
Posted on Monday, 01 August, 2011 - 10:57 am:   

thanks.
there was another question with a fraction as an answer and i had a mess that rounded to it. I will now go back to that and try 9.8 in that one too. wish they would make up their mind! on some questions they want g=10 used and 9.81 works for others. I see why i never liked applied :-)
Simba (Chris Bryant)
Veteran poster

Post Number: 2453
Posted on Monday, 01 August, 2011 - 06:54 pm:   

Hehe, yeah, I feel your pain - I used to just leave my answers in terms of g...
Vlad
Prolific poster

Post Number: 280
Posted on Monday, 01 August, 2011 - 08:07 pm:   

When doing an exam, first look at what the question asks. If the question itself tells you a value of g, use that, and if not, then look at the front of the paper for what the value of g should be. (be careful, sometimes the paper can say use g=9.81 at the front, but certain questions might tell you to use g=10 in that same paper) If all else fails, then probably to be safe you should first get an answer involving g, then say what value of g you are going to use and plug it in...
jem777
Frequent poster

Post Number: 76
Posted on Tuesday, 02 August, 2011 - 12:35 am:   

using the 9.8 gives the nice answer too. although i did use

s=ut+1/2at^2 first to get time, hence the root but it all fell out using

v=u+at.

i getting to like this applied a bit more now. when i was doing it years ago i did not fully understand the questions had many layers to them. I look at pure questions and it is obvious what to do but with these its as case of: resolve in right direction (ie direction which lessen the work) , find acc then time to find distance , moments about the right point, etc etc. And even then its a job sometimes to take the equations and get the answer out! I respect this a lot more now :-)

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