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 Sep 8th, 2009, 05:30 PM #1 Rtael Team WTAworldSenior Member   Join Date: Feb 2003 Location: Day-Dreaming Posts: 7,815 Physics problem (easy) Just in an introductory physics class and having trouble working this out... A stone is dropped from the roof of a building; 1.30 after that, a second stone is thrown straight down with an initial speed of 18.0 , and the two stones land at the same time. I have to figure out: How long did it take the first stone to reach the ground? How high is the building? What are the speeds of the two stones just before they hit the ground? I attempted using the formula x = x(0) + v(0)t + (1/2)at^2 and got 0 = x(0) + 0 + (1/2)(-9.8)(t^2) and 0 = x(0) + 18t + (1/2)(-9.8)(t-1.3)^2 and worked that out to t = 1.6 seconds unfortunately that was wrong, lol any help? __________________ "You Worry About Your Jump Rope and Yourself!" -Coral on Battle of the Sexes 2 Running for the cliff again...
 Sep 8th, 2009, 05:31 PM #2 Rtael Team WTAworldSenior Member   Join Date: Feb 2003 Location: Day-Dreaming Posts: 7,815 Re: Physics problem (easy) those units came out weird but it's 1.3 s and 18.0 m/s __________________ "You Worry About Your Jump Rope and Yourself!" -Coral on Battle of the Sexes 2 Running for the cliff again...
 Sep 8th, 2009, 05:43 PM #3 Sally Struthers Senior Member     Join Date: Jun 2004 Location: UnattractiveOn the inside Posts: 10,895 Re: Physics problem (easy) it has been a long time so if this is wrong don't blame me.. how high is hte building? 2as =(v1)^2 - (v0)^2 2(-9.8m/s^2)s = 0 - (18 m/s )^2 s=(- 324m^2/s^2)/(-19.6 m/s^2) s= 16.53m __________________ Blowing out someone else's candle does not make yours shine brighter.
 Sep 8th, 2009, 07:49 PM #4 Rtael Team WTAworldSenior Member   Join Date: Feb 2003 Location: Day-Dreaming Posts: 7,815 Re: Physics problem (easy) nope, not the right answer __________________ "You Worry About Your Jump Rope and Yourself!" -Coral on Battle of the Sexes 2 Running for the cliff again...
 Sep 8th, 2009, 08:31 PM #5 Sally Struthers Senior Member     Join Date: Jun 2004 Location: UnattractiveOn the inside Posts: 10,895 Re: Physics problem (easy) well what is the right answer? __________________ Blowing out someone else's candle does not make yours shine brighter.
 Sep 8th, 2009, 08:34 PM #6 vadin124 Senior Member     Join Date: Aug 2007 Posts: 1,854 Re: Physics problem (easy) 1 a = 9.8 v = 0 u = 18 v = u + at 0 = 18 + 9.8t -18 = 9.8t t = 1.83s (+1.30) t = 3.13s 2 s = ut + 1/2at^2 s = 18x1.83 + 1/2x9.8x1.83^2 s = 32.94 + 16.41 s = 49.35m 3 Stone 1 v = u + at v = 0 + 9.8 x 3.13 v = 30.67ms^-1 Stone 2 v = u + at v = 18 + 9.8x1.83 v = 35.93ms^-1 probably completely wrong, but i tried __________________ Marion Bartoli Tennis player of the Millenium Grand Slam Finalist | Career High Rank #9 | Queen of the Injury Time-Out Venus | Dokic | Kirilenko | Robson
 Sep 8th, 2009, 08:36 PM #7 Rtael Team WTAworldSenior Member   Join Date: Feb 2003 Location: Day-Dreaming Posts: 7,815 Re: Physics problem (easy) Also wrong. And I don't know what the right answer is...I'm doing it online and i have a certain number of tries to get it right. __________________ "You Worry About Your Jump Rope and Yourself!" -Coral on Battle of the Sexes 2 Running for the cliff again...
Sep 8th, 2009, 08:49 PM   #8
Senior Member

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Posts: 1,854
Re: Physics problem (easy)

Quote:
 Originally Posted by Rtael Also wrong. And I don't know what the right answer is...I'm doing it online and i have a certain number of tries to get it right.
masteringphysics sucks...our school tried it with us for a couple weeks, was rubbish though so haven't used it since...

i feel sorry for you
__________________
Marion Bartoli

Tennis player of the Millenium

Grand Slam Finalist | Career High Rank #9 | Queen of the Injury Time-Out

Venus | Dokic | Kirilenko | Robson

 Sep 8th, 2009, 09:03 PM #9 AleOrtu Senior Member     Join Date: Jun 2002 Location: Argentina Posts: 6,755 Re: Physics problem (easy) Remove the "(easy)" word in the title of the thrad __________________ Eleni Daniilidou - Iveta Benesova - Melanie Klaffner - Gisela Dulko - Daniela Hantuchova - Jelena Dokic - Karolina Sprem - Akenatón rules, Horemheb sucks Member #5452 Making Your Life Better Since 2002
 Sep 8th, 2009, 09:23 PM #10 FrOzon Senior Member     Join Date: Apr 2005 Location: Berlin, Germany Posts: 3,889 Re: Physics problem (easy) How long did it take the first stone to reach the ground? 2,88sec How high is the building? 40.7m What are the speeds of the two stones just before they hit the ground? Stone 1 = 28.3 m/s Stone 2 = 33.5 m/s
 Sep 8th, 2009, 09:23 PM #11 Sally Struthers Senior Member     Join Date: Jun 2004 Location: UnattractiveOn the inside Posts: 10,895 Re: Physics problem (easy) ok .. y =y0 +v0t -1/2gt^2 y = 0 + 18(1.3) -1/2(9.8)(1.3)^2 y = 15.119 meters question a y = y0 +vot - 1/2gt^2 15.119 m = 0+0 -1/2(9.8)t^2 t = 1.76 seconds question c stone 1 v= v0 -gt v = 0-9.8(1.76) v = 17.248 m/s stone 2 v= 18 -9.8(1.76+1.3) v = 11.99 m/s __________________ Blowing out someone else's candle does not make yours shine brighter. Last edited by Sally Struthers : Sep 8th, 2009 at 09:30 PM.
 Sep 8th, 2009, 09:34 PM #12 Apoorv Senior Member     Join Date: Jan 2005 Posts: 4,672 Re: Physics problem (easy) s = (0)*t + 0.5*9.8*t*t = 18*(t-1.3) + 0.5*9.8*(t-1.3)*(t-1.3) => 4.9t*t = 18t - 23.4 + 4.9t*t - 12.74t + 8.28 => 5.26t = 14.72 => t = 2.8 sec. (approx.) height, h = 0.5*9.8*2.8*2.8 = 38.416 m speed of stone 1, v = 0 + 9.8 * 2.8 = 27.44 m/s Speed of stone 2, v = 18 + 9.8 * 1.5 = 32.7 m/s Is this solution correct, I used to study physics about 8 years back. So it may be wrong. I just took g as positive. because it is in the direction of throw of stone i.e downwards. __________________ Sania Mirza Elena Dementieva
 Sep 8th, 2009, 09:35 PM #13 Sally Struthers Senior Member     Join Date: Jun 2004 Location: UnattractiveOn the inside Posts: 10,895 Re: Physics problem (easy) all different answers. no one knows what they are doing __________________ Blowing out someone else's candle does not make yours shine brighter.
Sep 8th, 2009, 09:49 PM   #14
FrOzon
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Re: Physics problem (easy)

Quote:
 Originally Posted by Apoorv s = (0)*t + 0.5*9.8*t*t = 18*(t-1.3) + 0.5*9.8*(t-1.3)*(t-1.3) => 4.9t*t = 18t - 23.4 + 4.9t*t - 12.74t + 8.28 => 5.26t = 14.72 => t = 2.8 sec. (approx.) height, h = 0.5*9.8*2.8*2.8 = 38.416 m speed of stone 1, v = 0 + 9.8 * 2.8 = 27.44 m/s Speed of stone 2, v = 18 + 9.8 * 1.5 = 32.7 m/s Is this solution correct, I used to study physics about 8 years back. So it may be wrong. I just took g as positive. because it is in the direction of throw of stone i.e downwards.
Almost correct! Just a small error in line 3: 23.4 - 8.28 = 15.12, then you get the same numbers as I do
(Also a physicist )

Sep 8th, 2009, 10:12 PM   #15
Apoorv
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Posts: 4,672
Re: Physics problem (easy)

Quote:
 Originally Posted by FrOzon Almost correct! Just a small error in line 3: 23.4 - 8.28 = 15.12, then you get the same numbers as I do (Also a physicist )
Thanks!! I had a feeling I am making a mistake in calculation but I was too lazy to check
__________________

Sania Mirza

Elena Dementieva