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Old Sep 8th, 2009, 04:30 PM   #1
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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?
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Old Sep 8th, 2009, 04:31 PM   #2
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Re: Physics problem (easy)

those units came out weird but it's 1.3 s and 18.0 m/s
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Old Sep 8th, 2009, 04:43 PM   #3
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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
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Old Sep 8th, 2009, 06:49 PM   #4
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Re: Physics problem (easy)

nope, not the right answer
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Old Sep 8th, 2009, 07:31 PM   #5
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Re: Physics problem (easy)

well what is the right answer?
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Old Sep 8th, 2009, 07:34 PM   #6
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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
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Old Sep 8th, 2009, 07:36 PM   #7
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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.
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Old Sep 8th, 2009, 07:49 PM   #8
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Re: Physics problem (easy)

Quote:
Originally Posted by Rtael View Post
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
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Old Sep 8th, 2009, 08:03 PM   #9
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Re: Physics problem (easy)

Remove the "(easy)" word in the title of the thrad
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Old Sep 8th, 2009, 08:23 PM   #10
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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
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Old Sep 8th, 2009, 08:23 PM   #11
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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
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Old Sep 8th, 2009, 08:34 PM   #12
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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.
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Old Sep 8th, 2009, 08:35 PM   #13
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Re: Physics problem (easy)

all different answers. no one knows what they are doing
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Old Sep 8th, 2009, 08:49 PM   #14
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Re: Physics problem (easy)

Quote:
Originally Posted by Apoorv View Post
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 )
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Old Sep 8th, 2009, 09:12 PM   #15
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Re: Physics problem (easy)

Quote:
Originally Posted by FrOzon View Post
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
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