hancyk
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a stone is thrown from the top of a cliff.
the height h in meters, of the stone above the ground level after t seconds is modeled by the function
h(t)=115+12.25t-4.9t^2.

a)give a physical interpretation of the meaning of the constant term 115 in the model.
b)write h(t) in the form A-B(t-C)^2, where A B and C are constant to be found.
c)using your answer to part b, or otherwise find with justification
i)the time taken after the stone is thrown for it to reach ground level.
ii)the maximum height of the stone above the ground and the time after which this
maximum height is reached
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InfiniteWill
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a) 115 is the height the stone is thrown from
b) dunno if right, but i got...
113.4375-4.9(t-5/4)^2
So A=113.4375, B=4.9, C=5/4
but doesn't work out for part c)i)
c)ii) maximum height above ground is the height the stone is dropped at? so 115? and time will be 0, since that's where the stone is dropped.

In general, I think the question is wrong...but I am not sure....Could you type out the whole question, pls?
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hancyk
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(Original post by neluxsan)
a) 115 is the height the stone is thrown from
b) dunno if right, but i got...
113.4375-4.9(t-5/4)^2
So A=113.4375, B=4.9, C=5/4
but doesn't work out for part c)i)
c)ii) maximum height above ground is the height the stone is dropped at? so 115? and time will be 0, since that's where the stone is dropped.

In general, I think the question is wrong...but I am not sure....Could you type out the whole question, pls?
this is the whole question
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InfiniteWill
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sorry I suppose I don't know how to do it
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Bradley00
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Oh, I had a lot of fun doing this question the other day, I will try my best to explain it. I will spoiler block it incase someone wants to do it without the answers. Also, if you could tell me where you got this question from because im having trouble finding it now, and the work is in my folder at school...

Spoiler:
Show

Part a - Your constant will be the place where the stone is thrown off, so the height of the cliff will be 115
Spoiler:
Show

Part b - Just do your normal completing the square, which if you do it right you should get something like this... 3925/32 - 4.9(t-5/4)^2
Spoiler:
Show

Part c.i - Make the equation = 0 because the height would be 0 when it reaches ground level, then make t the subject of the equation. You should get something like this... (35+5sqrt(785))/28.
Spoiler:
Show

Part c.ii - It would help if you drew the graph for this part, but the turning point of the graph will show you the maximum height and time. Which should be something like max height of 122.66m at 1.25 seconds
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Student guest
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Where did you get the question from???? It came up in my mock exam.
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systaniec324
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Here are the answers:

9)
a) 115 m is the height of the cliff which means that's the stone's initial height as that's when t=0.

b) h(t) = -4.9(t^2- 2.5t)+155
h(t) = -4.9(t - 1.25)^2 - (-4.9)(1.25)^2 +115
= -4.9(t - 1.25)^2 +122.65625
= 122.65625 - 4.9(t - 1.25)^2

c)
i) -4.9t^2 + 12.25t + 115 = 0 as the height is 0 when it's on the ground.

Find t using the completed square from part b or use the quadratic formula.

t=6.25 s , ignore any negative values as t can't be less than 0

ii) Using the completed square, we know that the max point of a curve is defined by the constant A. So max height of the stone is 123 m (3 s.f)
We also know that the constant C represents the time that the stone is in the air. Meaning time must be t=1.25 s.
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Skybluey217
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(Original post by systaniec324)
Here are the answers:

9)
a) 115 m is the height of the cliff which means that's the stone's initial height as that's when t=0.

b) h(t) = -4.9(t^2 - 2.5t)+155
h(t) = -4.9(t - 1.25)^2 - (-4.9)(1.25)^2 +115
= -4.9(t - 1.25)^2 +122.65625
= 122.65625 - 4.9(t - 1.25)^2

c)
i) -4.9t^2 + 12.25t + 115 = 0 as the height is 0 when it's on the ground.

Find t using the completed square from part b or use the quadratic formula.

t=6.25 s , ignore any negative values as t can't be less than 0

ii) Using the completed square, we know that the max point of a curve is defined by the constant A. So max height of the stone is 123 m (3 s.f)
We also know that the constant C represents the time that the stone is in the air. Meaning time must be t=1.25 s.
thankyou
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