Need help on a ques.
From: Gagan (gaganoops_at_hotmail.com)
Date: 10/19/04
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Date: 18 Oct 2004 23:21:48 -0700
Well i have spent the whole night over this problem but still don't
get what i am doing wrong.
(The prob is from Resnick,Halliday,Walker-Fundamentals of Physics 6th
edition).
Pg 70 41P
A football kicker can give the ball an initial speed of 25 m/s.
Within what 2 elevation angles must he kick the ball to score a field
goal from a point 50 m in front of goalposts whose horizontal bar is
3.44 m above the ground?
My sol.is
Along X axis
s = 50m
And taking the angle we have to find to be A.
Inital velocity = 25 cosA
s = v * t
50 = 25 cosA *t
t = 2 secA
Along Y axis.
INitial vel = 25 sinA
Using kinematial eq
s = ut + 0.5 a t^2
= 25 sinA * t + 0.5 (-10) t^2 (Here i have taken g = 10 for easier
cal.)
Now putting value of 6
= 50 tanA - 20 (secA)^2
sec^2 = tan^2 + 1
_________________________________
|s = -20 (tanA)^2 + 50 tanA - 20 |
_________________________________
NOw distance travelled on X axis must be less than equal to 3.44 or
the ball will not hit goalpost.
-20 tanA^2 + 50 tan A -20 <= 3.44
20 tana^2 -50 tan a + 23.44 >= 0
Solving (tana - 1.875)(tan a - 0.625) >= o
this gives me an answer opp to what i shud get(the ans is 31 -63
degress. However my ans doesnt inlude these and everything else. :;) )
Please tell me what i am doing wrong....
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