help!!!!projectile question (1 Viewer)

[weesa]

help anyone?

a baseball player hits a ball at a velocity of 60km/h str0iking the ball from 0.9m off the ground. through what angles should he hit the ball so that it just clears a 3m high wall 18m away?( ignore air resistence and take g as 9.8m/s)

 

[mojako]

60 km/h = 60000 m/h = 60000/3600 m/s = 50/3 m/s

Do the usual integration stuff and you'll find:
x = 50/3 cos@ t
y = -4.9t^2 + 50/3 sin@ t + 0.9

put (x=18, y=3) into the above equations:
18 = 50/3 cos@ t
t = 1.08 / cos@

3 = -4.9t^2 + 50/3 sin@ t + 0.9
3 = -4.9(1.08 / cos@)^2 + 50/3 sin@ (1.08 / cos@) + 0.9
0 = -4.9(1.08^2) sec^2@ + 50/3(1.08) tan@ + 0.9 - 3
0 = -4.9(1.08^2) (tan^2@ + 1) + 50/3(1.08) tan@ + 0.9
this is a quadratic in (tan@)... you can find tan@ using quadratic formula
then you can find @... there will be 2 possible values.

 

[weesa]

yea i did that too... i got 69.177...degrees and 27.4....degrees
but the back of the book says: 88.58 degrees, 8.06degrees

 

[Li0n]

i couldn't be bothered solving it, but as mojako said, there'll be two possible values
the book probably had the answer to the second value?