# -.874=sin2θ θ=?

1. Feb 19, 2009

### twenty5

BLAHH!!! I can't figure this one out!

how do I isolate θ??

Statement is...

-0.874 = sin2θ

jeeebuss can someone walk me through LOL! I need it to finish off a 4 hour physics question =/

2. Feb 19, 2009

### tiny-tim

Hi twenty5!

walk this way … if x = sinθ, then θ = sin-1x (or arcsin x)

3. Feb 19, 2009

### twenty5

so, if -0.874 = sin2θ

then...
sin-1(-0.874) = 2θ

then...

(sin-1(-0.874))/2 = θ?

4. Feb 19, 2009

### tiny-tim

Yes, except …

i] it's normal to have only positive numbers inside the sin-1

ii] you didn't say what the range of θ was … did they specify 0 < θ < π or 2π or what? … there may be several solutions, especially since you're divding the angle by 2

5. Feb 19, 2009

### twenty5

hmm not sure because I'm using this to do this... https://www.physicsforums.com/showthread.php?t=293717

the very final post should be mine =P

right, since the football player is kicking the ball above ground on a horizontal plane (straight field), and he is kicking it forward to a distance other than where he is standing, it has to be somewhere between 0o and 89o

so, the answer I ended up with was -30.46o... so would that mean it is... wrong? LOL! omg... can't believe I spent 4 -5 hours to end up with a wrong answer -__- so depressing..

6. Feb 19, 2009

### tiny-tim

hmm … your minus sign seems to have come in post #18, but you don't show how you got it …

yes, you need an answer between 0 and 90º, but your formula should give you two such answers.

and i'm going to bed now … :zzz:​