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So your saying it should be

Я бросила мяч в твою голову.

?

Oops. Sorry. :oops:
I messed that up real badly. голова is feminine. Arg.
I was thinking that мяч is the direct object and should be in accusative case; and твоей голове is the indirect object...

I am guessing here but . . .
Я бросила мяч в твоему голове.