I was reading an answer about torque acting on a rectangular current carrying loop kept in a uniform magnitude field B. Force acting on each sides is F1, F2, F3, F4. It's written here : Force F2, F4 acting on these sides has same magnitude F` = Blb sin(90-theta) = Blb costheta. And F1 and F3, have F=IlB sin90° = IlB.

(where l=length; b=breadth) I understand F1and F3. But I dont get how the magnitude of forces of F2, F4 become Blb costheta . Which law says that? Am i missing some knowledge?

If you need the diagram or the book text for more details , please ask for it.

I was reading an answer about torque acting on a rectangular current carrying loop kept in a uniform magnitude field B. Force acting on each sides is $F_1$, $F_2$, $F_3$, $F_4$. It's written here : Force $F_2$, $F_4$ acting on these sides has same magnitude $F' = Blb \sin(90-\theta) = Blb \cos\theta$. And $F1$ and $F3$, have $F=IlB\sin90° = IlB$.

(where $l$=length; $b$=breadth) I understand $F_1$ and $F_3$. But I don't get how the magnitude of forces of $F_2$, $F_4$ become $Blb \cos\theta$ . Which law says that? Am i missing some knowledge?

If you need the diagram or the book text for more details , please ask for it.