In projectile motion we have the following triangle form by the initial velocity:
Assuming that the angle of the launch is "A". From the right triangle that is formed we can use the following trigonometric function:
[tex]\sin A=\frac{v_y}{v}[/tex]
Now we multiply both sides by "v":
[tex]v\sin A=v_y[/tex]
Therefore, in order to determine the y-component of the velocity we need to use the function "sine".
