Answer to Question #79888 in Classical Mechanics for Irfan sukeri

Question

Need to know why do we take the negative angle from the y-axis(60degrees) instead of positive angle from x-axis(30degrees) . Example: tan(-60) = Velocity in Y-axis / Velocity in X-axis.

Answer 1

When we use the formula
tan⁡α=v_y/v_x
we take the angle from x-axis.
When we need to know the from the y-axis we use the formula:
tan⁡θ=v_x/v_y
where θ=90°-α.