https://youtu.be/ZRUbm7N7W48

Diagonal motion in a two-dimensional (2D) system occurs when an object moves simultaneously along both the horizontal (X) and vertical (Y) axes. The resulting path is the vector sum of these two independent movements.

Harmonic oscillation (x=sin(t), y=sin(t)):
When both movements share the same phase (phi = 0), x(t) equals y(t) at every instant. Instead of covering a 2D surface, the point oscillates back and forth along the line segment y = x at a 45° angle.

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