咩叫xxwith respect to xx(phy)

"A velocity of 5 m/s with respect to x" means "the velocity (both in magnitude and direction) of the object in question when viewed at the position of x ". It is , in fact, the same as velocity of the object relative to x.

Mathematically, if there are two objects, X and Y, with velocities Vx and Vy respectively, then the velocity of Y with respect to X (i.e. the relative velocity of Y to X) is the vector difference Vy - Vx.

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