Frame of Reference and Relative Velocity

Inertial Frame

A reference frame in which the observers are not subject to any accelerating force.

Reference Frame

A coordinate system or set of axes within which to measure the position, orientation, and other properties of objects in it.

Relative Velocity

The vector difference between the velocities of two bodies, or the velocity of a body with respect to another body which is at rest.

V⃗ba

Velocity of b relative to a.

V⃗ab

Velocity of a relative to b.

Galilean Principle of Relativity

The principle stating that all constant velocity reference frames are equivalent.

Relative Motion

Describes how the movement of an object is perceived differently depending on the frame of reference.

Velocity Vector

A vector that points in the direction of velocity and has a magnitude equal to the speed of the object.

At-Rest Frame

A frame of reference that is stationary and not accelerating.

Downstream

The direction in which a current flows; the direction of the movement of water in a river.

Resultant Speed

The combined speed of a moving object considering all directions of movement.