Relative velocity
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Relative Velocity. The velocity of one object or material relative to another Used in the calculation of fluid drag Involves the subtraction of velocity vectors. Relative Velocity.

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Relative Velocity

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Relative velocity

Relative Velocity

The velocity of one object or material relative to another

Used in the calculation of fluid drag

Involves the subtraction of velocity vectors


Relative velocity1

Relative Velocity

  • EXAMPLE 1: Two runners are in a race. Runner A is traveling at a velocity of 7 m/s (vA=7 m/s) while runner B is traveling at a velocity of 7.5 m/s (vB=7.5 m/s) in the same direction. What is their relative velocity?

    The relative velocity equals the difference between their velocities.

    Relative Velocity = vB – vA = 7.5 m/s – 7 m/s = .5 m/s

    This means that B is moving away from A at a velocity of .5 m/s


Relative velocity2

Relative Velocity

  • Example 2: The two runners in the previous problem are approaching each other (that is, their velocity vectors are in opposite directions). In this case vA=-7 m/s while vB=7.5 m/s.

    Relative Velocity = vB – vA = 7.5 m/s – (-7 m/s) = 14.5 m/s

    This is also called a closing velocity (it’s the velocity that results from two objects closing on each other).


Relative fluid velocity related to fluid drag

Relative Fluid Velocity – Related to fluid drag

  • This relates to the velocity of a fluid medium (like air or water) compared to an object moving within it.

    EXAMPLE 1: A runner is moving at a velocity of 7 m/s (vR = 7 m/s) and experiences a headwind of 3 m/s (vw = -3 m/s) (Since it’s a headwind it’s opposite in direction to the runner’s velocity) . What’s the relative wind velocity?

    Relative Wind Velocity = vR – vw = 7 m/s – (-3 m/s) = 10 m/s

    This produces the same effect as the runner standing still and facing into a wind that’s blowing at 10 m/s.


Relative fluid velocity related to fluid drag1

Relative Fluid Velocity – Related to fluid drag

EXAMPLE 2: A runner is moving at a velocity of 7 m/s (vR = 7 m/s) and experiences a tailwind of 3 m/s (vw = 3 m/s) (Since it’s a tailwind it’s in the same direction as the runner’s velocity) . What’s the relative wind velocity?

Relative Wind Velocity = vR – vw = 7 m/s – 3 m/s = 4 m/s


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