Relative Speed Calculator

Compare two constant speeds, choose same or opposite direction, and optionally estimate catch-up or meeting time from an initial separation.

Speed A

Speed B

Speed unit

Directions

Initial separation (optional)Uses the same distance unit implied by km/h→km, mph→mi, or m/s→m.

Interpretation

Catch-up / separation rate

—Relative speed

—m/s

—km/h

—Gap-closing time

—1D rule

Same direction uses the absolute difference of speeds. Opposite directions uses their sum. Gap-closing time assumes the objects are moving along one straight line toward a catch-up or meeting point at constant speeds.

Physics model preflight

Check the governing model before trusting the number

1D constant relative motion

same: |v₁−v₂| · opposite: v₁+v₂

Reading the current native inputs…

Speeds and separation are non-negative; motion is one-dimensional and constant-speed. This is an idealized educational model: unit consistency and valid inputs do not guarantee that omitted effects are negligible in a real system.

State the model before trusting the number

Classical formulas are only as good as their assumptions and units. Air resistance, damping, deformation, non-constant forces and measurement uncertainty are not silently invented when the selected model does not include them.

How to use this Relative Speed Calculator

Compare two constant speeds, choose same or opposite direction, and optionally estimate catch-up or meeting time from an initial separation.

Governing physics model

same: |v₁−v₂| · opposite: v₁+v₂ — 1D constant relative motion.

How to verify the result

For same-direction motion use |v₁−v₂|; for opposite directions use v₁+v₂. Separation divided by closing speed gives meeting time.

Domain and assumption boundary

Speeds and separation are non-negative; motion is one-dimensional and constant-speed. The calculator does not silently add drag, damping, deformation, varying fields, relativistic effects, measurement uncertainty, or geometry that the selected model does not contain.