Relative Velocity Calculator to Solve Motion in ...

What is Relative Velocity?

Relative velocity is the velocity of one object as observed from another moving object. It depends on the reference frame. In classical (Galilean) mechanics, velocities simply add or subtract. In relativistic mechanics (near light speed), Einstein's velocity addition formula is required.

🚂 Relative Velocity Calculator

Use our free calculator for instant, accurate results.

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📐 Formula

v_AB = v_A − v_B

v_AB = velocity of A relative to B. For 2D: apply vector subtraction component by component. For relativistic speeds: v_AB = (v_A − v_B)/(1 − v_Av_B/c²).

📝 Worked Example

Train A moves east at 60 km/h. Train B moves east at 100 km/h.
Relative velocity of A as seen by someone on B:
v_AB = 60 − 100 = −40 km/h (A appears to move west at 40 km/h)

📝 How to Use the Calculator

Enter Velocity ASpeed and direction of object A.

Enter Velocity BSpeed and direction of the observer/object B.

Select Mode1D (same direction), 2D (vector), or relativistic.

View Relative VelocityGet speed and direction of A as seen from B.

❓ FAQ

If two cars approach each other at 60 mph each, what is relative speed?

In classical mechanics: 60+60=120 mph. At relativistic speeds, you would use the Einstein formula to get a value slightly less than 2c.

What is a reference frame?

A coordinate system attached to an observer. Physics laws are the same in all inertial (non-accelerating) reference frames — this is the principle of relativity.

What is Relative Velocity?