# Relative velocity

The velocity of one object as seen from another moving object. You add or subtract velocity vectors.

Physics (university) · Part 01: Motion and vectors · Card 8 of 8 · Last updated 2026-10-19

## Analogy

Walking forward on a moving train: the platform sees your speed plus the train's.

## Example

**Boat in current**

Across: 4 m/s
Downstream: 3 m/s
speed = sqrt(16 + 9) = 5 m/s

## In practice

A boat steers 4 m/s straight across a river flowing 3 m/s. Its speed over the ground is 5 m/s.

## Questions people ask

### How do I find the velocity of A relative to B?

Subtract B's velocity from A's velocity as vectors. This gives what a person riding along with B would see for A. If two cars move at 30 and 20 m/s in the same direction, the faster car moves at 10 m/s relative to the slower one.

### Why does rain look slanted from a moving car?

The rain falls straight down relative to the ground, but you also move forward. Relative to you, the rain has a backward velocity equal to your speed added as a vector. The faster the car goes, the more slanted the rain looks.

## Related concepts

- [Vector components](https://studyvisually.com/learn/science/physics-university/physics-university-part-1/physics-university-part-1-concept-6)
- [Projectile motion](https://studyvisually.com/learn/science/physics-university/physics-university-part-1/physics-university-part-1-concept-7)

Web page: https://studyvisually.com/learn/science/physics-university/physics-university-part-1/physics-university-part-1-concept-8
Study this card in the app: https://studyvisually.com/app/learn?category=physics-university&card=physics-university-part-1-concept-8