 # Relative Speed Relative speed is one of the most interesting topics which is central to most physics concepts. When do you say that an object is in motion? If an object is lying completely still on the ground, is it in motion or at rest? To understand these questions one must learn the relative speed concept.

## What is relative speed?

We can define relative speed as the speed of a moving body with respect to another. When two bodies are moving in the same direction, the relative speed is computed by their difference. But when to bodies are moving in the opposite direction, the relative speed is calculated by adding the speed of both the bodies.

The difference between relative speed and relative velocity is that relative speed is the scalar quantity whereas relative velocity is the vector quantity.

### How to calculate relative speed?

Suppose two bodies are moving at different speed in the same direction.

Let the speed of 1st body be x km/hr
And the speed of the 2nd body be y km/hr.

So, their relative speed is = (x – y) km/hr [x > y]

Then,

The time after which both the bodies meet = distance travelled / relative speed

= d km/ (x – y) km/hr

Now, we already know that relative speed is the speed of one object with respect to another.

So, when time after which both the bodies is given,

Suppose, time = t hrs.

Then, the distance covered in ‘t’ hours = relative speed * time

= (x – y) km/hr * t hrs.

### Solved Example of Relative Speed

Question:Two vehicles are travelling from the same location at the speed of 6 km/hr and 4 km/hr respectively. Calculate the distance between the vehicles after 10 minutes given that both vehicles are travelling in the same direction.

Solution:The relative speed of the vehicles when they move in the same direction

= (6 – 4) km/hr

= 2 km/hr

Total time taken = 10 minutes

Therefore distance travelled = speed × time
= (2 × 10/60) km
= 1/3 km
= 1/3 × 1000 m
= 333.3 m

Therefore, the distance between the vehicles after 10 minutes is 333.3 m given that both vehicles are travelling in the same direction. 