# Understanding Relative Velocity: Explained in Simple Terms

• Aichuk
In summary, the concept of relative velocity involves the calculation of the velocity of one object in relation to another object. In this scenario, the two trucks are traveling towards each other at 70 m/s, resulting in a relative velocity of 140 m/s. However, depending on the frame of reference, this velocity can be seen as either positive or negative. For example, from the perspective of truck B, the relative velocity would be -140 m/s.

#### Aichuk

Umm, so as far as I understand velocity is speed in a direction. So if I'm going North at X Km/h and another guy goes south, he'll go at -X Km/h.

So anyways, the formula for relative velocity, for example V(AB) is V(A)-V(B). So let's say two trucks are going at 70 m/s towards each other (they're going to gonna crash :P).

So the velocity of Truck A relative to Truck B will be 70 m/s - (-70 m/s) = 140 M/S. Now since this is velocity, doesn't this imply that Truck A is going parallel alongside Truck B instead of going the opposite direction.

I'm really sorry if this is a stupid question, this is just a new concept and I don't fully understand it.

Maged Saeed
No ,,

You can understand it in other way.

Imagine that there is someone on truck A that is moving with 70 km/h . so he will see the other truck 'B' , which is moving with 70 km/h , going with 140 km/h .

Thus , this is the velocity of the truck B relative to A.

v=va-(-vb)

On the other hand , imagine that someone ' let call him X ' is on truck A , that is going parallel alongside truck B and they are moving in the same velocity ,

Now , 'X' will see truck B not moving ! ,,

So the relative velocity is v=va-(-vb)=0.

this is the idea.

Is it understandable now,
:)

Aichuk said:
So the velocity of Truck A relative to Truck B will be 70 m/s - (-70 m/s) = 140 M/S. Now since this is velocity, doesn't this imply that Truck A is going parallel alongside Truck B instead of going the opposite direction.
As I understand the concern in the OP, this comes down to a question about sign conventions.

The relative velocity of A relative to B is +140 meters/sec if we stay with the convention that North is positive and South is negative. Truck A is moving northward at 140 meters/sec according to truck B.

Edit: This +140 m/s is the calculated "closing velocity" expressed in terms of the ground frame of reference.

But if we adopt the point of view of truck B, it is easy to shift to a convention where front is positive and back is negative. Truck A is moving backward at 140 meters/sec according to truck B.

Edit: This is -140 m/s if expressed in terms of the truck-B-relative frame.

forward and northward in opposite directions. Hence the reversal in sign.

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Aichuk

## 1. What is relative velocity?

Relative velocity is a measure of the motion of an object in relation to another object. It takes into account the speed and direction of both objects and is usually measured in meters per second (m/s).

## 2. How is relative velocity different from absolute velocity?

Absolute velocity refers to the speed and direction of an object in relation to a fixed point, such as the Earth. Relative velocity, on the other hand, takes into account the motion of both objects and is measured in relation to each other.

## 3. Can relative velocity be negative?

Yes, relative velocity can be negative. This occurs when the two objects are moving in opposite directions, resulting in a negative value for the relative velocity.

## 4. How is relative velocity calculated?

Relative velocity is calculated by taking the difference between the velocities of the two objects and considering their direction of motion. It can be calculated using the formula vAB = vB - vA, where vAB is the relative velocity of object A with respect to object B.

## 5. What are some real-life examples of relative velocity?

One example of relative velocity is when driving a car on the highway. The relative velocity between your car and another car can change depending on your speed and direction of travel. Another example is when playing catch with a friend, where the relative velocity between you and the ball changes as you throw and catch it.