## On Velocity Addition (2)

Let's work on a simple math problem.

Two children are walking toward each other, when will they meet?

Assume they will meet after time t. During this time, the first child will have walked a distance of d1=v1*t, and the second child will have walked a distance of d2=v2*t. Since d1+d2=d, so we have v1*t + v2*t =d, thus (v1+v2) * t= d, and finally we get t=d/(v1+v2).

In this example, we used the following definitions and assumptions (maybe unknowingly):

**Definitions:**

Distance, Time, and Speed (Speed=Distance/Time)

**Assumptions:**

- Evenly distributed space (so we can have d=d1+d2)
- Evenly flowing time (so in the example, we don’t care it is today or tomorrow, at noon or in the evening)
- Any distance or time interval has the same measurement in all reference frames. (so the two children’s time are the same, and their distances walked can be added up).