Find Velocity of Two Moving Objects: Formula for Relative Speed Calculation

[missie]

what formula can i use when i wanted to find out the velocity of two moving object?

like 2 car going foward on the same road but different speed. Car 1 in front is at 30m/s and car 2 in back is going 60m/s. how can i find the velocity of car b relative to car a?

 

[Monitor16807]

Let's say car A was moving at 30m/s and car B was also at moving at 30m/s, if they looked at each other they wouldn't be moving, so their relative velocity is 0.
But if a car C was moving at 60m/s both car A and B would agree it was moving at 30m/s.

So the formula is quite simple: relative V=V1-V2=60m/s-30m/s=30m/s.
Also V2-V1 doesn't equal -30m/s, it only means I think you're going at 30m/s in the other direction.

Always think of it as if the first variable was standing still.