When Does Change in Velocity Use v=v2+v1 or v=v2-v1?

[fakecrash2011]

in this question change v=v2-v1: a car traveling at 15m/s [N] executes a gradual turn, so that it then moves at 18m/s[E]. Calcue change in veloctiy.

but in this question change v= v2+v1: A plane's velocity changes from 200km/h[N] to 300 km/h [N 30 W]. Find the plane's change on velocity.

How do i know when to use which formula?

 

[SammyS]

in this question change v=v2-v1: a car traveling at 15m/s [N] executes a gradual turn, so that it then moves at 18m/s[E]. Calculate change in velocity.

but in this question change v= v2+v1: A plane's velocity changes from 200km/h[N] to 300 km/h [N 30 W]. Find the plane's change on velocity.

How do i know when to use which formula?

Change in velocity is Δv = v₂ - v₁ . Never is v₂ + v₁

 

[fakecrash2011]

what i mean is how do you know when to add or subtract vectors?

 

[HallsofIvy]

The same way you know when to add or subtract numbers!