# Average velocity question?

Hey guys I just had a question regarding the equation to calculate average velocity. The first equation i've seen is: Vf+Vi/2, with Vf representing final velocity and Vi representing initial velocity. The second equation i've seen is Xf-Xi/Δt, with X representing position and Δt representing the elapsed time. What is the difference between the two? I get confused on which equation to use for a problem asking for average velocity.

## Answers and Replies

Doc Al
Mentor
The first equation, (Vf + Vi)/2, gives the average velocity when the acceleration is constant. It's not true in general.

The second, (Xf - Xi)/Δt, is the definition of average velocity and is always true. Whether it's useful or not depends on what you are given in the problem.

SammyS
Staff Emeritus
Science Advisor
Homework Helper
Gold Member
Hey guys I just had a question regarding the equation to calculate average velocity. The first equation i've seen is: Vf+Vi/2, with Vf representing final velocity and Vi representing initial velocity. The second equation i've seen is Xf-Xi/Δt, with X representing position and Δt representing the elapsed time. What is the difference between the two? I get confused on which equation to use for a problem asking for average velocity.
Notice the parentheses Doc Al added in his reply to you.

ok that cleared it up, thanks!