# Clarification between average velocity and average speed

1. Feb 3, 2013

### cmkc109

To find average velocity, you do total displacement over total time
To find average speed, you add up the speeds / total speeds there are? (Eg. (10+20)/2)

Am I correct?

2. Feb 4, 2013

### Staff: Mentor

Correct.

No, you take the total distance along the path traveled, over the total time. For "total distance", imagine tracing out the path with a string, then straightening the string and measuring its length with a ruler.

3. Feb 4, 2013

### cmkc109

thanks for your reply! but when do you use v1 + v2 /2 ? or did i remember this wrong?

4. Feb 4, 2013

### willem2

You can use (v1+v2)/2 for the average speed if you first moved with speed v1 and then with speed v2 for the same amount of time. (not for the same amount of distance).

For constant acceleration in a straight line, the average speed (and velocity as well) is $\frac {v_i + v_f}{2}$. the average of initial and final speed.

5. Feb 4, 2013

### Staff: Mentor

Hi. I think you forgot some parentheses.

You use that formula when acceleration is constant and you need to find the velocity at a time midway between the time that you measure velocity as v1 and the time you measure velocity as v2.