# Calculate the average speed and average velocity

1. Jul 11, 2015

### AHashemi

1. a man drives from a to b with car and speed of 20 km/h and returns from b to a immediately with speed of 30 km/h. what's the average speed and average velocity?

2. V=at+Vi etc

3. I think avg. speed will be 25 km/h and avg. velocity is -10 km/h.
Is that right?

thanks :)

2. Jul 11, 2015

### THE BEAST

See, one thing you need to really understand while doing kinematics in one direction is the difference between distance and displacement., velocity and speed.
One way of not getting fooled by it, the one i used a lot as a beginner is to remember the definition. Now, let's look at average velocity's definition. It means change in displacement in a given amount of time. So, if a person goes from initial point and comes back to it, then what do you think the change in displacement will be?

3. Jul 11, 2015

### SammyS

Staff Emeritus
Hello AHashemi. Welcome to PF .

In addition to what THE BEAST posted:

The definition for average speed is:

$\displaystyle \ \text{Average Speed}=\frac{\text{total distance traveled}}{\text{elapsed time}} \ .$​

It takes more time to travel from a to b than it does for the return trip. Therefore, the average is not a simple averaging of the two speeds ,

You need to find the total time for the round trip.