Cheryl runs a race on a 400.0 m circular track. She starts running east of the starting line and then circles the track and falls, stopping 1.00 m west of the starting line. Her time is 80.0 s. What was her average velocity?

Question

Cheryl runs a race on a 400.0 m circular track. She starts running east of the starting line and then circles the track and falls, stopping 1.00 m west of the starting line. Her time is 80.0 s. What was her average velocity?
I know the correct answer is -1.25 x 10-2 m/s.
But for the life of me I don't understand how they came up with that.

drwls · Answer

Remember that velocity is a vector, and that average velocity is based upon the total displacement (divided by time), not the distance covered.

She ends up one meter behind where she started. If displacement is positive towrds the east (the starting dorection), her displacement is -1.0 meter. Dividing by the time of 80 s, you get

Vav - -1.0/80 = -1.25*10-2 m/s