# Average velocity with a relay race

## Homework Statement

A relay race is run along a straight-line track of length 300.0 m running south to north. The first runner starts at the south end of the track and passes the baton to a teammate at the north end of the track. The second runner races back to the start line and passes the baton to a third runner who races 300.0 m northward to the finish line. The magnitudes of the average velocities of the first, second, and third runners during their parts of the race are 7.06 m/s, 7.00 m/s, and 7.51 m/s, respectively. What is the average velocity of the baton for the entire race? [Hint: You will need to find the time spent by each runner in completing her portion of the race.]

## Homework Equations

Vavg= delta r/delta t

delta r= rf-ri
delta t=tf-ti

## The Attempt at a Solution

7.06m/s=300m/T
T=42.4929 sec

7.00m/s=300m/T
T=42.85714 sec

7.51m/s=300m/T
T=39.9467 sec

After these I get confused which one is the final and initial displacement because there are three of them. Any help please?

Related Introductory Physics Homework Help News on Phys.org
Delphi51
Homework Helper
average velocity = total distance divided by total time.
The total distance is 900 meters.

So Delphi51 you say to get the answer it would be 900m/125.296sec= 7.518 m/s

But im doing my h.w. online and it marks it incorrect?

SammyS
Staff Emeritus
Homework Helper
Gold Member
NO! Average velocity is (Displacement)/(Time).

The Displacement is a VECTOR. The displacement for the race is: The VECTOR from the starting line to the finish line. At the end of the race, the baton is 300 meters NORTH of where it was at the start.

Delphi51
Homework Helper
The word "velocity" sometimes means displacement/time and sometimes means distance/time. When it means the former, you must express the velocity as a vector - magnitude and direction. So the final answer would be 300/7.518 North.