Velocity When a Runner Covers Half a Lap

[Bottomsouth]

Homework Statement

A runner covers one lap of a circular track 40.0 in diameter in 63.2 .

If she covered the first half-lap in 28.7 , what were her average velocity for that half-lap?

Homework Equations

We use perimeter of a cirle = 2*3.14*20m to get 125.6

The Attempt at a Solution

I know that if the runner runs around a full lap, he or she will be at 0 velocity. But at half a lap I tried 62.8/28.7 to get 2.188. I am sure that is the speed and not the answer. I know I am making a small mistake.

 

[PhanthomJay]

Hint: Average velocity is the change in displacement (a vector) divided by the change in time. So what's the change of the runners dsplacement, both in magnitude and direction?

 

[Bottomsouth]

No ideas? 1?

 

[PhanthomJay]

I gave you the hint. Take it from there.

 

[dnp33]

the runner ends in the same place he/she starts.

 

[PhanthomJay]

the runner ends in the same place he/she starts.

After one lap, she does, but not after half a lap.

 

[dnp33]

haha.
well apparently i can't read.

 

[Bottomsouth]

Got it, 1.4 m/s because displacement is the shortest distance. This would be 40 m / 28.7 s.