- #1

Balsam

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## Homework Statement

I'm just confused about this. I know that displacement=df-di, but using that formula didn't give me the correct displacement in this case.

In a 100m sprint, a runner starts from rest and accellerates to 9.6m/s[W] in 4.2 seconds. The runner runs at a constant velocity for the rest of the race, what is the total time?

I am trying to solve for the time that it takes for the runner to go from their first displacement, when they are accelerating, to the end of the race when they're running at a constant velocity. In a previous calculation, the first displacement is 20m[W]. So, the final displacement- from when the runner is traveling at a constant velocity until they finish the race, should be 80m[W] because the whole sprint it 100m long. But, if you do displacement=d2-d1 and try to solve for d2 that way, you get a displacement of 120m[W]. For my calculations, I plugged in all values in the west directions with negative signs.

Conventions-- West= negative, East= positive

-100=d2-(-20)

-100-20=d2

-120=d2

120m[W]=d2.

How can d2 be 120 m[W] if the sprint itself is only 100m long. Can someone please explain this to me?

## Homework Equations

displacement=d2-d1

## The Attempt at a Solution

I showed that above, under the problem statement. Can someone tell me where I went wrong? I'm really confused.