A car starts 10 m north of a reference point. It moves at a constant velocity over the next 5 s, reaching a position of 10 m south of the reference point. What is the car's average velocity?

  v = -4 m / s

Explanation:

The average speed of a body is defined by the distance traveled in a given time interval

     v = (x₂ -x₁) / (t₂ -t₁)

in this case the end point is x₂ = -10 m and the start point is x₁ = 10 m, the negative sign is because the point is below the reference that can be considered as zero point

let's calculate

     v = (-10 - 10) / 5

     v = -20 / 5

      v = -4 m / s

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