Answer:
18 m/s²
Explanation:
You want Dean's acceleration when his speed changes from 10 m/s to 100 m/s in 5 seconds.
Acceleration
Acceleration is the rate of change of velocity, the change in velocity divided by the corresponding change in time:
a = ∆v/∆t
a = (100 m/s -10 m/s)/(5 s) = (90 m/s)/(5 s) = 18 m/s²
Dean's acceleration is 18 m/s².
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Additional comment
Both velocity and acceleration are vector quantities. They have both magnitude and direction.
We assume the given speeds are in the same direction, so the direction of the velocity vector does not change. The acceleration is in that same direction.