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9. A car travels at a constant velocity of 70 mph for one hour. At the second hour, the car’s velocity was 60 mph. At the third hour, the car’s velocity was 80 mph. At what hour was their positive acceleration? At what hour was their negative acceleration?

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smmwaite

Answers

Part 1: At the third hour

Part 2: At the second hour


Explanation

The acceleration is considered positive when the change in velocity is increasing. At the second hour, the velocity was 60 mph. At the third hour the velocity was 80 mph.

∴ acceleration = (80 - 60)/(3 -2)

                        = 20/1

                        = 20 mph²   (positive)

The acceleration is negative when the velocity is decreasing.

At the first hour velocity was 70 mph. At the second hour the velocity was 60 mph.

∴ acceleration = (60 - 70)/(2 - 1)

                      = -10/1

                       = -10 mph²  (Negative)

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