2.1.1 Sign Convention

  • Let’s adopt the “rightward is positive” convention for the motions below

1 pospos

  • Since the sheep is moving rightward and speeding up, its velocity is represented by more and more positive numbers.
  • Since the velocity is becoming more positive, the acceleration is positive.

1 pospos

  • Since the sheep is moving leftward and speeding up, its velocity is represented by more and more negative numbers.
  • Since the velocity is becoming more negative, the acceleration is negative.

1 pospos

  • Since the sheep is moving rightward but slowing down,  its velocity is represented by less and less positive numbers.
  • Since the velocity is becoming less positive, the acceleration is negative.

1 pospos

  • Since the sheep is moving leftward but slowing down,  its velocity is represented by less and less negative numbers.
  • Since the velocity is becoming less negative, the acceleration is positive.
  • In a nutshell, the speed increases if the velocity and acceleration have the same sign, and decreases if the velocity and acceleration have opposite signs.

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