- Displacement, velocity and acceleration are all vector quantities. They thus have both magnitude and direction.
- Rectilinear motion refers to motion which occurs strictly along a straight line. Since only two directions are possible, a positive sign indicates one direction, and the negative indicates the opposite direction.
- When velocity is negative
- “decreasing” velocity actually implies increasing speed.
- positive acceleration actually implies decreasing speed and vice versa.

- For example, let’s adopt the “
**rightward is positive**” convention for the motions below.

- Since the sheep is moving rightward and speeding up, its velocity is represented by more and more
**positive**numbers. - Since the velocity is increasing, the acceleration is
**positive**.

–

- Since the sheep is moving leftward and speeding up, its velocity is represented by more and more
**negative**numbers. - Since the velocity is “decreasing”, the acceleration is
**negative**.

–

- Since the sheep is moving rightward but slowing down, its velocity is represented by less and less
**positive**numbers.

- Since the velocity is decreasing, the acceleration is
**negative**.

–

- Since the sheep is moving leftward but slowing down, its velocity is represented by less and less
**negative**numbers. - Since the velocity is “increasing”, 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.