In acceleration and deceleration formulas, a minus sign is used to indicate a decrease in velocity or speed. When an object is accelerating, its velocity or speed is increasing, so the acceleration is positive. When an object is decelerating, its velocity or speed is decreasing, so the acceleration is negative.

Examples:

Acceleration: If a car starts from rest and accelerates to 60 mph in 10 seconds, its average acceleration is:

$$a = \frac{\Delta v}{\Delta t} = \frac{60 \text{ mph} - 0 \text{ mph}}{10 \text{ s}} = +6 \text{ mph/s}$$

The acceleration is positive because the car is increasing its speed.

Deceleration: If the car then brakes and comes to a stop in 5 seconds, its average deceleration is:

$$a = \frac{\Delta v}{\Delta t} = \frac{0 \text{ mph} - 60 \text{ mph}}{5 \text{ s}} = -12 \text{ mph/s}$$

The deceleration is negative because the car is decreasing its speed.