Acceleration is the rate of change of velocity. Usually, acceleration means the speed is changing, but not always. When an object moves in a circular path at a constant speed, it is still accelerating, because the direction of its velocity is changing.

Acceleration:
Acceleration is a fundamental concept in physics that refers to the rate at which an object changes its velocity. It is defined as the change in velocity per unit of time. In simpler terms, it is the rate at which an object speeds up, slows down, or changes direction.

Formula:
The formula to calculate acceleration is:
Acceleration (a) = Change in velocity (Δv) / Time taken (Δt)

Units of Measurement:
The units of acceleration are typically meters per second squared (m/s^2) in the International System of Units (SI). This means that for every second, the velocity of the object changes by the given amount.

Negative and Positive Acceleration:
Acceleration can be both positive and negative. Positive acceleration occurs when an object is speeding up, while negative acceleration (also known as deceleration or retardation) occurs when an object is slowing down. It is important to note that negative acceleration does not mean that the object is moving in the opposite direction, but rather that it is slowing down in the direction of motion.

Examples:
Here are a few examples to illustrate the concept of acceleration:
1. When a car accelerates from rest to a certain speed, it experiences positive acceleration.
2. When a ball is thrown upwards, it initially experiences positive acceleration due to the force of gravity but then experiences negative acceleration as it slows down and eventually starts falling back down.
3. When a car applies brakes to come to a stop, it experiences negative acceleration.

Key Points:
- Acceleration is the rate at which an object changes its velocity.
- It is calculated as the change in velocity divided by the time taken.
- The units of acceleration are meters per second squared (m/s^2).
- Positive acceleration occurs when an object is speeding up, while negative acceleration occurs when an object is slowing down.
- Acceleration can be seen in various everyday situations, such as a car accelerating or decelerating, a ball being thrown, or a person running and changing their speed.