Structure of the Atom | Class 9 Science Notes

4.1.4 Average acceleration

Acceleration is a measure of how quickly the velocity of an object changes with time. Average acceleration is defined as the change in velocity divided by the time interval over which this change occurs. It is a vector quantity, having both magnitude and direction.

Mathematically, average acceleration a = (final velocity - initial velocity) / time interval.

If the velocity increases, acceleration is in the direction of velocity; if velocity decreases, acceleration is opposite to the velocity direction. For example, when a vehicle speeds up, acceleration is positive; when it slows down (decelerates), acceleration is negative.

The SI unit of acceleration is metre per second squared (m/s²).

An example involving a bus moving on a highway illustrates this: the bus accelerates from 36 km/h (10 m/s) to 54 km/h (15 m/s) in 10 seconds, resulting in an acceleration of 0.5 m/s². When the bus brakes and stops in 5 seconds, the acceleration is -3 m/s², indicating deceleration.

Acceleration can result from changes in the magnitude of velocity, its direction, or both. For instance, in uniform circular motion, the speed remains constant but the direction changes continuously, resulting in acceleration.

Another example is a freely falling object, where the velocity increases by approximately 9.8 m/s every second, indicating a constant acceleration due to gravity (g = 9.8 m/s²).

It is important to note that an object moving at constant velocity has zero acceleration, regardless of how fast it is moving.

📊 Diagram: Fig. 4.8: Direction of average acceleration when magnitude of velocity is (a) increasing, and (b) decreasing; Fig. 4.9: A bus moving on a long straight highway; Fig. 4.10: An object dropped from a height

🧪 Activity: Activity 4.2: Research and calculate the magnitude of average acceleration for various cars accelerating from 0 to 100 km/h, recording the time intervals and calculating acceleration.

🔗 Connection: This section prepares students to understand motion graphs and kinematic equations that describe motion with constant acceleration, discussed in the next sections.