Acceleration Class 9 Physics Lesson – Motion

Hello guys, in this article we are going to cover the most essential parameter of motion i.e. acceleration. We must have seen the port provided on the right side of handle of bike or scooty! What is use of it? If you continuously change the setting of fan regulator, what do you observe? While enjoying trip with family in car, driver has to change his speed during its motion. Which quantity is introduced here in motion? The answer is acceleration.

So let’s discuss the acceleration in detail with the following example.

Suppose that you started cycling on large square marked on ground from a point P, then goes to points Q, R and S. During this due to different surface of road, sometime you need to slow down and then speed up instantly. So is your speed is constant throughout the motion? No! As you are taking different time to trace the each sides of the square, your speed is also different or changing. When the speed of body is changing with respect to time, acceleration is formed.

Acceleration is defined as rate of change of velocity. Or Change in velocity per unit time is also known as acceleration.

Where, v1 =initial velocity,v2 = final velocity

Since the velocity is vector quantity, acceleration is also vector quantity.

SI unit of acceleration is m/s2.

In general, u= initial velocity

v = final velocity

and, t = time required to change the velocity then acceleration is written as

Acceleration is of following types:-

1) Positive acceleration: When the velocity of object is increases i.e. initial velocity is less than its final velocity, and then the acceleration is known as positive acceleration.

e.g. Suppose car has started its motion with initial velocity of 0 km/h and after reaching on highway in few minutes speed becomes 80 km/h. In this case the velocity increase and acceleration becomes positive.

2) Negative acceleration: When the velocity of object is decreases i.e. initial velocity is greater than its final velocity, and then the acceleration is known as negative acceleration.

e.g. Suppose car running with initial velocity of 80 km/h and suddenly driver applied brakes so that car slows down to 20 km/h. In this case the velocity decrease and acceleration becomes negative.

Negative acceleration is also known as deceleration or retardation.

3) Zero acceleration: When the velocity of object remains constant throughout i.e. initial velocity is equal to its final velocity, and then change in velocity is zero and hence the acceleration is also zero.

e.g. Suppose that you are enjoying the journey of train of which is running at constant speed of 55 km/hr from past two hours, in this case change in speed is zero and hence the acceleration is zero.


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