Question

When motion and acceleration are in opposite direction then-

Answer 1

Hint: We know that acceleration is the rate of change of velocity for a given interval of time. Think of your daily encounters with motion and acceleration. Then you could recall what happens when they are directed in opposite directions. Also, mention these examples while answering the question.

Complete answer:
In the question, we are asked what happens when the motion of a body and the acceleration of the body are directed opposite to each other. In order to answer this, let us go through our practical experiences where we encountered acceleration being directed opposite to each other.
Imagine the case where you throw a ball up in the air. When you throw a ball up, the body’s velocity is directed in the upward direction and hence the motion is in upward direction. But, what direction is the acceleration? Acceleration on this body is exerted due to gravity which we know is directed downward toward the ground. While going up, we have seen the ball to have a decrease in velocity until it reaches the maximum height where its velocity is zero and then it comes back to the ground with an increase in velocity. In its motion back to ground from maximum height the motion is directed in the same direction as the acceleration and hence the velocity of the ball increases. Therefore, speed of the body decreases when the acceleration is in the opposite direction to motion of the body.
The acceleration that is directed opposite to the direction of motion is said to be negative acceleration or in other words, we could call it deceleration.
Another example is when you apply brakes while driving a car. When you are applying the brake, the acceleration is directed opposite to the motion of the car and hence the car slows down.
Therefore, when motion and acceleration are in opposite directions then the body slows down, that is, the velocity decreases.

Note:
From the definition of acceleration we know that it is the time rate of change of velocity. We say the acceleration is positive when the velocity of the body increases and the acceleration is said to be negative, that is the body decelerates, when the velocity decreases. When acceleration is directed opposite to that of motion, the body actually decelerates, that is, it slows down.