Question

What happens when the motion is positive and acceleration is negative?

Answer 1

We need to understand the relation between the direction of motion or the displacement involved in the motion of a body and the direction of acceleration applied to it while in motion. This idea will help us solve the given problem correctly.

Complete answer:
We are given a situation in which a body is moving in a positive direction with respect to the observer whereas an acceleration is applied in the opposite direction or in the negative direction. We need to understand the situation to analyse whether this motion is possible or not.

Consider the body to be moving with an initial velocity ‘v’ in the positive direction. Now, an external force started acting on the body from the direction opposite to its motion. We know that according to Newton's second law of motion, the force should create an acceleration on the body.
i.e.,
$F=ma$

But the force is acting in the direction opposite to the motion, therefore, the acceleration will also be in the opposite direction.

In such a situation, the body will start to slow down instead of speeding up. If the force remains even after the body comes to rest, then the body starts to move in the direction of the force.

So, we can understand that, the body can decelerate in the presence of a negative acceleration thus moving in the positive direction itself.

Note:
The opposite forces acting on a moving body are used practically for many purposes. The braking itself is the process of negative acceleration to stop the motion. Also, the frictional forces act in the opposite direction as that of the motion.