Question

What is an example of a vertical motion with a positive velocity and a negative acceleration ?

Answer 1

A positive velocity indicates that the object is travelling in the same direction, while a negative velocity indicates that the object is moving backwards.According to the principle we also know that as an object slows down, the acceleration is in the opposite direction of the velocity, according to the theorem.

Complete step by step answer:
Throwing a ball straight up into the air is a simple example. According to the convention that upward is positive and downward is negative, the ball has a positive initial velocity (from the throw) that gradually decreases due to acceleration in the opposite (negative) direction, which is caused by the earth's gravitational force pressing down on the ball.

This is valid because we can no longer accelerate the ball until it has left our hands. As a result, the only force (of concern) acting on it is gravity, resulting in a $- g$ acceleration of the ball. Newton's second rule is in agreement with this.
${{\vec F}_{net}} = m\vec a \\\ \therefore {F_g} = mg$
Where the gravitational force is in the negative (downward) direction, the acceleration due to gravity is also negative, resulting in ${a_{ball}} = - g$.

Note: This is not the case, for example, with a rocket fired outward, since the rocket retains a thrust force against gravity (and drag, and so on), resulting in a positive acceleration.We can define motion as the change of position of an object with respect to time. A book falling off a table, water flowing from the tap, rattling windows, etc all exhibit motion.