Throw a ball up in the air. You have provided it with an initial velocity (how else will it go up) and therefore also imparted it Kinetic Energy (KE) right at the beginning. Observe the ball move up.
As the ball moves up, you see the velocity of the ball is reducing since the force of gravity is acting against it. In Physics, we say that this force of gravity is doing negative work on the ball.
The ball has now reached the top, its velocity is zero. Basically force of gravity has done enough negative work to reduce the velocity to zero and therefore its KE has also becomes zero. Let’s say this total work done by gravity in upward journey is W1 (it would be a negative sign e.g. -4J or -10J)
But what has happened to the initial KE. We have been told in smaller classes that energy can neither be created nor destroyed, it can only change form. This is true here as well. The KE of the ball keeps reducing as it moves up but another form of energy keeps increasing. This other form of energy is Potential Energy (PE). This energy is determined by the distance between the ball and the earth (often Physics books term it as earth - particle system configuration). Thus the PE at start is zero and keeps increasing till all KE has converted to PE by the time it reaches the top of the flight. The gravitational force that did negative work on the ball increased the PE of the ball.
Now imagine the downward path of the ball. It is ready to go back to earth because it has the PE stored. As it moves down, the force of gravity is helping it increase its velocity and therefore the KE. We say that the force of gravity is doing positive work on the ball (let us say it is W2 and it will be positive, like +4J or +10J etc.). The KE keeps increasing and the PE keeps reducing, till all PE is converted to KE when the ball hits the ground. You would have guessed that the velocity at which the ball hits the ground would be same as that at which it was projected.
Now in this entire journey of the ball, the force of gravity would be termed as a conservative force. But why?
A force would be termed a conservative force if it meets the following conditions-
- The system consists of 2 or more objects. In this case it is ball and earth
The force acts between these 2 objects
- Force does work on the object when the configuration of the objects changes and transfers energy between KE and PE. It is W1 in this case and the change in configuration is nothing but the change in distance between the ball and earth.
When the configuration is reversed, the force reverse the energy transfer. In this case it is work W2 during the balls downward journey
When W1 + W2 = 0, in a closed loop path, we say that the force in play is a conservative force.
Thus is the force has done work of say -10 J in upward journey, it would have done a work of +10J in the downward journey
You may like to watch this video made by me to understand this concept deeper