why should the object be moved at constant velocity when we define potential energy
Answers
Explanation:
I can see that you are confused about potential energy and work done on the body. It is not true that there is no work done when the body moves with constant velocity. There is work and the amount of that work is equal to the energy you used to move it.
Consider the lifting of a 5 kg body 4 m away from the ground. The amount of work done on the body is force exerted times the height. The force you exerted is equal the the weight of the body. Weight is the body’s mass times the acceleration due to gravity which is 9.8 m/s^2.
The weight = 49 N. Since it moved 4 m, then the work = 196 Nm.
Why did the body move by 4 m since the net force acting on it is zero?
When the net force is zero, the body moves at constant velocity according to Newton’s first law of motion. If it is at rest it will remain at rest. If the net force is greater than zero then the body will accelerate upward.
You can compute for the acceleration using F = m * a where F is the net force. If the net force is zero the body undergoes zero acceleration. Zero acceleration means that the velocity of the moving body is constant.
How much potential energy is possessed by the 5 kg body at a height of 4 m from the ground? The potential energy of the body is computed by m * g * h and it is equal to the work done on it. It is 196 Nm or 196 joules.
Why does the potential energy increase if the height is increased? It increases because you did more work on it.
Another misconception I will clarify here is about net work. Many are confused about positive and negative work. They claimed that since the work done in lifting the body upwards is equal to the work done by gravity the net work done is zero joules.
There is no such thing as net work since work is a scalar quantity. It is not a vector quantity like force. When you lift a body to a height of 4 meters you are the one doing the work. Gravity does not do work. The total work done here is the weight of the body times 4 m. The work done by gravity is zero. It is a positive work.
Gravity only do work when the body is falling. If the body falls for 4 meters then it did a negative work of weight times 4 meters. You did zero work on the body.
Positive and negative work are not simultaneously done. You cannot add the work done upward and downward to get zero.