# Problem in understaning potential energy

• Vatsal Goyal

## Homework Statement

If we lift a block with constant velocity, by applying a force mg upwards, is the work done zero?

## The Attempt at a Solution

The work done must be zero as the resultant force is zero, what I don't understand is how does the block get potential energy if no work is being done?

I assume that m is the mass of the block.

Why do you think that it should gain potential energy?

You should write down some relevant equations and show some work if you want any hints.

Why do you think that it should gain potential energy?
It should gain some energy, as when we will release the block, the block will gain kinetic energy, indicating it must have had some form of energy at the height which is converted into kinetic energy.

## Homework Statement

If we lift a block with constant velocity, by applying a force mg upwards, is the work done zero?
The work done by all forces is zero, that means the KE does not change (work-energy theorem).

## The Attempt at a Solution

The work done must be zero as the resultant force is zero, what I don't understand is how does the block get potential energy if no work is being done?
How is the potential energy of a body at a point due to a force defined?

It should gain some energy, as when we will release the block, the block will gain kinetic energy, indicating it must have had some form of energy at the height which is converted into kinetic energy.
The block has some potential energy initially due to the height that it starts at.. Do you think that the force increases it? If so, why?

How is the potential energy of a body at a point due to a force defined?
It is defined as the energy a body has due to its position or configuration, and measured by the work that could be done by a body passing from its present position to some zero position.

The block has some potential energy initially due to the height that it starts at.. Do you think that the force increases it? If so, why?
Yes, I think it would, suppose the block is at height h in the beginning. We lift it by a distance x with velocity u. Now, if we drop it, when it reaches height h, it will have velocity v>u as only force of gravity is acting on it, hence total energy at h is greater than what it was before, then at height h+x, it must have had some extra energy, which is giving the extra kinetic energy at h.

It is defined as the energy a body has due to its position or configuration, and measured by the work that could be done by a body passing from its present position to some zero position.
Potential energy due to a conservative force is defined as the work done by that force when the body passing from its present position to the position of zero potential energy.

• Delta2
Yes, I think it would, suppose the block is at height h in the beginning. We lift it by a distance x with velocity u. Now, if we drop it, when it reaches height h, it will have velocity v>u as only force of gravity is acting on it, hence total energy at h is greater than what it was before, then at height h+x, it must have had some extra energy, which is giving the extra kinetic energy at h.
Yes. The first step is to calculate the values of x and u. They may be positive, negative, or zero.

## Homework Statement

If we lift a block with constant velocity, by applying a force mg upwards, is the work done zero?

## The Attempt at a Solution

The work done must be zero as the resultant force is zero, what I don't understand is how does the block get potential energy if no work is being done?

Consider a car moving at constant 100mph on level ground. The net force acting on the car is also zero (it's not accelerating or decelerating). Do you think the engine is doing zero work?