Work done in lifting and lowering an object

[Herman Trivilino]

The lifting force is whatever force is necessary to overcome gravity, it can't be less.

If the object is moving upward and slowing down, the lifting force is less than $mg$.

 

[Herman Trivilino]

But there is a confusion here in W=Fd , d is the distance up to which the force is applied and up to which the body travels. It’s not a momentary push.

What's a "momentary push"? It has to last for some length of time, and during that time the object moves a distance $d$.

 

[rudransh verma]

What's a "momentary push"? It has to last for some length of time, and during that time the object moves a distance $d$.

Oh! It’s the same thing.

 

[rudransh verma]

You can use a very high force for a very short distance and throw the book on to the shelf. Or you can use a force barely stronger than the weight and lift the book smoothly. Either way, the work done by the force is the same.

When using high forces for very short distance mean momentary push. In my book resnik they are not using this thing but constant force throughout the motion and displacement. Even though after a set time both will produce same change in energy.

 

[Ibix]

In my book resnik they are not using this thing but constant force throughout the motion and displacement.

If the book starts and stops, they cannot be using constant force.