Question

I'm confused...we know that net work equals change in kinetic energy. So let's say we slowly lift a book on a table 3 meters up, then leave it at rest again (except we're still holding it). Since velocity in the initial and final times is 0, there is no kinetic energy and thus no work. But there IS a change in potential energy, and change in potential energy means work. What am I doing wrong? thanks.

Answer 1

When you are slowly lifting the book( I guess that means you lift it with infinitesimally small speed, i.e. the book is NOT accelerating) Then even as the book is lifted up its in equilibrium. This means your hand must apply a force on the book equal to the weight force of the book. This force must be in the vertically upward direction. The book is also getting displaced upwards. Thus the force you apply on the book does work which is equal in magnitude but opposite in sign to the work done by the gravitational force. Thus the net work performed is zero . Hence the change in Kinetic Energy is also zero. You must include the work performed by all forces on the object if you want to compute the change in kinetic energy. Its the net work done on the object which is equal to the change in Kinetic Energy.

Answer 2

S.S. Shreyas you are wrong...

Answer 3

How exactly may i know

Answer 4

And by the way. That is the correct answer. Do tell me if you have a better one

Answer 5

dood u r saying that work done is zero ...then tell me that how it got potential energy with zero work

Answer 6

I only said that the NETwork done on the body by the hand l(lifting the block) AND the gravitational force is zero. Work is done by the force provided by the hand which is why the body gains potential. You seem to be forgetting that the potential energy of a body is the negative of the work done by conservative forces other than gravity.

Answer 7

Change in kinetic energy is the NET work done by all the forces acting on the body. This includes the work done by the gravitational force (which is the negative of the potential gained).

Answer 8

Well! slow or fast, it makes no difference unless you want to include air resistance. Anyway, the book accelerated, to some maximal velocity (work done by hand - work done by gravity > 0) then you slowed the book down to zero velocity (work done by hand - work done by gravity < 0) net work is zero. Of course, I assume we're neglecting air resistance and so on.

Answer 9

I had also been confused with this problem for a very long time. Well, I got the answer in Halliday's <Principles of Physics>, mind you.