Question

we know work done is scalar quantity but it is negative and zero also . how

Answer 1

Yes, work done is a scalar quantity because it has only magnitude and no direction.

✔Work done is negative when it is opposite in the direction off the applied force.

✔It is zero when there is no displacement.

{Hope this helps you}

Answer 2

It is to denote where the work is going.

First Scalars can be zero, there is no issue in that.

And work is just a way to give or take energy (like heat)

When is work negative?

When F.s is negative, i.e force and displacement are in opposite direction. What it means is this work is reducing the energy of the system (object).

Like work done by friction is always negative.

If I am giving you money then I am losing money.

Same way if someone does work ON you, he will lose energy.

So the work he did ON you is positive. The work done you did (of course you did work! Newton's Third law says if he is pushing you, you are pushing him back)  on him is negative (you took his energy!).

Same goes for heat. If you add heat to a body it's positive, if you take it away , it's negative.

There is one more quantity (Electric Current) which can have direction (I mean to show where the electrons are going) but it is a scalar.

Hope this helps