Work done is the product of force and displacement. we know that sometimes displacement can be negative. so in this case work done is negative
but then how can it be scalar and scalar quantity kabhi negative nahi hoti
it is a scalar quantity because it is the dot product of force and displacement
u know what is meant by dot product?
but scalar quantity can't be negative
sign convention does not indicate the direction

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If the displacement is in a direction opposite to the force,
i.e., θ = 180°, then cos 180° = -1
              W = - F * S
The work done is negative. This is usually the case when the force
opposes the motion or it tries to stop a moving body.
Example :-
When a body moves on a surface , the force of friction between the body
and the surface is in a direction opposite to the motion of the body. Therefore, 
the work done by the force of friction is negative.
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anurag can u draw and send me the graph for freely falling body
I mean displacement time graph
free falling and disp. time graph is different and in comment how can i sent u
I have posted this que . answer there