1.Work Done by Gravitational Force

Case a: If a ball is thrown up then gravitational force is acting downward and displacement is upwards so angle between them is 180

W=Fs cos(180⁰)

W= mg h (-1)=-mgh

Case b:  If ball is thrown down from a building . mg is acting downwards and displacement is also downwards so the angle is 0.

W= mg h cos0= mgh

Case c: If a ball is rolling on a horizontal plane , angle between mg and displacement is 90 so

W= mg * h cos90=0

2. Work done by Normal force

Case a: If a block is moving on horizontal plane .Normal Force acts at the point of contact and

acts perpendicular to the surface .Angle between Normal Force and Displacement is 90 so Work done is Zero.

Case b:  In the diagram shown below Normal force is acting between two objects .Normal Force acting on 300 kg block is along +X axis  and displacement is also in same direction so Work done by Normal Force on 300Kg is W= N s cos0            i.e.  Work Done is positive.

 

Work Done by Normal Force applied by 300 Kg Object on 200 Kg Object .

In this case Normal force is along –X axis and displacement is along +X axis so Work done is negative .