give examples of the situations of work done and work not done (15-20)
Whenever you do anything like standing, reading... people say you do work but actually physics will say you don't, because in physics work is defined when force makes a body move by a distance. In the above cases of reading and standing you neither apply force nor do you displace yourself.
Work done = force x displacement.
Mathematically,
Work done (W) by a force (F) in displacing a body through S is given by,
W = (F)(S) cosθ
Where, θ is the angle between F and S.
When force and displacement are in same direction then the work done is simply, W = FS.
Example: work done in displacing a box on a floor is the product of force applied and the displacement.
When force is perpendicular to displacement, θ = 90o and cos90 = 0
So, work done = 0
Example: work done by a coolie against gravity in walking with heavy bag on his head is zero. Here, gravitational force on the bag is downward but the displacement of the bag is horizontal, which is perpendicular to the weight of the bag.