a) Work done is zero when displacement is zero. This happens when a man pushes a wall. There is no displacement of the wall. Thus, there is no work done.

b)It is true that the hammer drives the nail into the wood only when it is lifted and then struck.

Analysis -The iron hammer on its own has no kinetic energy, but it has some potential energy (because of its weight). To drive a nail into the piece of wood (which is work), he has to lift the iron hammer up, (this increases its potential energy because if its high position) and force it to move at great speed downwards (now has kinetic energy) to hit the nail.The sum of the potential and kinetic energy that the hammer acquired to drive in the nail is called the Mechanical energy, which results in the work done.

c) If adogandhorsearerunningwith thesame speed, kinetic energyof horseis more than the kinetic energy of thedogbecause the mass ofa horseis more than thedog.

d)A teacher moving around in class has net displacement, whereas the child has zero displacement.

Questions answer not clear