Which of the following has more inertia: (a) a rubber ball and a stone of the same size? (b) a bicycle and a train? (c) a five-rupees coin and a one-rupee coin?

Inertia is the same thing as mass, more or less. Inertia is the tendency of the object to want to stay in place if at rest, or the object's tendency to stay in motion if a force is applied to it. A train's inertia is massive because it takes a lot of force to move such a large mass, or to slow it down. This is in comparison to the other objects (rubber ball, stone, bike, and the coins).

r=rho l/a where a is the area, l is the length and rho is resistivity so the greater the area the lesser the resistance thats why the earth pin is thicker bcos current follow the shortest path, so less resistive .

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.