Which type of energy is stored while stretching the catapult? How this energy is obtained?
While stretching the catapult, potential energy is stored in it. This energy is obtained by doing work against the restoring force of the string.
No work is done when a person is standing by carrying a load of 50kg for one hour, why?
When a person carries load and stands for any interval of time, total work done is zero. By definition, work is defined as the scalar product of Force and displacement. However, in this case, there is no displacement because the person is stationary which results in 0 work done.