What is Law of machine of simple screw jack?
the law of the machine is an expression which gives the relationship between the load on a machine and the applied effort
A paintbrush is not a machine in the sense that the word "machine" is used in elementary physics. Machines are used to do work, usually by providing leverage in some way. In that way, Atwood's machine, a collection of ropes and pulleys, is a machine, but the gasoline engine is not. This is merely an issue of terminology and classification and has historical roots. It is not a statement of a basic law of science.
Only the simpler concepts of the 2nd law can be stated at a child's level. Many of them have no simple explanation because they involve equations that can only be written as differentials and integrals. Some of the simple parts of it though are: To warm up something cold, you have to use something that is warmer. To cool something down, you have to use something that is cooler. No matter how hot they are…