Basic concepts of classical physics.
Some people doubt the correctness of certain physical principles. Some are inventing modifications of physics. It's good to know excatly which principles they do accept as correct, to see whether there is any common ground for discussion. Which of these do you accept as correct?
- Newton's first and second laws, embodied in Fnet = ma. The net force, Fnet on a body of mass m causes it to have acceleration a.
- Newton's third law. FAB = -FBA. If body A exerts a force on body B, then B exerts an equal size and oppositely directed force on A.
- The definition of torque, t = r´F, where ´ is the vector (cross) product operator.
- The rotational analogue of Newton's law: tnet = Ia where I is moment of inertia and a is angular acceleration.
- The definitions of displacement, velocity, acceleration and force as vectors, and the fact that the law of vector addition applies to them.
- The definition of momentum, p = mv. It is a vector and the law of vector addition applies.
- The definition of work, W = F•x, where x is displacement and • is the vector scalar (dot) product operator. Work is a scalar.
- The definition of kinetic energy, Ek = (1/2)mv2. Kinetic energy is a scalar.
- The work-kinetic energy principle: Wnet = DEk in a closed non-dissipative system. The Wnet is the net work done on the system by all external influences.
- Conservation of net mass in a closed system (classical physics).
- Conservation of net momentum in a closed system.
- Conservation of net energy in a closed system.
- Conservation of net angular momentum in a closed system.
- The thermal energy principle: Q = DU in a closed system, where U is the internal thermal energy and Q measures the thermal energy transfer in or out of the system. (In older books thermal energy was called "heat".)
- The first law of thermodynamics. DU = Q - W where U is internal energy, Q is the thermal energy added to the system, and W is the work done by the system. This is the usual sign convention.
- The second law of thermodynamics. This has several equivalent statements, but one is that no heat engine can have an efficiency greater than that of a Carnot engine operating between the same two temperatures. That maximum efficiency of any engine is therefore e = (Th - Tc)/Th, where the Ts are absolute temperatures of the reservoirs. Note that this efficiency is always less than one since absolute temperatures are always positive (greater than zero).
- The third law of thermodynamics. The entropy of a closed system never decreases over time.
Aside from any possible carelessness on my part, these are all correct. Furthermore, they are so logically connected that you can't change or discredit any one of them without changing the others. Before you try, see Things to consider before you rewrite classical physics.