A farmer and a housewife may have never studied biology, yet both of them know some of its general principles through experience. State any two principles which they know.

Newton's laws of motion are applicable in all inertial reference frames. Some physical quantities, when measured by observers in different reference frames, have exactly the same value. Such physical quantities are called invariant. In Newtonian mechanics mass, time and force are invariant quantities . On the other hand, some physical quantities, when measured by observer in different reference frames, do not have the same value. Sigmae physical quantities are called not invariant . In Newtonian mechanics displacement, velocity and work ( which is the dot product of force and displacement) are not invariant. Also kinetic energy (=1/2mv^(2)) is not invariant. Physicists believe that all laws of physics are invariant in all inertial frames, i.e. the work-energy principle states that the change in the kinetic energy of a particle is equal to the work done on it by the force. Although, work and kinetic energy are not invariant in all reference frames, the work-energy principle remains invariant. Thus even though different observers measuring the motion of the same particle find different values of work and change in kinetic energy, they all find the work energy principle holds in their respective frames. Which of the following quantities is invariant?