In fluid mechanics, Mach number (M or Ma) is a dimensionless quantity representing the ratio of speed of an object moving through a fluid and the local speed of sound. M = v/vsound where, M is the Mach number, v is the velocity of the source relative to the medium, and vsound sound is the speed of sound in the medium. Mach number varies by the composition of the surrounding medium and also by local conditions, especially temperature and pressure. The Mach number can be used to determine if a flow can be treated as an incompressible flow. If M < 0.2-0.3 and the flow is (quasi) steady and isothermal, compressibility effects will be small and a simplified incompressible flow model can be used.

This case is similar to propeller aircraft in which the propeller is mounted on the aircraft.when propeller turns air is blown and as the momentum of air changes it imparts reaction on the blades of the propeller,which in turn moves the air craft.

The water droplets in the sky behave like tiny prism. Rays of sunlight entering in to the droplets undergoes one reflection and two refraction. The rainbow appears semicircular for an observer with inner edge violet and outer edge red.

Formula for Couple acting on body (T)=F*d =Ma*d [F=M*a] =M*dv/t*d [a==(vf-vi)/t)] =M*(d/t/t)*d =Md^2/t^2 =ML^2T^-2