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kellysatchmo · Oct 19, 2013, 03:19 PM

A helicopter of mass 500kg hovers when its rotating blades move through an area of 30 m^2 and gives an average speed V to the air. Estimate V assuming the density of air is 1.3kg/m^3 and g = 10 N/kg...

Celine91 · Oct 21, 2013, 06:15 AM

m = ρ*V*A

V = m / ( ρ* A )
V = 500 / (1.3 * 30 ) = 12.8205 m/sec

ebaines · Oct 21, 2013, 09:33 AM

Originally Posted by Celine91

m = ρ*V*A

Not quite right. Check your units - as written the left side of your equation is in Kg and the right side is Kg-m/s^2, or newtons. Hint - you left out the aceleration due to gravity, g.

Celine91 · Oct 21, 2013, 11:30 AM

yeah sorry forgot the time variable!

(m/t) = ρ*V*A... right side kg/sec (kg/m^3 * m/sec * m^2 = kg/sec)

and V=gt so t can be replaced with V/g
this makes the formula (m*g/V) = ρ*V*A... simplify and solve for V

V = square root of ( m*g/ ρ * A) = 11.3 m/sec