- #1
johnconnor
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Homework Statement
Fan blades of radius r are mounted directly on the axle of a d.c. electric motor. assuming that the fan imparts a velocity v to a cylinder of air of radius equal to that of the blades, and that the density of the air is ρ, obtain expressions for:
i. the rate at which momentum is transferred to the air
ii. the thrust on the motor
iii. the rate of working of the motor
iv. the rate at which kinetic energy is supplied to the air
Comment on any apparent conflict between your answers.
The Attempt at a Solution
i. ∏r2 . v . ρ . v
explanation: rate of supply of mass x velocity of air imparted
ii. ∏r2ρv
explanation: since thrust = force (Newton's third law)
iii. ∏r2ρv . v
explanation: rate at which momentum is transferred to air x distance traveled by the momentum per second
iv. The answer is half that of iii. Could anyone please explain to me why? Does it have anything to do with the centre of "mass" of the air cylinder pushed? Something like "half the amount of energy the cylinder of air gains becomes the potential energy"? It doesn't really make sense to me.
Please help. Thank you!