Calculating mass of air from Speed and Density?

[ntk_k]

For the hovering helicopter, it is assumed that all the air beneath the blades is pushed vertically downwards with the same speed of 4.0m/s. No other air is disturbed.

The density of air is 1.2kg m^-3

How can you calculate, for the air moved downwards by the rotating blades,

i) the mass per second
ii) the rate of change in momentum ??

 

[ideasrule]

You can't unless you know the area of the blades. Do you?

 

[ntk_k]

The length of each blade of the helicopter is 0.70m. The area that the blades sweep out as they rotate is 1.5 m^2.

 

[dacruick]

could you not just calculate the area of the cylinder? which would be an area of 6m^3. then find out the mass of the air in the 6m^3. I believe that would be the mass per second.

As for the change in momentum, I am unclear as to what the question is asking. Are you asking for the change in momentum of the helicopter? Or the change in momentum of the air?

 

[dacruick]

in case of any ambiguity, the 6m^3 arises from 1.5m^2(area of the blades), multiplied by the metres of air displaced in one second.(4m)

 

[kabwe]

For the hovering helicopter, it is assumed that all the air beneath the blades is pushed vertically downwards with the same speed of 4.0m/s. No other air is disturbed.

The density of air is 1.2kg m^-3

How can you calculate, for the air moved downwards by the rotating blades,

i) the mass per second
ii) the rate of change in momentum ??[i don't know]

 

[kabwe]

You can't unless you know the area of the blades. Do you? the area = 85.9645m^2

 

[kabwe]

You can't unless you know the area of the blades. Do you? [yes area = 85.9645m^2]

 

[kabwe]

the area = 85.9645m^2