# Newton's laws question

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#1
A bird of mass 0.45kg hovers by beating its wings of effective area 0.28m^2. What is:
i) The downwards force of the bird on the air as it beats its wings?
ii) The upwards force of the air on the bird
iii) The velocity imparted to the air if the density of the air is 1.29 kg/m^3
Not sure if this is correct:
i) just 0.45g
ii) same as i
iii) using F=ρAv^2 v=sqrt(0.45g/(1.29x0.28))
0
2 months ago
#2
To i)

The downwards force is the gravutational force in this concern, so F = m*g

To ii)

Think about it again. Upwards force can't be equal tp downwards force.
0
2 months ago
#3
(Original post by Kallisto)
To i)

The downwards force is the gravutational force in this concern, so F = m*g

To ii)

Think about it again. Upwards force can't be equal tp downwards force.
i) and ii) are the Newton 3rd Law action and reaction and so must be equal.
1
2 months ago
#4
(Original post by Stonebridge)
i) and ii) are the Newton 3rd Law action and reaction and so must be equal.
Oh, I did not think about the 3rd law, thanks. I just wondered why this law is valid, as I thought that the bird has to overcome the gravitational (downwards) force during flying to the air all the time and that this force has to be substracted to the upwards force then, but alright! in short it is:

F(up) = -F(down) <=> m*g = -m*g

Without any flow resistance.

Last edited by Kallisto; 2 months ago
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