Camera drone and speed of air below it

[hello478]

Homework Statement

image below

Relevant Equations

f=ma

my answer =
F = 1.6*9.81 - 1.2*9.81 = 3.924 N -> resultant force of the drone moving (weight minus the upward force)
M= 1.2kg , mass of drone
a= 3.31 m/s^2 , acceleration of drone
u=0 m/s , initial speed of drone
t=1s , time
v = 3.27 m/s, final speed of drone
and its not even in the options...

 

[Orodruin]

It is impossible to follow what you are actually trying to do in your attempt. Please write out units and explain what you are doing in each step.

 

[hello478]

It is impossible to follow what you are actually trying to do in your attempt. Please write out units and explain what you are doing in each step.

done

 

[haruspex]

F = 1.6*9.81 - 1.2*9.81

Where does 1.6kg come from? You seem to have added the mass of the drone to the mass of the air it pushes down in one second. In what way do those add? You can't add a mass to a mass per unit time.
If, instead, you had been told it pushes down 1440kg per hour, would you have done 1.2+1440?

 

[Frabjous]

You have a mass of air, m, accelerated to a velocity, v, in 1 s. What is the force associated with this?

 

[Lnewqban]

my answer =
F = 1.6*9.81 - 1.2*9.81 = 3.924 N -> resultant force of the drone moving (weight minus the upward force)

Note that the hovering drone is not moving in any direction.
Therefore, the resultant force in any direction must be null or zero.

The mass of air that each propeller accelerates downwards (from zero to the value of V that the problem is asking about), generates an upwards force.

Still, the drone does not move, because all the forces acting on it balance each other.

Hint:
Drones depart from the balanced condition of hovering and move upwards when the pilot simultaneously increases the rotational speed of all the propellers (via manipulation of the left lever of the remote control).

Could you tell what happens to the individual mass of air, its acceleration, and induced force in that case?