question_answer

A helicopter is moving to the right at a constant horizontal velocity. It experiences three forces \[{{\vec{F}}_{gravitational}},\] and force on it caused by rotor \[{{\vec{F}}_{rotor.}}\]Which of the following diagrams can be a correct free-body diagram representing forces on the helicopter?

A)

B)

C)

D)

Correct Answer: C

Solution :

\[{{\vec{F}}_{gravitation}}\] acts downwards. \[{{\vec{F}}_{\operatorname{drag}}}\]act opposite to the direction of the motion. Now let us how \[{{\vec{F}}_{\operatorname{rotor}}}\]acts.
[a]	[b]
Wings of the helicopter will push the air perpendicular to their plane of rotation as shown in Fig. [a]. From the third law, air will apply force \[{{\vec{F}}_{\operatorname{rotor}}}\]On the wings as shown in Fig. [b].
There is no force\[ma\]. Basically, \[ma\]is the resultant of all the forces acting on a body. Moreover, here a is zero because velocity of the helicopter is constant.