Question

An airplane flying in the air with a constant velocity of 240 m/s horizontally experiences a force due to gravity, directed downwards with a magnitude of 4×10^6 N, and a force generated by the engines, directed forwards with a magnitude of 3 × 10^6 N.

a. In light of Newton’s first law, why must there be a third force acting on the airplane? Calculate the magnitude of this third force, and explain its physical origin. Draw this third force on the free-body diagram.

b. If the airplane were flying straight up instead, and all other things the same, what would the net force on the airplane be?

Answer 1

Solution :

Given :

v = 240 m/s

F_gravity = 4 x 10⁶ N

F_Thrust = 3 x 10⁶ N

.

Part (a) Solution :

The forces acting on An airplane flying in the air with a constant velocity will be : Weight, lift, thrust and drag are the acting forces on an airplane.

• Thrust opposes drag. The engine creates thrust and moves the plane forward.
• Drag opposes thrust.
• Lift opposes weight. Newton's Laws and Bernoulli's Principle generate lift.
• Weight opposes lift. Weight and lift are equal when a plane flies level at constant velocity.

For equilibrium : The magnitude of the Lift force will be equal to the weight of the plane.

Thus : F_Lift = F_gravity = 4 x 10⁶ N

.

Part (b) Solution : If the airplane were flying straight up :

F_Net = F_Thrust - F_gravity = (3 x 10⁶ N) - (4 x 10⁶ N) = - 1 x 10⁶ N

Thus, Net force will be in downward direction. The plane will be Slowing down.