Force equilibrium in inclined steady flight

Steps:

• Airplane moves at constant speed, so net force is zero along flight path and perpendicular to it.
• Along flight path (inclined at θ): thrust T forward balances drag R backward and weight component W sin θ downward along path, giving T = R + W sin θ.
• Perpendicular to flight path: lift L upward perpendicular balances weight component W cos θ, giving L = W cos θ.
• These equations ensure no acceleration in either direction.

Why A is correct:

• It matches Newton's first law for equilibrium, resolving weight into components parallel (W sin θ) and perpendicular (W cos θ) to the flight path.

Why the others are wrong:

• B: Incorrectly swaps sin θ and cos θ components for L and T.
• C: Wrong sign in T equation subtracts W sin θ instead of adding the opposing component.
• D: Swaps sin θ and cos θ while using incorrect subtraction for T.

Final answer: A