A paraglider is flying horizontally at a constant speed. Assume that only two fo

Question

A paraglider is flying horizontally at a constant speed. Assume that only two forces act on it in the vertical direction, its weight and a vertical lift force exerted on its wings by the air. The lift force has a magnitude of 1600 N. For both questions, take the upward direction to be the +y direction

a) What is the magnitude and direction of the force that the paraglider exerts on the earth?
______N direction ?

b) If the lift force should suddenly decrease to 900 N, what would be the vertical acceleration of the glider? m/s^2

Explanation / Answer

(a) 1800 Newtons, downward. (b) In the formal physics sense, it depends on the mass of the glider. But more correctly, the answers to both questions actually depend on the lift-drag ratio of the airframe, which your teacher undoubtedly did not consider. First of all, since we are talking about an unpowered glider, the lift force cannot be vertical if the glider is flying horizontally at a constant speed. Rather, the lift force must be inclined at a slight angle (a) to the vertical, such that sin(a) * Lift = cos(a) * Drag. This condition is required for constant horizontal speed. Since the glider is flying horizontally (neither rising nor sinking), that means that the total vertical component of forces must be cos(a) * Lift + sin(a) * Drag. (Usually lift is large and drag is small, and angle a is very small, so this is pretty close to Lift=1800N; but not quite).

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