點解一個降落傘可以降慢一個人由高空跌落去的速度(10點)

A parachute is usually a soft fabric device used to slow the motion of an object through an atmosphere by creating drag.

Drag (physics)
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An object falling through a gas or liquid experiences a force in direction opposite to its motion. Terminal velocity is achieved when the drag force is equal to force of gravity pulling it down.

In fluid dynamics, drag is the force that resists the movement of a solid object through a fluid (a liquid or gas). Drag is made up of friction forces, which act in a direction parallel to the object's surface (primarily along its sides, as friction forces at the front and back cancel themselves out), plus pressure forces, which act in a direction perpendicular to the object's surface (primarily at the front and back, as pressure forces at the sides cancel themselves out). For a solid object moving through a fluid or gas, the drag is the sum of all the aerodynamic or hydrodynamic forces in the direction of the external fluid flow. (Forces perpendicular to this direction are considered lift). It therefore acts to oppose the motion of the object, and in a powered vehicle it is overcome by thrust.
In astrodynamics, depending on the situation, atmospheric drag can be regarded as inefficiency requiring expense of additional energy during launch of the space object or as a bonus simplifying return from orbit.


http://en.wikipedia.org/wiki/Parachute


http://en.wikipedia.org/wiki/Drag_%28physics%29

I have tried my best to help you.

因為在降落傘落地前，會接觸到空氣，從而產生fraction 減慢了下降的速度。

由 Physics 角度去分析，

降落傘的作用是加大表面積和加大與空氣的摩擦力(Fraction)

Fraction大，你的降落速度自然會減慢，

就可以降慢一個人由高空跌落去的速度