[ثابت احمد]

[align=center] بسم الله الرحمن الرحيم [/align]
هذه مقدمه (introduction) عن درس free falling أتمنى أن تستفيدوا منها
[align=left]Free Fall and the Acceleration of Gravity
Introduction to Free Fall
A free-falling object is an object which is falling under the sole influence of gravity. That is to say that any object which is moving and being acted upon only be the force of gravity is said to be "in a state of free fall." This definition of free fall leads to two important characteristics about a free-falling object:
• Free-falling objects do not encounter air resistance.
• All free-falling objects (on Earth) accelerate downwards at a rate of approximately 10 m/s/s (to be exact, 9.8 m/s/s)

Because free-falling objects are accelerating downwards at a rate of 10 m/s/s (to be more accurate - 9.8 m/s/s), a ticker tape trace or dot diagram of its motion would depict an acceleration. The dot diagram at the right depicts the acceleration of a free-falling object. The position of the object at regular time intervals - say, every 0.1 second - is shown. The fact that the distance which the object travels every interval of time is increasing is a sure sign that the ball is speeding up as it falls downward. Recall from an earlier lesson, that if an object travels downward and speeds up, then its acceleration is downward.

This free-fall acceleration was witnessed in class in a demonstration involving a strobe light and a dripping jug of water. The room was darkened and water dripping from a medicine dropper was illuminated with a strobe light. The strobe light was adjusted such that the stream of water was illuminated at a regular rate - say every 0.2 seconds. Subsequently, instead of seeing a stream of water free-falling from the medicine dropper, you saw several consecutive drops. These drops were not spaced equally far apart; instead the spacing increased with the time of fall (similar to what is shown in the diagram at the right). Not only was it a "cool demo," it served to illustrate the nature of free-fall acceleration.[/align]:baa_cut: