|
These points may be proved quite easily. Fly a kite from a fixed position in a gentle breeze. Left to its own devices it will tend towards the vertical position. Because of this position or angle the kite will be sluggish in rising, and at the same time will be driven backwards. It may eventually assume the vertical position, and consequently make one of those annoying dives to earth. In this event, the upward thrust has been made of no effect and the kite responds to a downward pull.
Again, fly a kite in a strong wind. The kite will be eager to climb and to attain an overhead position. This is due to the fact that the greater wind pressure and the stronger pull on the line have caused the kite to assume an angle at which it responds readily to the upward thrust.
But if for some reason the kite is prevented from maintaining the correct angle, and it moves too far in the direction of the horizontal position, then it becomes unsteady in flight and unless the position can be rectified, the kite will respond to the downward pull, and this is the next force to be considered.
Downward pull is due to the weight of the kite, and weight is an important factor. A kite is heavier than air, and ways have to be found to overcome this disadvantage. Obviously, one thing to do is to make the kite as light as possible. A kite which is not made according to this rule is overloaded, and the effect of this will be the same as when an aeroplane is overloaded. Nevertheless, even when a kite is as light as possible in relation to its size it is still subject to the influence of downward pull.
This pull is called the force of gravity. Everything falls to the ground if it is not held up by some means. It is the earth which attracts things. If there were no such attraction, then everything would be shot off into space by the force of the earth's rotation. Gravity then is force exerted by the earth, to which all bodies are subject; a force manifested in a downward pull.
At this point reference may be made to what is called the centre of gravity. This may be defined as the point where the downward pull is concentrated; or again, as that point in a body about which the whole weight is evenly balanced. The centre of gravity may be found by the rules of geometry; or by experiment.
|
