# ## GRAVITY ### - Free Fall

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Suppose two objects, one light ,another heavy  are dropped off a very tall building from the same height at the same time.  Which objects -  will hit the ground first ?

Suppose also that air resistance could be eliminated such that none would experience any air drag during the course of their fall. Which object - will hit the ground first

An object that falls through a vacuum is subjected to only one external force, the gravitational force, expressed as the weight of the object. The weight equation defines the weight W to be equal to the mass of the object m times the gravitational acceleration

W = m * g

the value of g is 9.8 meters per square second on the surface of the earth. The gravitational acceleration g decreases with the square of the distance from the center of the earth. But for many practical problems, we can assume this factor to be a constant. An object that moves because of the action of gravity alone is said to be free falling. If the object falls through the atmosphere, there is an additional drag force acting on the object.  The motion of any moving object is described by Newton's second law of motion, force F equals mass m times acceleration a:            F = m * a    and                    a = F / m

For a free falling object, the net external force is just the weight of the object:

F = W               so                a = W / m = (m * g) / m = g

The acceleration of the object equals the gravitational acceleration. The mass, size, and shape of the object are not a factor in describing the motion of the object. So all objects, regardless of size or shape or weight, free fall with the same acceleration. In a vacuum, a beach ball falls at the same rate as an airliner. The remarkable observation that all free falling objects fall with the same acceleration was first proposed by Galileo, nearly 400 years ago.

### AIRPLANE

When the airplane moves forward , due to air viscosity, circulation of air takes place. Now the upper surface of the wing is made more curved than the lower one. The air over the upper wing surface has to cover more distance , compared to that of lower one during the same interval of time.  So the wind velocity on the upper surface is higher and so the velocity head. According to the Bernoulli's theorem

Pu + (1/2)pv2u + pgh =Pl  +  (1/2)pv2l + pgh

where P= air pressure, suffix u stands for upper, l stands for lower, p=air or fluid density , g =gravity , v= velocity of air, h=height

the sum of gravitational head ,pressure head and velocity head always remains constant. As the velocity head increases on the upper side, its pressure head decreases.  The pressure on the upper surface will be less than the lower side. This difference of pressure between the upper and lower produces thrust which causes the lift.