Gravitational force
In nature, every object is attracted by every other object. This phenomenon is called gravity. The force associated with gravity is called gravitational force. The gravitational force is the weakest force that exists in nature. The gravitational force is always attractive.
Acceleration Due to Gravity
In fundamental physics, gravity or gravitational force is the universal attractive force acting between all the matters that exist or exhibit. It is the weakest known force. Therefore no internal changes in an object occurs due to this force. On the other hand, it has control over the trajectories of bodies in the solar system and in the universe due to its vast scope and universal action. The free fall of objects on Earth and the motions of celestial bodies, according to Newton, are both determined by the same force. It was Newton who put forward that the moon is held by a strong attractive force exerted by the Earth which makes it revolve in a straight line. He was sure that this force is similar to the downward force which Earth exerts on all the objects on it.
Suppose that a projectile of mass m moves in a vertical
plane in the atmosphere near the surface of the earth under
the influence of two forces: a downward gravitational
force of magnitude mg, and a resistive force FR that is
directed opposite to the velocity vector v and has magnitude
k v2 (where v = |v| is the speed of the projectile;
see Fig. 4.1.15). Show that the equations of motion of the
projectile are
mx'' = -kvx' , my'' = -kvy' -mg,
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