Gravity By Dr M Abbas Jamil Gravity Gravity

Gravity By: Dr. M. Abbas Jamil

Gravity: � Gravity is the force by which all bodies are attracted to the earth. Gravity is the mutual attraction between the earth and an object. � Gravity is one of the four fundamental forces (interactions) in nature. � Sir Isaac Newton theorized the Law of Gravitation in 1687. � Gravity is greater when mass is larger and distance is shorter. � Gravitational force is always directed vertically downward toward the center of the earth.

Law of Gravitation: � The Law of Gravitation states that any two masses exert an attractive force on each other. � Gravitation force depends on two things: � The mass of the two objects � The distance between the two objects � The acceleration due to gravity is directly proportional to the mass of the objects and inversely related to the distance squared. F = G m 1 m 2/r 2

Forces: � The Force of Gravity acts continuously upon the Human Body. � The effects of Gravity can be counterbalanced when a Force equal and opposite is employed such as: � Support of Plinth � Buoyancy of water � Isometric Muscle contraction

� Example: � From the standing position heels can be raised from the ground by the contraction of calf muscles working in opposition to resistance of gravity. They are called antigravity muscles. � The heels will remain raised as long as force of contraction of muscles is equal to gravity. � The heels will be lowered to the ground by the action of gravity if the muscles relax.

Centre of Gravity: � Center of gravity (COG) is the balance point of an object at which torque on all sides is equal. � It is also the point at which the planes of the body intersect. � In the human body, the COG is located in the midline at about the level of, though slightly anterior to the second sacral vertebra of an adult. Because body proportions change with age, the COG of a child is higher than that of an adult.

� Base of support (BOS) is that part of a body that is in contact with the supporting surface. � Line of gravity (LOG) is an imaginary vertical line passing through the COG toward the center of the earth.