lecture 1 part 2 (gravitational force

Gravitational force :
There is a force of attraction between any two objects, so our weight is due to the attraction between the earth and our bodies.
Medical effect of gravitational force:
The formation of varicose venis in the legs as the venous blood travels against force of gravity on its way to the heart. Another medical effect is on the bones. gravitational force on the skeleton , if the person become weightless he may loss some bone mineral and may be serious problem.














To maintain stability the vector sum of all forces applying at the CG must be zero.


Many of muscles and bones systems of the body acts as levers, levers are classified as, first, second and third. The last are the most common in the body. Three examples for lever systems, W is the applied weight, F is the force supporting the pivot point of the lever system, and M is the muscles force.










The biceps muscle pulls the arm upwards by muscle contraction with a force M the opposing force is the weight of the arm H
























The lower arm can be hold by the biceps muscle at different angles ?.
The deltoid muscle at the shoulder pulls the arm upwards by muscle contraction with a force T at a fixed angle ? with respect to the arm the opposing force is the weight of the arm H













Consider arm at an angle hold by the deltoid muscle (a » 15°)











Frictional force :
If a body of mass m is in constant motion no acceleration or deceleration occurs so


Acceleration a can be caused by leg muscle force F and Deceleration can be caused by friction.

















Friction occurs between a moving surface and a surface at rest:

F= N

N is the normal force
is coefficient for kinetic friction: for rubber-concrete = 0.8, joints between bones » 0.003
As smaller the coefficient as less resistance by frictional force. Examples of frictional force in the body are: the lung movement in the chest, the intestine have slow rhythmic motions the food move toward its final destination. All of these organs are lubricated by slipping mucus covering to minimize friction.
Retarding force:
Stokes has shown that for spherical object of radius (a) , the retarding force (Fd) and terminal velocity are related by :



When the particle is moving at constant speed the retarding force in equilibrium with the difference between gravitational force and toward buoyant force thus

Gravitational force :
Buoyant force :

Retarding force


Sedimentation velocity:


These equations is valid only for spherical particles , some of disease such as gout , R.B.C. The effective radius increases as increased sedimentation velocity.
Other diseases such as hemolytic jaundice and sickle cell anemia R.B.C change shape or break , so the radius decrease thus the rate of sedimentation is ……………..?