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Lecture 1: Forces on and in human body

الكلية كلية طب حمورابي     القسم الكلية ذات القسم الواحد     المرحلة 1
أستاذ المادة نادية حسين صاحب طاهر       11/06/2018 07:39:38
Lecture Objectives:
The fundamental origins of forces.
Types of problems involving forces on body.
Effects of the acceleeration.

Physicists recognize fundamental forces:
The gravitational force: (Newton’s law); these laws state that there is a force of attraction between any two objects; our weight is due to attraction between the earth and our body.
The electrical force: The repulsive or attractive interaction between any two charged bodies is called as an electric force. Similar to important at molecular and cellular levels force, e.g. affecting the binding together of our bones and controlling the contraction of our muscles, its impact and effects on the given body are described by Newton’s laws of motion.
The nuclear force:
Is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nuclear force almost identically.
Types of problems involving force on body.
One of the important medical effects of gravity force is the formation of varicose veins in the legs, as the venous blood travels against the force of gravity on its way to the heart.
The forces produced by muscles are caused by electrical charges attracting opposite electrical charges. Cells in the body have an electrical potential different across the cell membrane.
Frictional and energy loss resulting from friction appear everywhere in our everyday life. Some diseases of the body, such as arthritis increase the friction in bone joint.

Force of fraction F is described by:
Ff = ? N
N: is normal force, ?: is coefficient of friction between two surfaces.
Frication must be overcome when joints move, but for normal joints it is very small. If a disease of the joint exists, the frication may become large.
Muscle forces involving levers:
For the body to be at rest and equilibrium (static), the sum of the forces acting on it in any direction and the sum of the torques any axis must both equal zero. Levers are classified as, first, second, and third. The last are most common in the body.



4M – 30 W = 0
M = 7.5 W Neglect the weight of forearm
M = 3.5 H + 7.5 W Include forearm
The arm can be raised and held out horizontally from shoulder by the deltoid muscle, by taking sum of torques about shoulder joint, the tension T can calculate from: T= 2W1 + 4W2 /sin ?



Dynamics:
Newton’s second law is: Force equals mass times acceleration
F = m a a = acceleration= dv/dt
The force equals the change of momentum over a short internal of time.
F = ?( mv) / ?t

Effects of the acceleeration.
An apparent increase or decrease in body weight
Changes in internal hydrostatic pressure
Distortion of the elastic tissues of the body
The tendency of the solids with different densities suspended in a liquid to separate
Stokes has shown that for spherical object of radius (a), retarding force (Fd) and terminal velocity (v) are related by:
Fd = 6 ? a ? v ? = viscosity
When the particle is moving at constant speed the retarding force
is equilibrium with the difference between gravitational force and
upward buoyant force (the weight of the liquid the particle displaces)
thus we have:

Force of gravity Fg = 4/3 ? a3? g

Buoyant force Fb = 4/3 ? a3 ?? g

Retarding force Fd = 6 ? a ? v

In equilibrium Fg - Fb = Fd


We obtain the Sedimentation Velocity:
V= 2a2/ 9 ? [g (?- ??]

This equation is valid for spherical objects. In some forms of diseases such as rheumatic fever, rheumatic heart disease, and gout RBC clumps together, and the effective radius increase thus increased sedimentation velocity occur.


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