Forces on and in the Body PDF

Forces on and in the Body Medical physics Forces Forces Physicists recognize four fundamental forces. In order of their relative strength from weakest to strongest. They are: Gravitational force Electrical force Weak nuclear force Strong nuclear force Forces Forces Only the gravitational and electri...

Forces on and in the Body Medical physics Forces Forces Physicists recognize four fundamental forces. In order of their relative strength from weakest to strongest. They are: Gravitational force Electrical force Weak nuclear force Strong nuclear force Forces Forces Only the gravitational and electrical forces are importance in our study of the forces affecting the human body. The electrical force important at molecular and cellular levels, e.g. affecting the binding together of our bones and controlling the contraction of our muscles. Gravitational force, though very much weaker than the electrical force by a factor 1039. Forces How Forces Affect the Body? The muscular forces That cause the blood to circulate and lungs to take in air. In the bones Many crystals of bone mineral require calcium. 1. A calcium atom will become part of crystal if it gets close to a natural place for calcium and electrical forces are great enough to trap it. 2. It will stay in that place until local conditions have changed and electrical forces can no longer hold it in place. One of the important medical effects of gravity is the venous blood travels against the force gravity on its way to the heart. Forces Frictional forces Frictional force refers to the force generated by two surfaces that contacts and slide against each other. Friction 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. Forces Frictional forces Force of friction Ff is described by: Ff = μ N Friction must be overcome when joints move, but for normal joints it is very small. If a disease of the joint exists, the friction may become large. N: Is normal force. μ: Is coefficient of friction between two surfaces Forces Frictional forces Synovial fluid in the joint is involved in lubrication and the saliva we add when we chew food acts as lubricant. 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. Forces Frictional forces Example: A large block of ice is being pulled across a frozen lake. The block of ice has a mass of 300 kg. The coefficient of friction between two ice surfaces is small: μk = 0.05 What is the force of friction that is acting on the block of ice? Solution: On a flat surface, the normal force on an object is given by N = mg. With this, we can find the force of friction as follows: Ff =μN Ff =μmg Substituting the values in the above equation we get, Ff =0.05 × 300 kg × 9.8 m/s2 = 147 kg-m/s2 or 147 N. Dynamics Dynamics Forces on the body where acceleration, The Newton's second low, force equals mass times acceleration. F = ma The force equals the change of momentum Δ(mv) over a short internal of time. F = Δ(mv) / Δt. m: mass a: acceleration Dynamics Dynamics Example: If there is a block of mass 2 kg, and a force of 20 N is acting on it in the positive x-direction, and a force of 30 N in the negative x-direction, then what would be its acceleration? Solution: To calculate its acceleration, we first have to calculate the net force acting on it. Fnet = 20 N – 30 N = -10 N Mass = 2kg ∴ Acceleration = −10 / 2 = −5 m/s2 Dynamics Dynamics Accelerations can produce a number of effects such as: 1. An apparent increase or decrease in body weight 2. Changes in internal hydrostatic pressure 3. Distortion of the elastic tissues of the body 4. The tendency of the solids with different densities suspended in a liquid to separate. Dynamics Dynamics What happen If the acceleration become large? If the acceleration become large may pool in various regions of the body, the location of the pooling depends upon the direction of acceleration. If a person is accelerated headfirst, the lack of blood flow to the brain can cause blackout. Tissue can be distorted by acceleration, if the forces are large, tearing or rupture can take place. Dynamics Dynamics Each of our major organs has its own resonant frequency (or natural period) which depends on its mass and elastic forces that act on it. Pain or discomfort occurs if particular organ is vibrated vigorously and its resonant frequency. Dynamics Dynamics Tendency of the suspended solids of different densities to separate when accelerated: The speed at which small objects fall through a liquid depends on their size, viscosity, acceleration due to gravity g, we can artificially increase g by centrifuge. Dynamics Dynamics Tendency of the suspended solids of different densities to separate when accelerated: Let us consider sedimentation of the small spherical objects of density ρ in the solution of the density ρo in the gravitational field g. In some forms of disease such as rheumatic heart disease, the red blood cells clump together and the effective radius increases; thus an increased sedimentation velocity occurs Stokes has shown that for a spherical object or radius a, the retarding force Fd and terminal velocity V are related by Fd= 6πηaV. η: is viscosity of the liquid. a : radius of spherical object.