Medical Physics - Forces in the Body PDF

MEDICAL PHYSICS 2- Forces On and In The Body PRESENTED BY: HUSSAIN HASAN MS.C. IN MEDICAL PHYSICS Fundamental forces Force :- is a such a common concept that unless we are physicists or engineers we just use our intuitive feeling about it. Force controls all motion in the world; we are usually una...

MEDICAL PHYSICS 2- Forces On and In The Body PRESENTED BY: HUSSAIN HASAN MS.C. IN MEDICAL PHYSICS Fundamental forces Force :- is a such a common concept that unless we are physicists or engineers we just use our intuitive feeling about it. Force controls all motion in the world; we are usually unaware of important forces in the body, for example, the muscular forces that cause the blood to circulate and the lungs to take in air. Physicists recognize four fundamental forces. In order of their relative strength from weakest to strongest. They are: 1- Gravitational force 2- Electrical force 3- Weak nuclear force 4- Strong nuclear force  Gravitational force  Our weight is due to the attraction between the earth and our bodies.  The gravitational force is much smaller  on the moon.  From Newton’s law of gravitation: There is a force attraction between any two objects. F = force between two objects G = gravitational constant M1, M2 = masses of the objects D = distance between objects (center of mass)  Medical effects of Gravitational force 1. The formation of varicose veins in the legs as the venous blood travels against the force of gravity on its way to the heart. 2. The medical effect of gravity on the skeleton (on the bones), in some way, contributes to healthy bones, if a person becomes weightless, such as in an orbiting satellite, he may lose some bone mineral. Forces on the body 1-Body in equilibruim(statics) 2- Body is accelerated ( dynamics ) 3-friction is involved in both static & dynamics Static :-when objects are stationary they are in a state of equilibrium. The sum of the forces in any directions is equal zero , and the sum of the torques is equal zero. Many of the muscle and bone act as levers. CENTER OF GRAVITY CENTER OF GRAVITY  Levers A lever is a rigid bar free to rotate about a fixed point called the fulcrum.The position of the fulcrum is fixed so that it is not free to move with respect to the bar There are three classes of levers: 1-Class 1 lever, the fulcrum is located between the applied force and the load. 2-Class 2 lever, the fulcrum is at one end of the bar; the force is applied to the other end; and the load is situated in between. 3-A Class 3 lever has the fulcrum at one end and the load at the other. The force is applied between the two ends. 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. EXAMPLE: THE FOREARM AS LEVER SYSTEM  The biceps muscle pulls the arm upwards by muscle contraction with a force M the opposing force is the weight of the arm H at its center of gravity (CG)!  Biceps can be strengthened by weight W lifting this adds another force which has to be compensated by the muscle force The Effect of the Arm Angle on the Muscle Force If the forearm is at an angle (θ) to the  horizontal as shown in Figure below: The muscle and the bone system. a)  The forces and dimensions. b)   The Effect of the Arm Angle on the Muscle Force If we take the torque about the joint, we find that M remains constant as (θ) change. The length of the biceps muscle (L) changes with (θ) change. The figure below shows that the resting length of (L) a muscle is close to its optimum length for producing force. At about (L/2) of the length, it cannot shorten much more and the force can produce drops significantly. A very large stretch of about (2L) produces tearing of the muscle.  The Tension of Raising the Right Arm (a) The deltoid muscle and bones are involved. (b) The forces on the arm. (T) is the tension in the deltoid muscle fixed at the angle (α), (R) is the reaction force on the shoulder joint, (W1) is the weight of the arm located at its center of gravity, and (W2) is the weight in the hand The Tension of Raising the Right Arm  The arm is raised and held out horizontally from the shoulder by the deltoid muscle as shown in the figure below.  By taking the sum of the torques about the shoulder joint, the tension (T) can be calculated from:  T = (2 W1 + 4 W2) / sin α  If α = 16 °, the weight of the arm W1 = 68 N, and the weight in the hand W2 = 45N, then T = 1145 N.  The force needed to hold up the arm is surprisingly large. Frictional forces :-  If a body of mass m is in constant motion no acceleration or deceleration occurs  Acceleration a can be caused by leg muscle force F  Deceleration can be caused by friction, muscle force or external forces  Friction occurs between a moving surface and a surface at rest:  N is the normal force  mk is coefficient of friction:  for rubber-concrete: mk  0.8  joints between bones: mk  0.003  As smaller the coefficient as less resistance by frictional forces Where µ is the coefficient of friction depends upon two materials in contact. Reduced the friction in the body. 1-The synovial fluid in the joint is involved in the lubrication 2- The saliva we add when we chew food acts as a lubricant. 3- Heart beats it moves ,the lungs move inside the chest ,all of these organs are lubricated by slippery mucus covering to minimize friction.  Dynamics From Newton second law. Force equals mass times acceleration. F= m a  This is not the way Newton originally wrote the law force equals the change of momentum ∆ (m v ) Over a short interval of time ∆t F = ∆(m v)/ ∆t Example :-A 60 Kg person walking at 1 m/ sec bumps into a wall and stops in about 0.05 sec. what is the force developed on impact. ∆ (mv)=(60kg)(1 m/sec) - (60kg) (0m/sec)= 60 kg m/sec F=∆(mv)/∆t F=60 (kg m/sec)/0.o5 sec = 1200 N Accelerations can produce a number of effects such as:- 1- An apparent increase or decrease in the body weight. 2- changes in interval hydrostatic pressure. 3- distortion of the elastic tissues of the body. 4- the tendency of solid with different densities suspended in a liquid to separate. The tendency of suspended solids of different densities of separate when accelerated is not important in the body but it is utilized in the common laboratory centrifuge. centrifuge :  is a way to increase apparent weight.it is especially useful for separating a suspension in a liquid.it speeds up the sedimentation that occurs at a slow rate under the force of gravity.  When we drop pebbles into a pond they fall to the bottom at constant speed. The speed depends:-   their size.  viscosity of the fluid.  acceleration due to gravity. We can artificially increase g by spinning the fluid in a centrifuge. This will cause very fine particles from the fluid. Let us consider first sedimentation of small spherical object of density ρ in a solution of density ρo in gravitational field (g). The falling object reach a maximum (terminal)velocity due to viscosity effects. Stoke has shown that for a spherical object of radius a ,retarding force Fd and terminal velocity V are related. Fd = 6 π a ŋ V (ŋ : viscosity Kg/m.sec ) when the particle is moving at a constant speed ,the retarding force is in equilibrium with the difference between the down word gravity force and up word buoyant force. Thus we have.  the force of gravity ==» Fg = 4/3 π a3 ρ g  the buoyant force ==» Fb = 4/3 π a3 ρo g  the retarding force ==» Fd = 6 π a ŋ V Fd =Fg –Fb We obtain the expression for terminal velocity V =(2 a2/9 ŋ ) g ( ρ – ρo )  In some forms of disease such as rheumatic fever rheumatic heart , the red blood cells clump together and effective radius increases thus an increased sedimentation velocity occurs. In other diseases such as hemolytic jaundice and sickle cell anemia the red blood cell change shape or break. The radius decreases. Thus the rate of sedimentations of these cells is slower than normal.  To determination the percent of RBC in the blood Since the sedimentation velocity is proportional to the gravitational acceleration We can increase g by means of a centrifuge , which provides an effective acceleration geff. geff =4 π2 f 2 r where f is the rotation rate in revolutions per second  r is the radius of the centrifuge. The increase of any of these.lead to more dense packing of RBC or smller hematocrit. One standard method utilized centrifuge for 30 min at 3000 rpm with r=22 cm. A normal hematocrit is 40 to 60 A value lower than 40 indicates anemia. A high value may indicates polycythemia Vera.