Medical Physics: Types of Forces PDF

3 Types of Forces By: Abdullah Munqith Types of Force 1- Some effect of Gravity on the Body The force of gravity on the surface of the Earth, normally denoted (g), has remained constant in both direction and magnitude since the formation of the planet As a result, both plant and animal life have evo...

3 Types of Forces By: Abdullah Munqith Types of Force 1- Some effect of Gravity on the Body The force of gravity on the surface of the Earth, normally denoted (g), has remained constant in both direction and magnitude since the formation of the planet As a result, both plant and animal life have evolved to rely upon and cope with it in various ways. Gravity hurts: you can feel it hoisting a loaded backpack or pushing a bike up a hill. But lack of gravity hurts, too: when astronauts return from long-term stints in space, they sometimes need to be carried away in stretchers When a person becomes “weightless,” such as: 1- in an orbiting satellite, he or she loses some bone mineral. This may be a serious problem on very long space journeys. Types of Force 2- Long-term bed rest is similar in that it removes much of the force of body weight from the bones which can lead to serious bone loss. One of the important medical effects of gravity is the formation of varicose veins )‫ (الدوالي‬in the legs as the venous blood travels against the force of gravity on its way to the heart. Yet gravitational force on the skeleton also contributes in some way to healthy bones. Blood feels gravity, too. On Earth, blood pools in the feet. When people stand, the blood pressure in their feet can be high about 200 mmHg (millimeters of mercury). Types of Force In the brain, though, it's only 60 to 80 mmHg. In space, where the familiar pull of gravity is missing, the head-to-toe gradient vanishes. Blood pressure equalizes and becomes about 100 mmHg throughout the body. That's why astronauts can look odd: their faces, filled with fluid, puff up, and their legs, which can lose about a liter of fluid each, thin out. Types of Force Our bodies expect a blood pressure gradient.Higher blood pressure in head raises an alarm. The body has too much blood within two or three days of weightlessness astronauts can lose as much as 22% of their blood volume as a result of that errant message This change effects heart too, if you have less blood then your heart doesn't need to pump as hard ,it's going to atrophy. Types of Force 2- Electrical Forces in the Body Electric Force: force exerted by two charged objects. Positive/Negative Charges: two unlike charges will exert a force that attracts each other while two like charges will exert repelling forces on each other. Protons/Electrons: positive & negative particles inside atoms. Control and action of our muscles is primarily electrical. The forces produced by muscles are caused by electrical charges attracting opposite electrical charges. Types of Force Each of the trillions of living cells in the body has an electrical potential difference across the cell membrane. This is a result of an imbalance of the positively and negatively charged ions on the inside and outside of the cell wall. The resultant potential difference is about 0.1 V, but because of the very thin cell wall it may produce an electric field as large as 107 V/m, an electric field that is much larger than the electric field near a high voltage power line. Only the gravitational and electrical forces are of importance in our study of the forces affecting the human body. Types of Force The electrical force is important at the molecular and cellular levels, e.g., affecting the binding together of our bones and controlling the contraction of our muscles. The gravitational force, though very much weaker than the electrical force by a factor of 1039, is important as a result of the relatively large mass of the human body (at least as compared to its constituent parts, the cells). For example, in the bones there are many crystals of bone mineral (calcium hydroxyapatite) that require calcium. A calcium atom will become part of the crystal if it gets close to a natural place for calcium and the electrical forces are great enough to trap it. Types of Force It will stay in that place until local conditions have changed and the electrical forces can no longer hold it in place. This might happen if the bone crystal is destroyed by cancer. We do not attempt to consider all the various forces in the body in this chapter; it would be an impossible task. Types of Force specialists who deal with forces are: ✓ physiatrists (specialists in physical medicine) who use physical methods to diagnose and treat disease ✓ orthopedic specialists who treat and diagnose diseases and abnormalities of the musculoskeletal system ✓ Physical therapists. ✓ Chiropractors who treat the spinal column and nerves. ✓ Rehabilitation specialists. ✓ Orthodontists who deal with prevention and treatment of irregular teeth. Types of Force Electric eels and some other marine animals are able to add the electrical potential from many cells to produce a stunning voltage of several hundred volts. This special “cell battery” occupies up to 80% of an eel’s body length! Since the eel is essentially weightless in the water, it can afford this luxury. Land animals have not developed biological electrical weapons for defense or attack. Types of Force 3- Frictional Forces Frictional force refers to the force generated by two surfaces that contacts and slide against each other. F = 𝝁𝐍 F: Friction force 𝝁: 𝑪𝒐𝒆𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒕 𝒐𝒇 𝒇𝒓𝒊𝒄𝒕𝒊𝒐𝒏 𝑁: 𝑁𝑜𝑟𝑚𝑎𝑙 𝑓𝑜𝑟𝑐𝑒 Types of Force ❖ A few factors affecting the frictional force: These forces are mainly affected by the surface texture and amount of force impelling them together. The angle and position of the object affect the amount of frictional force. If an object is placed flat against an object, then the frictional force will be equal to the weight of the object. If an object is pushed against the surface, then the frictional force will be increased and becomes more than the weight of the object. Types of Force ❖ Types of Frictional Forces The friction that takes place between solid surfaces is classified as Static, Kinetic, Rolling, and Sliding Friction. The friction that takes place between fluids and gases are termed as fluid friction. Hence, friction is broadly classified as: ✓ Dry Friction Dry Friction Rolling Friction Sliding Friction Static Friction Kinetic Friction ✓ Fluid Friction Types of Force ✓ Dry friction Dry friction describes the reaction between two solid bodies in contact when they are in motion (kinetic friction) and when they are not (static friction). Both static and kinetic friction is proportional to the normal force exerted between the solid bodies. The interaction of different substances is modeled with different coefficients of friction. By this, we mean that certain substances have a higher resistance to movement than others for the same normal force between them. Each of these values is experimentally determined. Types of Force ✓ Fluid Friction Is the force that obstructs the flow of fluid? It is a situation where the fluid provides resistance between the two surfaces. If both the surfaces offer high resistance then it is known as high viscous and, generally, we call them greasy. For example to avoid creaking sounds from doors, we lubricate the door hinges which leads to the smooth functioning of door hinges. Types of Force Q/ A large block of ice are 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? Ff = μN Where N is a normal force and μ is the coefficient of friction between the two surfaces. N = mg Where m is the mass g=9,8 m/s2 Ff = μN Ff = μmg Ff = 0.05 × 300 kg × 9.8 m/s2 = 147 kg-m/s2 or 147 N. The force of friction acting in the opposite direction as the block of ice is pulled across the lake is 147 N. Types of Force Some diseases of the body, such as arthritis, increase the friction in bone joints. Friction plays an important role when a person is walking. A force is transmitted from the foot to the ground as the heel touches the ground. This force can be resolved into vertical and horizontal components. The vertical reaction force, supplied by the surface, is labeled N (a force perpendicular to the surface). The horizontal reaction component, FH, must be supplied by frictional forces. Types of Force The maximum force of friction Ff is usually described by: The value of ƒ depends upon the two materials in contact, and it is essentially independent of the surface area. This is how large the frictional force must be in order to prevent the heel from slipping. If we let N~W, we can apply a frictional force as large as f=W Types of Force Normal walking (a) Both a horizontal frictional component of force, FH, and a vertical component of force N with resultant R exist on the heel as it strikes the ground, decelerating the foot and body. The friction between the heel and surface prevents the foot from slipping forward. At (b) when the foot leaves the ground, the frictional component of force, FH, prevents the foot from slipping backward and provides the force to accelerate the body forward. a person slips on an icy, wet, or oily surface where( f )is less than 0.15. This is not only embarrassing; it may result in broken bones. Slipping can be minimized by taking very small steps. Types of Force N = mg cos θ When: μs (coefficient of static friction) = 0.6 μk(coefficient of Kinetic friction) = 0.3 If θ = 30 o F = Nμ o If the body does not move then F = N μs = mg cos 30o μs Fnet = FAid - Fopposing Fnet = mg sin θ - mg cos μs If mg cos θ μs > mg sin θ mg ‫بالقسمه على‬ μs>sinθ/cosθ> tanθ 0.6 > 0.57 NO move static case Types of Force o If there is little push to the body then Fnet = mg sin θ - mg cos 30o μk If mg cos 30o μk > mg sin θ μk >sinθ/cosθ> tanθ 0.3 > 0.57 friction force less thanPushing force ‫حساب التعجيل للجسم المتحرك‬ a = Fnet / m = mg sin θ - mg cos 30o μk / m = g (sin30o - cos30o) μk = g (sin30o - cos30o) 0.3 m ‫بلقسمة على‬ Types of Force 4- Forces, Muscles, and joints Skeletal muscles have small fibers with alternating dark and light bands, called striations—hence the name striated muscle. The fibers are smaller in diameter than a human hair and can be several centimeters long. The other muscle form, which does not exhibit striations, is called smooth muscle. The fibers in the striated muscles connect to tendons and form bundles. Types of Force 4.1-Muscle Forces Involving Levers For the body to be at rest and in equilibrium (static), the sum of the forces acting on it in any direction and the sum of the torques about any axis must both equal zero. Many of the muscle and bone systems of the body act as levers. Levers are classified as first-, second-, and third-class systems. W is a force that is usually the weight F is the force at the fulcrum point M is the muscular force. Types of Force Example: If w = 10N, H = 20N the effort exerted by the biceps muscle (M), and the reaction due to the upper arm (R), calculate M and R? M x 4 = H x 14 + W x 30 = 20 x 15 + 10 x 30 = 280 + 300 = 580 M = 580 / 4 = 145N the Effort Resolving vertically gives R + 20 + 10 = 145 → R = 114N Reaction due to upper arm ▪ Notes The mechanical advantage of a system is defined by MA = load effort In the example above the effort has to be bigger than the load, because it acts closer to the fulcrum than the load does. Although this makes the mechanical advantage less than one, it means that large movements of the arm are produced by only small contractions of the muscles. Types of Force Most of the joint in body are third – class levers. Since all third – class levers have a mechanical advantage which is less than one, it follows that most of joints are designed for speed of movement rather than for lifting heavy loads. Types of Force ✓ Static equilibrium is a state where bodies are at rest; dynamic equilibrium is a state where bodies are moving at a constant velocity (rectilinear motion). In both cases the sum of the forces acting on them is zero. If only two forces (the weight of body acts downward, and reaction force of the ground acts upward) act on a body in the state of either static or dynamic equilibrium, they have equal magnitude but opposite direction. Types of Force ✓ 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 Δt or F = (Δ (mv))/Δt Types of Force 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. a) If the acceleration become large may pool in various regions of thebody, the location of the pooling depends upon the direction of acceleration. If a person is accelerated head first the lack of blood flow to the brain can cause blackout. b) Tissue can be distorted by acceleration, if the forces are large, tearing or rupture can take place.