Forces of Human Body - PDF

Forces Physics for Biomedical Sciences Dr Irene Polycarpou  2 The Body: A Mobile Physics Lab Motion and balance Fluids and pressure Energy Acoustics Optics Electricity The musculos...

Forces Physics for Biomedical Sciences Dr Irene Polycarpou  2 The Body: A Mobile Physics Lab Motion and balance Fluids and pressure Energy Acoustics Optics Electricity The musculoskeletal system is a structure and machine that provides balance and motion. Functions of the cardiovascular system (breathing and pulse) are governed by laws of fluids and pressure. The brain is a computer with an electrical connection called the nervous system.  3 The Musculoskeletal System Its primary purpose is to provide movement for the body. It Includes... Muscles – generate force Tendons – transfer force to bones Bones – move if enough force is transmitted Joints – allow bones to move In Biology, these components form the musculoskeletal system. In Physics, these components form a system of simple machines.  4 Forces What are the forces? Kick a ball and it'll fly up into the air before falling back down to the ground. That's an example of everyday forces. What exactly is a force? Definition of force: A force is a pushing or pulling action that can make things move, change direction, or change shape. Forces have both a magnitude and a direction (i.e. they can be represented as vectors). They give rise to accelerations through Newton’s second law: F=ma Forces can be represented as vectors  5 Remember the Newton laws: Newton’s Laws  7 Newton’s FIRST law Every object continues in its state of rest or continues to move in a uniform speed in a straight line UNLESS it is compelled to change that state by a net force acting on the object Note what is not stated or implied by Newton’s first law: 1) it does not mean that every moving object has a force acting on it. 2) it does say that a stationary object has zero net force acting on it.  8 Remember the Newton laws:  9 Remember the Newton laws: 10  11 How do forces affect the body? We are aware of forces on the body such as the force involved when we bump into objects. We are usually unaware of important forces inside the body. Forces inside the human body: Muscular forces: cause the blood to circulate and the lungs to take in air. Molecular forces: force that determines if a particular atom or molecule will stay at a given place in the body.  12 How do forces affect the body? Molecular forces For example, in the bones there are many crystals of bone mineral (calcium hydroxyapatite, HAp) 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. 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.  13 Internal forces External forces 1. Gravitational force 1. Gravitational force 2. Electromagnetic forces 2. Static force 3. Strong and weak nuclear forces 3. Dynamic force Forces that act on one part of an object or Forces that act on an object or system system from another part of the same from outside of it. object or system. Can cause acceleration or deformation Do not change the overall motion of the of the object or system. object or system. Static: e.g. friction, static electricity Dynamic: e.g. electromagnetic,  14 Forces in the body 1. Gravitational forces Newton’s Universal law of gravity. There is a force of attraction between any two objects ✓ Our weight is due to the attraction between the Earth and our bodies. 2. Electromagnetic forces Attractive and repulsive forces between static electrical charges as well as magnetic force produced by moving electrical charges (electrical current). Electrical forces are immense compared to gravitational force. For example, the electrostatic force between an electron and a proton in a hydrogen atom is about 𝟐. 𝟐𝟕 𝒙 𝟏𝟎𝟑𝟗 times greater than the gravitational force between them.  15 The main force acting on the body is the gravitational force! Stability of the body against the gravitational force is maintained by the bone structure of the skeleton. Gravitational force W applies at the center of gravity CG of the body! CG depends on body mass distribution! to maintain stability CG must be located between feet, if feet are far apart forces in horizontal direction Fx have to be considered  16 The influence of the gravitational force within the human body One of the main applications of gravitational force in a human body is visible in blood circulation.  17 Role of valves in veins 1. Muscle contracts 2. Forces blood flow along the vessel 3. Valves keep the blood flowing towards the heart, against the gravitational force  18 What happens when the valves do not work?  19 What happens when the valves do not work? Blood flows in a reverse direction within the veins, along the direction of the gravitational force, called an incompetent vein.  20 Role of gravitational force within the body ✓ Blood circulation ✓ Bone health ✓ Body weight  21 What does gravity have to do with bone health? Imparts mechanical resistance to the body's activities. → Perceived by osteocytes and translated into cellular signals that regulate the balance between tissue formation (growth) and tissue resorption (breakdown), termed bone remodelling.  26 What happens to our bones in space? Mass does not change on Earth vs. in space. Weight changes depending on the gravitational pull. Astronaut in the International Space Station (ISS) ❑ Downward gravitational pull of about 0.89 g, but the station itself is simultaneously accelerating downward at 0.89 g. Everyone and everything inside the station experiences the same gravity and acceleration → sum of the forces on the astronauts is close to zero. ❑ Microgravity – condition where people or objects appear to be weightless  27 What happens to our bones in space? Calcium that is stored in the bones is broken down and released into the blood stream. Decreased bone density → higher concentration of calcium in blood.  28 What happens to our bones in space? Serious problem on very long space journeys. 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. Running in Space! (youtube.com)  29 In space, the amount of weight that bones must support is reduced to almost zero. At the same time, many bones that aid in movement are no longer subjected to the same stresses that they are subjected to on Earth. Over time, calcium normally stored in the bones is broken down and released into the bloodstream. The high amount of calcium found in astronaut's blood during spaceflight (much higher than on Earth) reflects the decrease in bone density, or bone mass. Gravity and the human body - Jay Buckey (youtube.com) Adding forces To practice: https://phet.colorado.edu/sims/html/forces-and-motion-basics/latest/forces- and-motion-basics_en.html Net force The net force is the vector sum of all the forces that act upon an object. That is to say, the net force is the sum of all the forces. The net force depends on the magnitudes and directions of the applied forces. Net force  33 Forces: How many forces? Most of the time, there are several different forces working at once, all pulling or pushing with different strength, often in different directions. The effects of all these forces add together or subtract from one another to produce an overall force (or perhaps no force at all). Resultant force: The resultant force is the sum of all the force vectors acting on the body. It is the single force which has the same effect as the combination of forces. Forces are vectors (They have both magnitude and direction) and so add as follows: In one dimension, note direction using a + or – sign then add like scalar quantities (regular numbers with no direction associated with them) The net force is the resultant of Find the this vector addition: resultant force in this Fnet = F = F1 + F2 + F3 +  situation: Bold letters represent vectors. The units of Force are Newtons, or the Fnet = 2N abbreviation N, which represent the SI units: kg-m/s2  34 Forces: Example 35  36 Forces: Example 37 Equilibrium When you feel dizzy might someone tell you that you lost your equilibrium. Equilibrium: means that the object remains at rest or continues to move with a constant velocity (i.e. it is not accelerated). When your body is in equilibrium, it is in a state where it is physically balanced.  Equilibrium We also use the word equilibrium when talking about balance. ❑Only common way equilibrium comes up is when looking into the motion of an object. Different types of motion = different types of equilibrium. ✓ Equilibrium means that the object remains at rest or continues to move forward with a constant velocity, i.e. it is not accelerated Common types of motion: Translational Rotational Translational Motion Occurs when there is movement in a straight line. From one point to another point. Rotational motion Occurs when there is movement around an axis. An object revolves around an axis. Rotational + translational motion = rolling States of Equilibrium are associated with the types of motion  Translational equilibrium Vector sum of all the external forces acting on the body is zero. Object in translational equilibrium when experiencing zero overall acceleration → not moving or moving at a constant velocity. Translational = only changes of position are considered; changes of orientation of the object with respect to the axes are ignored. Both cars are in translation equilibrium.  Example of translational equilibrium The earth pulls down with force W. The floor pushes up on the right foot with force N1 and on the left foot with force N2. Draw the free body diagram to determine what the condition for translational equilibrium tells us about the forces. The equilibrium condition gives: N1+N2-W = 0 or N1+N2 = W Total force on the floor pushing up on both = pull of the earth → translational equilibrium STATIC EQUILIBRIUM DYNAMIC EQUILIBRIUM  49 Another example : The cast and the forearm together weigh 98.0 N. Assuming the upper arm exerts a horizontal force of 24.0 N to the right on the forearm, determine the force exerted by the sling on the neck. Force equilibrium problems like this can be analyzed by drawing a free-body diagram of the point of attachment which must be in equilibrium. Then you apply the force equilibrium condition. 24.0 N 98.0 N 50  51 Another example : Find the tensions required to support the mass m Play with this example: http://hyperphysics.phy-astr.gsu.edu/hbase/fcab.html  52 Another example : Find the tensions required to support the mass m Play with this example: http://hyperphysics.phy-astr.gsu.edu/hbase/fcab.html  Conditions for equilibrium ✓First condition ✓Second condition The net external force on Object must avoid the system must be zero. accelerated rotation → Expressed as an equation, maintain constant angular this is simply velocity. A rotating body or system net 𝐅=0 can be in equilibrium if its rate of rotation is constant and remains unchanged by TRANSLATIONAL the forces acting on it. ROTATIONAL EQUILIBRIUM EQUILIBRIUM x  x PIVOT POINT  x PIVOT POINT  x PIVOT POINT To rotate an object you need to apply a torque.  Rotational equilibrium The object does not rotate or continues to rotate at a constant rate (with a constant number of rotations per second). The condition for rotational equilibrium is that the sum of all torques is zero: Rigid rod free to rotate about a pivot at point X: F3 Forces F1 and F2 are applied to the rod at x distances r1 and r2. The F1 F2 pivot exerts the force F3 on r r the rod needed to maintain 1 2 translational equilibrium. Translational equilibrium requires F1+ F2=F3  What is torque? https://demonstrations.wolfram.com/TorqueExertedOpeningADoor/ The turning or twisting effectiveness of a force. Has both magnitude and direction. Opening a door: Push on the side farthest from the hinges. Pushing on the side closest to the hinges requires more force. Although the work done is the same in both cases (the larger force would be applied over a smaller distance) people generally prefer to apply less force, hence the usual location of the door handle.  Same force is much more effective at rotating an object such as a nut or a door if our hand is not too close to the axis. That is why we have long- handed wrenches and why doorknobs are not next to hinges.  Quiz: Yes No Published by Gerald Hicks Published by Gerald Hicks Published by Gerald Hicks  Torque Torque is a measure of the force that can cause an object to rotate about an axis. Torque is a vector quantity. The direction of the torque vector depends on the direction of the force on the axis.  Force vs Torque Forces describe changes in linear motion – which means changes in velocities. Torque describes how these same forces can change angular motion – which means changes in angular velocities.  How is torque calculated? Torque is the cross product between a force and the distance of the force from a fulcrum (the central point about which the system turns). Torque - magnitude Figure 1 Units: Nm [Newton meters] Torque increases as the force increases and as the distance increases. Torque - direction The direction of the torque vector is found by convention using the right hand grip rule. If a hand is curled around the axis of rotation with the fingers pointing in the direction of the force, then the torque vector points in the direction of the thumb. Interpretation of the torque 70 71  Torque - direction Produces a counterclockwise angular acceleration = positive Produces a clockwise angular acceleration = negative Exerting lesser force for same effect. Summary for the torque  76 The expression for the torque of the bicep muscle on the forearm is Schematic view of the muscle system used to bend the elbow. Biceps bend the elbow to lift, triceps straighten it. 78 B ful h di i s… A 25 N force is applied to a bar that can pivot around its end as shown below The force is r = 0.75m away from the end and at an angle θ=60°. What is the torque on the bar? -16 N.m The torque is clockwise and therefore is negative. B ful h di i s… Another example: A uniform meterstick (1m stick) is balanced at its midpoint with several forces applied as shown below. If the stick is in equilibrium, the magnitude of the force Xi w s (N) is…... Applying rotational equilibrium the magnitude of the force X in newtons (N) is 50 Newton  81 Equilibrium in medicine The equilibrium condition can be used to understand many problems in medicine (e.g. clinical orthopedics) Example: 1. Forces that sometimes cause the Achilles tendon at the back of the heel to break  82 Force in the Achilles Tendon The Achilles tendon connects the calf muscles (the gastrocnemius and the soleus) to the calcaneus at the back of the heel. Calculate the force exerted by this tendon on the calcaneus when a person standing on the ball of one foot. FT= The force exerted by the tendon on the foot FB= The force of the leg bones (tibia and fibula) on the foot W= The force of the floor upward, which is equal to the weight of the body. Note: Measurements in a few people suggest that the angle the Achilles tendon makes with the vertical is about 7°.  83 Translational equilibrium requires that: FT cos(7°) + W – FB cosθ = 0 FT sin(7°) – FB sinθ = 0 7° Note: Measurements in a few people suggest that the angle the Achilles tendon makes with the vertical is about 7°.  84 Note: For rotational equilibrium we Rotational equilibrium requires that: need to know the torques. We assume that the torques are taken about the The torque equation is: point where FB is applied to the foot. 10W – 5.6 FT cos(7°) = 0 This equation can be solved for the tension in the tendon: FT = 10W / 5.6cos(7°) = 1.8W The tension in the Achilles tendon is nearly twice the person’s weight To find the: Fby=Fbcosθ From the previous slide we know: FT cos(7°) + W – FB cosθ = 0 & FT sin(7°) – FB sinθ = 0 7° (1.8)(W)(0.993) + W = FB cosθ 2.8 W = FB cosθ and FB = 2.8 W (1.8)(W)(0.122) = FB sinθ The force exerted on the 0.22 W = FB sinθ leg by the talus is nearly three times the body weight  85 The tension in the Achilles tendon is nearly FT = 1.8W twice the person’s weight, while the force exerted on the leg by the talus is nearly three FB = 2.8 W times the body weight. One can understand why the tendon might rupture. Question: (a) Is it possible for an object to be in translational equilibrium (the first condition) but not in rotational equilibrium (the second condition)? Illustrate your answer with a simple example. (b) Can an object be in rotational equilibrium yet not in translational equilibrium? Justify your answer with a simple example. Be careful: A body can be in translation equilibrium and not in rotation equilibrium and vice versa.  88 To maintain stability the sum of the forces acting on it in any direction and the sum of the torques about any axis must both equal zero.  89 Levers in the human body: The Musculoskeletal System Includes levers Definition of lever: A rigid rod, or arm, that turns about a pivot point, called the fulcrum. Force, or effort, is applied to the lever arm to move a load, also called the resistance. Types of Levers: Levers are classified as first-, second-, and third-class systems. Levers can increase mechanical advantage You can increase mechanical advantage by: - Moving the fulcrum closer to the resistance and farther from effort force. Published byTheodora Price  91 Levers in the human body The human body is a complex machine that includes simple machines – LEVERS Many of the muscle and bone systems of the body act as levers (e.g. arms, legs work as levers to move and lift objects). When a person is moving his legs- arms-any part of his body he contracts or he extends his muscles. In facts, the bone in moving work as A LEVER. What type of levers do we have in human body? Most human body levers are third class levers, while first-class levers are least common.  92 Levers in the human body can not be changed but they can be utilized more efficiently.  94 First class lever in human body The head atop the spinal cord, where the weight of the head is balanced by the downward effective force of the muscles. The triceps brachii pull on the ulna about the elbow pivot balanced by the forces on the forearm. With the upper arm Effort down, the triceps brachii can balance an upward force Load pushing the hand up Fulcrum  96 Second class lever in human body On the lower leg when someone is standing on his toes. Force is applied on the muscles by the weight of our body at the toes as an axis. Doing push ups using triceps.  Third class lever in the body I. The elbow joint: when lifting a book, the elbow joint is the fulcrum across which the biceps muscle perform the work. II. Biceps curls: while lifting a dumbbell, the elbow joint acts as an axis with force applied on our hands by the weight we are lifting III. Raising the weight with the arm held straight  100 In a first class lever, the weight and muscle act on opposite sides of the fulcrum and are in the same direction In a second class lever, the muscle and weight act on the same side of the fulcrum, and the weight is nearer to the fulcrum In third class levers, the muscle and weight are again on the same side of the fulcrum, but now the muscle is nearer to the fulcrum than the weight  101 https://www.youtube.com/watch?v=d1wS_OlJzmI Exercise for HOME Exercise for HOME Exercise for HOME Exercise for HOME Thank you!