_⁨محاضره 1 فيزياء نظري جديد⁩.pdf

Forces on and in the body What is a Force? A force is a push or pull acting on an object that changes the motion of the object. Types (Classes) of Forces Force – Forces that act through direct contact between two objects  Applied Forces, Friction  Field forces act through empty space  No phys...

Forces on and in the body What is a Force? A force is a push or pull acting on an object that changes the motion of the object. Types (Classes) of Forces Force – Forces that act through direct contact between two objects  Applied Forces, Friction  Field forces act through empty space  No physical contact is required Forces that can act over distances  Gravity, Electromagnetic Force (EMF)  Contact Classes of Forces  Contact forces Examples a, b, c  Field forces Examples d, e, f Newton’s Laws of Motion  1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.  2nd Law – Force equals mass times acceleration.  3rd Law – For every action there is an equal and opposite reaction. 1st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 1st Law  Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on by an unbalanced force. Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces. 3rd Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift. Law The 4 Fundamental Forces in nature  1) Gravitational Force: an attractive force that exists between all objects.   It is the weakest force. Ex: The moon is held in orbit by the Earth’s gravity.  2) Electromagnetic Force: forces resulting from electric charge.   This force gives materials their strength, their ability to bend, squeeze, stretch or shatter. It is very large compared to gravity 4 Types of Forces  3) Strong Nuclear Force: holds the particles in the nucleus of an atom together.  It is the strongest force but only acts over the distance of a nucleus.  4) Weak Force: form of electromagnetic force.  Involved in the radioactive decay of nuclei Forces in and on the body 1.Muscular forces that cause the blood to circulate and the lungs to take in air . 2. Molecular forces (in bone , calcium atom ) . 3. Electric forces . 4. Gravitational forces . Medical effects of gravitation forces Medical effects of gravitation forces 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 , and the second effects is on the bone . If a person becomes weightless such as in orbiting satellite , he may lose bone mineral . Long term bed rest removes much of the force of the body weight from bones . Gravity and Motion  On Earth, gravity is a downward force that affects all objects.  When you hold a book, you exert a force that balances the force of gravity.  When you let go of the book, gravity becomes an unbalanced force and the book falls.  The effects of spaceflight on the human body are complex and largely harmful over both short and long term. Significant adverse effects of longterm weightlessness include muscle atrophy and deterioration of the skeleton (spaceflight osteopenia). Other significant effects include a slowing of cardiovascular system functions, decreased production of red blood cells (space anemia), balance disorders, eyesight disorders and changes in the immune system.Additional symptoms include fluid redistribution (causing the "moon-face" appearance typical in pictures of astronauts experiencing weightlessness),loss of body mass, nasal congestion, sleep disturbance, and excess flatulence. Forces on the Body 1. 1. Static force : where the body is in equilibrium 2. Dynamic force : where the body is accelerated . Static Force When objects stationary ( static ) they are in state of equilibrium . The sum of the forces in any direction is equal to zero . In the human body , many of the muscles ,joints and bones systems act as levers . Lever Systems: Bone-Muscle Relationships  The operation of most skeletal muscles involves the use of leverage and lever systems.  Partnership between the muscular and skeletal system levers  A lever is a machine consisting of a beam or rigid rod pivoted at a fixed hinge, or fulcrum. A lever is a rigid body capable of rotating on a point on itself. On the basis of the location of fulcrum, load and effort, the lever is divided into three types Lever Systems: Bone-Muscle Relationships  In     our body: Bones are levers. Joints are the fulcrums. Muscle contraction provides effort at the insertion on the bone. Anything that is being lifted (bone, tissue, anything else) is the load. Lever Systems: Bone-Muscle Relationships  Mneumonic to remember Levers: 123  FLE Or ARF  A lever allows a given effort to move a heavier load, or to move a load farther or faster, than it otherwise could.  Class 1 lever the pivot lies between the effort and the load second-class lever A lever that has its point of resistance (load) between its fulcrum (point of support or axis of rotation) and point of effort (force application). In the human body, a second class lever is used when a person stands on tip-toe. More Concepts  Mechanical  Levers designed for force  Mechanical  advantage disadvantage Levers designed for speed/ROM 42 MECHANICAL ADVANTAGE The general formula for the mechanical advantage (MA) of levers: MAlever = Effort (force) arm Resistance (load) arm Or F For 1st class lever R Mechanical advantage or disadvantage? How does mechanical advantage affect movement of the lever? 44 For 1st class lever Advantage: Small effort moves big resistance Disadvantage: Big movement required to move resistance a small distance 45  Speed/ROM  Examples?  Common    traits? Rigid bar Fixed point Lever movement vs. resistance movement 46 F R 47 For 2nd class lever Advantage: Small effort moves big resistance Disadvantage: Big movement required to move resistance a small distance For 3rd class lever For 3rd class lever the load is always farther from the fulcrum than the effort ,that means they will always increase the amount of effort required by the same factor. Even when the effort is larger than the load as for third class levers, it will come out to be less than one. 2nd class lever always have the effort farther from the pivot than the effort, so they will always allow a smaller effort to move a larger load, giving a greater than one. 1st class lever can either provide mechanical advantage or increase range of motion , depending on if the effort arm or load arm is longer, so they can have mechanical advantages of greater, or less, than one. We can model the human spinal column as a pivoted rod. The pivot corresponds to the joint between the sacrum and the lowest lumbar vertebra. The various muscles of the back are equivalent to a single muscle producing a force T, at a point two thirds up the spine. The sacrum exerts a force R on the spine When you bend over to lift something with the spine horizontal, the force T acts at an angle of 12.0o to the horizontal. The weight of the upper body, which is about 65% of your body weight, acts about halfway along the spine. The weight which you are lifting acts near the top of your spine, from where the shoulder is. If the spine is to be in equilibrium, so there is no net torque or force, then the torque due to T must balance the torques due to the weights. These weights are about 0.5 m to 1 m away from the pivot point, so they will exert a large torque. (Remember that torque is force  distance). The force due to the muscles, T, must be large to balance these torques. When you bend your knees to lift a weight, keeping your back straight, the weight of the body is almost directly over the pivot and hence exerts little or no torque. The closer you hold the weight to your body, the smaller the torque that it will exert. A 10 kg weight lifted against your body will exert a torque of only a few N. The same weight, lifted with the back horizontal, will exert a torque of around 100 N. To balance this torque, the muscles must exert a force of more than 1000 N! This is why lifting incorrectly is so dangerous, and results in so many back injuries. Wheel & Axle: Another movement amplifier! Wheel and Axle Mechanical Advantage is f/r, force arm ÷ resistance arm Force applied to wheel: MA greater than 1 (force amp) Force applied to axle: MA less than 1 (movement amp) Frictional Force Friction and energy loss due to friction appear every day in our life . The maximum force of friction F is F = µ N Where N is a Normal force . µ Is the coefficient between the two surfaces. . The value of µ depends upon the two materials in contact , and it is essentially independent of the surface area , as shown in Table 1. Table 1 Friction during walking (a) Heel contact stage and decelerating the foot and (b) toe-off stage and accelerating the foot When the foot touch the ground the force between the heel and surface prevent the foot from slipping forward fig(a).while when it leaves the ground , The force prevents the toe from slipping backward fig (b). The value of the horizontal force component of the heel as it strikes the ground when a person is walking is given by: F= 0.15W Dynamics force According to second law of Newton , the force is equal F = ma momentum = mv The change in momentum Δ(mv) over a short interval of time is F = Δ(mv ) Δt Example 1 for dynamic force A 60 Kg person walking at 1 m/sec bumps into a wall and stops in a distance of 2.5 cm in about 0.05 sec . what is the force developed on impact ? Δ(mv) = (60 Kg ) (1m/sec) – (60 Kg ) ( 0 m/sec) = 60 Kg m/sec the force developed on impact is F = Δ(mv ) Δt F = 60Kg m/sec 0.05 F = 1200 Kg m/sec² F = 1200 Newton Example 2 A. A person walking at 1 m/sec hits his head on a steel beam . Assume his head stops in 0.5 cm in about 0.01 sec . If the mass of his head is4Kg , What is the force developed ? Q.2 a. Δ(mv)=(4Kg)(1m/sec)-(4Kg)(0m/sec) = 4 Kg m/sec F = Δ(mv ) Δt F = 4 kg m/sec 0.01 F = 400 Newton 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 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