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Forces and Newton\'s Laws


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Forces and Newton\'s Laws

  1. 1. Forces and Newton’s Laws of Motion
  2. 2. Force : The Cause of Motion Every acceleration is caused by forces acting on a body Force is a push or a pull 15 N The strength of a force determines the magnitude of the acceleration Arrows are used to represent 5N forces. The length of the arrow is proportional to the magnitude of the force.
  3. 3. Newton’s Laws of Motion First Law What happens in the absence of force Second Law The effects of applying a force to an object Third Law Forces are caused by interactions of two objects
  4. 4. First Law of Motion: Inertia An object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force (unless acted on by an external unbalanced force). The net force is the vector sum of all of the forces acting on an object. r ∑ The SI unit of force is the Newton (N). F
  5. 5. First Law of Motion: Inertia Unbalanced force: difference between downward and upward forces Balanced force: net force is zero. Terminal velocity is constant.
  6. 6. First Law of Motion: Inertia Inertia: is the natural tendency of an object to remain at rest in motion at a constant speed along a straight line; “Resists changes in motion”
  7. 7. First Law of Motion: Inertia
  8. 8. Newton’s Second Law When a net force acts on an object, the object r accelerates in the same direction as that force. r a= ∑ F The acceleration is directly proportional to the net m force and inversely proportional to the mass of the object. The bigger the force, the greater the acceleration; the larger the mass, the smaller the acceleration. r r ∑ F = ma Force = mass (kg) x acceleration (m/sec2) 1 newton = 1.0 kg-m/sec2
  9. 9. Newton’s Second Law
  10. 10. Newton’s Second Law of Motion
  11. 11. Newton’s Second Law of Motion A free-body-diagram is a diagram that represents the object and the forces that act on it. The net force in this case is: 275 N + 395 N – 560 N = +110 N and is directed along the + x axis of the coordinate system.
  12. 12. Newton’s Second Law of Motion If the mass of the car is 1850 kg then, by Newton’s second law, the acceleration is a= ∑ F = + 110 N = +0.059 m s 2 m 1850 kg
  13. 13. Problem A 440-g can of food is given a shove on a frictionless level surface and is observed to accelerate at a rate of 1.5 m/sec2. What is the force of the shove? r r ∑ F = ma 0.66 N
  14. 14. Third Law : Action – Reaction For every action, there is an equal and opposite reaction. Whenever one body exerts a force on a second body, the second body exerts a force back on the first that is equal in magnitude and opposite in direction. Fa = - Fb
  15. 15. Third Law : Action – Reaction
  17. 17. FORCES a push or pull a vector quantity any influence that is capable of producing a change in the state of motion The magnitude is determined by its strength Unit = m x a ; 1kg m/s; 1 Newton (N)
  18. 18. Two General Types of Forces Fundamental Gravitational Force Strong Nuclear Force Electroweak Force Non-fundamental Friction Tension in a rope Normal or support forces
  19. 19. Gravity Attractive force that acts between any two objects in the universe. Gravitational attraction of the earth endows objects with weight and causes them to fall to the ground when dropped.
  20. 20. Newton’s Law of Universal Gravitation Between any two objects in the universe there is an attractive force (gravity) that is proportional to the masses of the objects and inversely proportional to the square of the distance between them. F = G x m1 x m2 d2 Gravitational constant = 6.673 x 10-11 N-m2/kg2
  21. 21. The Gravitational Force What is the magnitude of the gravitational force that acts on each particle, assuming m1= 12kg, m2 = 25kg and r = 1.2m? m1m2 F =G 2 r ( = 6.67 ×10 −11 N ⋅ m kg 2 2 ) (12 kg )(25 kg ) (1.2 m )2 = 1.4 ×10 −8 N
  22. 22. The acceleration due to gravity g = G x ME RE2 ME g =G 2 RE ( −11 = 6.67×10 N⋅ m 2 kg ) 2 (5.98×10 kg) 24 (6.38×10 m) 6 2 = 9.80m s 2
  23. 23. Weight and Mass Force between two objects in the earth : gravitational attraction Weight: force of gravity on an object Unit is Newton Mass: amount of matter in the body Unit is kg Weight depends on where you are. Where would you weigh less, sea level or mountain top?
  24. 24. The Gravitational Force Relation Between Mass and Weight M Em W =G 2 r ME W = mg g =G 2 r A cantaloupe has a mass of 0.5 kg. What does it weigh? 4.9 newtons
  25. 25. Apparent Weight The apparent weight of an object is the reading of the scale. It is equal to the normal force the man exerts on the scale.
  26. 26. Gravity and Varicose Veins Like all things on earth, circulating blood is subject to gravity. When in a standing position, “blood is redistributed to regions below the heart and venous return is reduced” (Gisolf, 2004). Many complications arise from abnormal blood vessel flow, one of which --- is recognized as the bane of middle-age life in women: varicose veins.
  27. 27. Gravity and Varicose Veins The cause of varicose veins is very much related to the gravitational influence on blood flow. In a standing position, blood pools in the peripheries. If the valves become weak, blood can leak back into the vein and collect there. This is called venous insufficiency. In order to test for the extent of the valve’s weakness one can perform a Doppler test or a Trendelenburg test.
  28. 28. Gravity and Varicose Veins FACTORS TREATMENT Increasing Age Support Genes Leg elevation Hormonal changes. Injection sclerotherapy is a Pregnancy surgical treatment in which the Obesity, leg injury, prolonged veins are injected with a standing and other things that chemical that closes them weaken vein valves completely. Sun exposure, which can cause Ligation and stripping (which spider veins on the cheeks or means tying and pulling out) is a nose of a fair-skinned person technique used to remove the surface veins either partly or SYMPTOMS of varicose veins can altogether. range from the mild aching, itching and swelling of the legs and ankles
  29. 29. Gravity and Varicose Veins PREVENTION avoid standing still for long periods of time take regular exercise, such as walking maintain a healthy weight wear properly fitted compression stockings to prevent further deterioration of existing varicose veins.
  30. 30. Gravity Drainage Gravity drainage simply means the removal of either air, water or solid from the body through the use of gravity and negative pressure. This process is due to gravity because as the catheter bag is placed in a level lower Examples of this are than the level of the bladder, and catheters, nasogastric air and fluid is caused to move from tube, chest tube drainage an area of higher level to a low level. It also creates a negative and postural drainage. pressure in the bag that forces the urine out.
  31. 31. Gravity Drainage It is mainly used to relieve The chest tube drainage system is composed of gastric pressure by draining the chest tubes and an outlet. The outlet maybe: air, liquid and solids in the one bottle water system, two bottle or the short stomach term catheterization and lastly using the three bottle or water pleural drainage system. Fluid flowing back into the patient may be a source of infection, in addition to reducing capacity for gas exchange.
  32. 32. Gravity Drainage Postural drainage aims to remove secretions by gravity In this procedure, the patient is twisted and turned so as to facilitate the movement of the hands. The patient is positioned with the area of the lung with retained secretions higher than the airway though which those secretions will drain Postural drainage therapy is often used in conjunction with aerosol administration and other respiratory care procedures.
  33. 33. The Normal Force Definition of the Normal Force The normal force is one component of the force that a surface exerts on an object with which it is in contact – namely, the component that is perpendicular to the surface.
  34. 34. The Normal Force FN − 11 N − 15 N = 0 FN = 26 N FN + 11 N − 15 N = 0 FN = 4 N
  35. 35. Centripetal Force Ac = v2/r Fc = mv2/r An object stays in a circular path as long as this force acts. If this disappears, the object moves in a straight line Central-seeking force
  36. 36. Force of Friction It is the force generated by the properties of the interface between the object and the surface Ff = µN N = Normal force – force perpendicular to the surface µ = coefficient of friction
  37. 37. Static and Kinetic Frictional Forces When an object is in contact with a surface there is a force acting on that object. The component of this force that is parallel to the surface is called the frictional force. Note that the magnitude of the frictional force does not depend on the contact area of the surfaces.
  38. 38. Types of Force of Friction Depends on the physical state of motion Static force of friction: If at rest / stationary Ff,s = µsN ; net F > Ff,s = object set to motion Kinetic force of friction: if in motion - Ff,k = µ kN ; keeps it in motion
  39. 39. Static and Kinetic Frictional Forces When the two surfaces are not sliding across one another the friction is called static friction. The magnitude of the static frictional force can have any value from zero up to a maximum value. 0 < µs < 1 is called the coefficient of static friction.
  40. 40. Static and Kinetic Frictional Forces Static friction opposes the impending relative motion between f k = µ k FN two objects. Kinetic friction opposes the relative 0 < µs < 1 sliding motion motions that actually is called the coefficient of kinetic friction. does occur.
  41. 41. Friction and Osteoarthritis Osteoarthritis – Inflammation of joints Joints – Synovial membrane Synovial fluid Almost same consistency of water µ - 0.005 to 0.02 Force of friction Ff = µ W W = mg
  42. 42. The Tension Force Tensions - forces exerted by strings or ropes, cables Cables and ropes transmit forces through tension. A “massless” rope will transmit tension undiminished from one end to the other. If the rope passes around a massless, frictionless pulley, the tension will be transmitted to the other end of the rope undiminished.
  43. 43. Traction Systems • A traction apparatus exerts a force on one of the patient’s extremities net force. • a combination of pulleys with a weight to exert force Traction System Rope/cord – transmits tension Pulleys – shifts direction of tension Force = Weight = Tension applied
  44. 44. Traction Systems F=3T F/3=T
  45. 45. Purposes of Traction Treatment of fractured extremities To lessen muscle spasm To reduce fracture To provide immobilization To maintain proper alignment of bones To prevent and correct deformity For support
  46. 46. Traction Systems - Stryker Frame
  47. 47. Balanced Skeletal Traction (Balkan Frame) • Weights • Traction Weight – 10% of body weight Indicated for fracture of hip and femur • Suspension Weight – 50% of Traction weight • E.g. a patient = 160 lbs – TW = 16 lbs – SW = 8 lbs
  48. 48. Russell Traction
  49. 49. Relationship of MUSCLES to FORCE and TENSION Muscles are attached - tendons – bones Tendons – rope-like function; Tension- tendency of a rope to be pulled apart due to forces applied at each end Force –shortening of muscles Pulling effect > contractions - > tension > pulling effect > force Contractions – shortens distance between points of attachments