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Balanced and unbalanced forces
Circular motion

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  1. 1. Dynamics 1 The Apple Tree
  2. 2. 2 Newton’s Apple Tree The descendant of the tree that grew in Sir Isaac Newton’s garden at Woolsthorpe Manor in Linconshire, England It is reputed to have inspired his Law of Universal Gravitation Dynamics
  3. 3. Forces Dynamics 3  A force is a push or pull acting upon an object as a result of its interaction with another object.  A force can….  move a stationary object  stop a moving object  change the shape / size of an object  change the direction / speed of an object  Force is a vector quantity .  The S.I. unit for force is kg m/s2 or newton, N
  4. 4. Type of Forces Dynamics 4 Upthrust – the upward force from a liquid (or gas) that makes some things float Weight – force with which the earth, moon, or other massively large object attracts another object towards itself
  5. 5. Dynamics 5 Tension – force which is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends Friction – force exerted by a surface as an object moves across it or makes an effort to move across it
  6. 6. Dynamics 6 Thrust – the forward force from an aircraft engine Air Resistance (Drag) – a special type of frictional force which acts upon objects as they travel through the air.
  7. 7. B A L A N C E D A N D U N B A L A N C E D F O R C E S F R I C T I O N C I R C U L A R M O T I O N Dynamics 7 Dynamics
  8. 8. S T A T E N E W T O N ’ S T H I R D L A W Dynamics 8 Balanced and Unbalance Force
  9. 9. Newton’s Third Law of Motion Dynamics 9  When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction — a force on the chair and a force on your body. These two forces are called action and reaction forces and are the subject of Newton's third law of motion.  Formally stated, Newton's third law is: "For every action, there is an equal and opposite reaction."
  10. 10. D E S C R I B E T H E E F F E C T O F B A L A N C E D A N D U N B A L A N C E D F O R C E S O N A B O D Y Dynamics 10 Balanced and Unbalance Force
  11. 11. D E S C R I B E T H E W A Y S I N W H I C H A F O R C E M A Y C H A N G E T H E M O T I O N O F A B O D Y Dynamics 11 Balanced and Unbalance Force
  12. 12. Balanced Forces Dynamics 12  When two forces acting on an object is balanced, the object can either be stationary or moving at constant speed.  Two forces are balanced when their magnitude is the same but they act in opposite directions.
  13. 13. Example Dynamics 13  A box is at rest on a table. Weight Reaction
  14. 14. Dynamics 14  An object that is suspended by a rope. Weight Tension
  15. 15. Dynamics 15  An airplane flying at a constant velocity. Upthrust Weight Thrust Drag
  16. 16. Dynamics 16  A trolley being pushed at a constant velocity Weight Reaction Pushing Force Friction
  17. 17. Unbalanced Forces Dynamics 17  When the forces acting on an object is unbalanced, the object’s speed will be change.  The resultant of two unbalanced forces can be found by considering their direction.  Example: When the driving force on a car is 100 N to the left but the friction force is 50 N to the right, then the resultant force is 50 N to the left
  18. 18. Dynamics 18  An airplane is slowing down at constant height. Up thrust Weight Thrust Drag Resultant Force
  19. 19. Dynamics 19  An airplane descending at a constant velocity. Up thrust Weight Thrust Drag Resultant Force
  20. 20. Example Dynamics 20  A man pulling a cart Reaction Friction Weight Pulling Force Resultant Force
  21. 21. Dynamics 21  F1 car decelerating Friction Thrust Normal WeightResultant Force
  22. 22. Dynamics 22  Jet Fighter taking off Up thrust Weight Thrust Drag Resultant Force
  23. 23. Dynamics 23  Sinking ship Up thrust Weight Resultant Force
  24. 24. R E C A L L A N D U S E T H E E Q U A T I O N F O R C E = M A S S × A C C E L E R A T I O N Dynamics 24 Balanced and Unbalance Force
  25. 25. Dynamics 25 Forces are Unbalanced (Resultant Force) There is an acceleration The acceleration depends directly upon the net force The acceleration depends inversely upon the mass of the object
  26. 26. Mass, Force and Acceleration Dynamics 26  A relationship between these factors is given by the formula force = mass × acceleration F = ma where F = force in newtons m = mass in kg a = acceleration in m/s2
  27. 27. Problem Solving Dynamics 27 1. Figure below shows the forces acting on three objects. For each, say whether the forces are balanced or unbalanced. If the forces are unbalanced, calculate the resultant force and give its direction. Unbalance force Resultant force = 80 – 60 = 20 N Direction = to the right
  28. 28. Dynamics 28 Balance force Resultant force = 0 N
  29. 29. Dynamics 29 Unbalance force Resultant force = 320 – 270 = 50 N Direction = downward
  30. 30. Dynamics 30 2. What is the acceleration of the model car below.
  31. 31. Dynamics 31 1. Find the resultant force 2. Use the equation Resultant force = 18 N − 10 N = 8 N F = m × a 8 N = 2 kg × a a = 4 m/𝑠2
  32. 32. Dynamics 32 3. Figure a. What is the resultant force on the car above? b. What is the car’s acceleration? c. If the total frictional force rises to 1500 N, what happens to the car?
  33. 33. Dynamics 33 a) Find the resultant force b) Find the acceleration c) No resultant force. Car will move at constant velocity Resultant force = 1500 N − 500 N = 1000 N F = m × a 1000 N = 800 kg × a a = 1.25 m/𝑠2
  34. 34. Dynamics 34 4. A block of mass 20 kg is pulled along the ground by a force, F of 60 N. The frictional force is 10 N. Calculate the acceleration of the block. Resultant force = 60 N − 10 N = 50 N F = m × a 50 N = 20 kg × a a = 2.5 m/𝑠2
  35. 35. Dynamics 35 5. A car has a mass of 800 kg. its engine provides a forward force of 400 N. There is a frictional force of 160 N, acting to oppose the car’s motion. What is the resultant force acting on the car? What is its acceleration? Resultant force = 400 N − 160 N = 240 N F = m × a 240 N = 800 kg × a a = 0.3 m/𝑠2
  36. 36. Dynamics 36 6. What force is needed to give a car of mass 600 kg an acceleration of 2.5 m/s2 F = m × a F = 600 kg × 2.5 m/𝑠2 F = 1500 N
  37. 37. Dynamics 37 7. What acceleration will result when a 12 N resultant force is applied to a 3 kg object. F = m × a 12 = 3 kg × a a = 4 m/𝑠2
  38. 38. Dynamics 38 8. A resultant force of 16 N causes a mass to accelerate at the rate of 5 m/s2. Determine the mass. F = m × a 16 = m × 5 m/𝑠2 m = 3.2 kg
  39. 39. Dynamics 39 9. A car of mass 1000 kg travelling at 10 ms-1 is brought to rest in 5 seconds. Find a) the average deceleration, b) the braking force. a = v − u t a = 0 − 10 5 = −2 m/s2 F = m × a F = 1000 kg × 2 m/s2 F = 2000 N
  40. 40. Dynamics 40 10. When a force of 6 N is applied to a block of mass 2 kg. It moves along a table at constant velocity. a) What is the force of friction? 11. When the force is increased to 10 N, what is b) the resultant force? c) the acceleration? Force of Friction = 6 N Resultant force = 10 N − 6 N = 4 N F = m × a 4 N = 2 kg × a a = 2 m/𝑠2
  41. 41. Dynamics 41 d) the velocity, if it accelerates from rest for 10 seconds? a = v − u t 2 = v − 0 10 v = 20 m/s
  42. 42. Dynamics 42 Forces Balanced Unbalanced Body remain stationary Body moves with constant velocity Object velocity changes Direction of moving object changes
  43. 43. Dynamics 43 1. A newton is a unit of force. Which quantity is measured in newtons? A. acceleration B. density C. mass D. weight
  44. 44. Dynamics 44 2. A spring is stretched by hanging a piece of metal from it. 3. What is the name given to the force that stretches the spring? A. friction B. mass C. pressure D. weight
  45. 45. Dynamics 45 3. The diagram shows a bird in flight. 4. In which direction does the weight of the bird act? D
  46. 46. Dynamics 46 4. Which property of an object cannot be changed by a force? A. its mass B. its motion C. its shape D. its size
  47. 47. Dynamics 47 5. A force acts on a moving rubber ball. 6. How many of the following changes could happen to the ball because of the force?  a change in direction  a change in shape  a change in mass  a change in speed A. 1 B. 2 C. 3 D. 4
  48. 48. Dynamics 48 6. Which is a statement of Newton’s third law of motion? A. Every force causes a reaction. B. If there is no resultant force on a body then there is no acceleration. C. The forces acting on a body are always equal and opposite. D. To every action there is an equal but opposite reaction.
  49. 49. Dynamics 49 7. Below are four statements about the effects of forces on objects. Three of the statements are correct. Which statement is incorrect? A. A force can change the length of an object. B. A force can change the mass of an object. C. A force can change the shape of an object. D. A force can change the speed of an object.
  50. 50. Dynamics 50 8. In which of these situations is no resultant force needed? A. a car changing direction B. a car moving in a straight line at a steady speed C. a car slowing down D. a car speeding up
  51. 51. Dynamics 51 9. Two forces act on an object. In which situation is it impossible for the object to be in equilibrium? A. The two forces act in the same direction. B. The two forces act through the same point. C. The two forces are of the same type. D. The two forces are the same size.
  52. 52. Dynamics 52 10. Which statement about a moving object is correct? A. When an object is accelerating, the resultant force acting on it must equal zero. B. When an object is moving at a steady speed, the air resistance acting on it must equal zero. C. When an object is moving at a steady speed, the resultant force acting on it must equal zero. D. When an object is moving, there must be a resultant force acting on it.
  53. 53. Dynamics 53 11. A person just supports a mass of 20 kg suspended from a rope. What is the resultant force acting on the mass? A. 0 N B. 10 N C. 20 N D. 200 N
  54. 54. Dynamics 54 12. The propeller on a boat pushes water backwards with a force of 2000 N. The boat moves through the water against a total resistive force of 1800 N. 1. According to Newton’s third law, what is the forward force on the propeller due to the water? B
  55. 55. Dynamics 55 13. An aeroplane is in equilibrium. 14. The diagram shows the forces acting on the aeroplane.
  56. 56. Dynamics 56 1. Which statement about the forces is correct? A
  57. 57. Dynamics 57 14. A wooden plank rests in equilibrium on two boulders on opposite sides of a narrow stream. Three forces of size P, Q and R act on the plank.
  58. 58. Dynamics 58 1. How are the sizes of the forces related? A. P + Q = R B. P + R = Q C. P = Q = R D. P = Q + R
  59. 59. Dynamics 59 15. A girl and a boy are pulling in opposite directions on a rope. The forces acting on the rope are shown in the diagram.
  60. 60. Dynamics 60 1. Which single force has the same effect as the two forces shown? A. 50 N acting towards the girl B. 350 N acting towards the girl C. 50 N acting towards the boy D. 350 N acting towards the boy
  61. 61. Dynamics 61 16. An aircraft, flying at a constant height, is gaining speed. 17. The four forces acting are
  62. 62. Dynamics 62 1. What is correct? C
  63. 63. Dynamics 63 17. A tractor pulls a trailer at a constant speed. 18. The tractor exerts a forward force of 1600 N on the trailer. 19. What is the force exerted by the trailer on the tractor? A. 0 N B. 1600 N backwards C. 1600 N forwards D. 3200 N forwards
  64. 64. Dynamics 64 18. When a block of wood of mass 2 kg is pushed along the horizontal flat surface of a bench, the friction force measured is 4 N. 19. When the block is pushed along the same bench with a force of 10 N, it moves with a constant A. speed of 3 m/s. B. speed of 5 m/s. C. acceleration of 3 m/s2. D. acceleration of 5 m/s2.
  65. 65. Dynamics 65 19. A force of 20 N pushes an object of mass 5.0 kg along a rough horizontal surface where the frictional force is 5.0 N. 20. What is the acceleration of the object? A. 1.0 m/s2 B. 2.0 m/s2 C. 3.0 m/s2 D. 4.0 m/s2
  66. 66. Dynamics 66 20. How is the motion of a body affected by balanced and unbalanced forces acting on it? C
  67. 67. E X P L A I N T H A T F R I C T I O N I S A F O R C E T H A T I M P E D E S M O T I O N A N D P R O D U C E S H E A T I N G Dynamics 67 Friction
  68. 68. Friction Dynamics 68  Friction is a force that opposes motion between two surfaces that are in contact. Motion
  69. 69. Dynamics 69  The frictional force between two surfaces on a horizontal plane  Always act against the direction  Depends on the material on contact.  Depends on the nature of the surfaces in contact.  Increases as the speed of the object increases.
  70. 70. Dynamics 70 Advantage of Friction Walking Writing Sitting To stop
  71. 71. Dynamics 71 Disadvantage of Friction Make movement difficult Heats part Wear things out Waste energy
  72. 72. Dynamics 72 Reducing Friction Smoothing surface Lubricating Use rotating object Streamlining
  73. 73. D I S C U S S T H E E F F E C T O F F R I C T I O N O N T H E M O T I O N O F A V E H I C L E I N T H E C O N T E X T O F T Y R E S U R F A C E , R O A D C O N D I T I O N S ( I N C L U D I N G S K I D D I N G ) , B R A K I N G F O R C E , B R A K I N G D I S T A N C E , T H I N K I N G D I S T A N C E A N D S T O P P I N G D I S T A N C E Dynamics 73 Friction
  74. 74. 74 A car driver sees a family of ducks crossing the road in front of her. She brakes for 1·5 s and took 1·8 s to stop. Dynamics
  75. 75. Stopping Distance Dynamics 75  Stopping Distance = Thinking Time + Braking Distance  Thinking Distance  Whilst you are reacting to the hazard, the car is still moving! During your thinking time, you are not slowing down. We call the distance moved during this time the thinking distance.  Braking Distance  With the brakes applied, the car slows down. The distance that the car moves whilst braking is called the braking distance.
  76. 76. Dynamics 76  Stopping is made up of two parts: thinking and braking.  Thinking distance is the distance travelled during the thinking time.  Braking distance is the distance travelled during the braking time.  Stopping distance is the sum of the thinking and braking distances.  When speed doubles, thinking distances doubles and braking distance is four time as far
  77. 77. 77 Braking Factors - Brakes Worn brakes won't work as well, so you'll need to brake for longer. Modern brakes are also better than old ones - they can apply bigger forces without causing skidding. Dynamics
  78. 78. 78 Braking Factors - Tyres Tread patterns are designed to push water out from between the tyre and road. Good tyres can reduce braking distance by many metres! Worn tyres (with little tread) will have good grip in the dry but in the wet will lead to much longer braking distances... Dynamics
  79. 79. 79 Braking Factors – Road Surface Different types of surface provide different levels of grip, especially in the wet. If the road is wet, braking distance will always be longer. Oil spills on the road, gravel, etc. all reduce grip and increase braking distances. Dynamics
  80. 80. 80 Braking Factors - aerodynamics The worse the car's aerodynamics, the better it will be at slowing down during braking! The reason is that the airflow at and around the car (drag or air resistance) is an additional force acting to slow you. Dynamics
  81. 81. 81 Braking Factors - Mass The larger the total mass of the vehicle, passengers and luggage, the more kinetic energy it will have at a given speed. This increases the braking distance as it is harder to slow down. Dynamics
  82. 82. Dynamics 82 1. When a body moves across a rough surface, a frictional force is produced.
  83. 83. Dynamics 83 1. Which statement about this force is always true? A. It acts in the direction of the motion. B. It is equal in value to the force producing the motion. C. It makes the body recoil in the opposite direction after stopping it. D. It opposes the motion across the surface.
  84. 84. Dynamics 84 2. A wooden block is pushed across a table at constant speed. 1. Which statement is correct? A. The frictional force increases as the block moves at constant speed. B. The frictional force is equal and opposite to the pushing force. C. The frictional force is greater than the pushing force. D. The frictional force is less than the pushing force.
  85. 85. Dynamics 85 3. The wheel of a moving car is driven by the engine. The car is accelerating in the direction shown. 4. In which direction does the frictional force act on the wheel? B
  86. 86. Dynamics 86 4. Three horizontal forces act on a car that is moving along a straight, level road.
  87. 87. Dynamics 87 1. Which combination of forces would result in the car moving at constant speed? C
  88. 88. Dynamics 88 5. A train is travelling along a horizontal track at constant speed. Two of the forces acting on the train are shown in the diagram.
  89. 89. Dynamics 89 1. A force of air resistance is also acting on the train to give it a resultant force of zero. 2. What is this air resistance force? A. 40 000 N backwards B. 80 000 N backwards C. 40 000 N forwards D. 80 000 N forwards
  90. 90. Dynamics 90 6. A car is travelling at constant speed along a road and drives over a large patch of oil. The driver applies the brakes to stop the car. Compared to braking on a dry road, what may happen? A. The car slows down more quickly because of the greater friction between the tyres and the road. B. The car speeds up at first because of the reduced friction between the tyres and the road. C. The car takes longer to slow down because of the reduced friction between the tyres and the road. D. The car takes longer to slow down because the thinking distance of the driver is greater.
  91. 91. Dynamics 91 7. A car travels along a road. The driver stops the car by pushing his foot down on the brake pedal. 8. What does not change if he pushes harder on the brake pedal? A. the braking distance B. the braking force C. the stopping distance D. the thinking distance
  92. 92. D E S C R I B E Q U A L I T A T I V E L Y M O T I O N I N A C I R C U L A R P A T H D U E T O A C O N S T A N T P E R P E N D I C U L A R F O R C E , I N C L U D I N G E L E C T R O S T A T I C F O R C E S O N A N E L E C T R O N I N A N A T O M A N D G R A V I T A T I O N A L F O R C E S O N A S A T E L L I T E Dynamics 92 Circular Motion
  93. 93. Changing Velocity Dynamics 93  Velocity is speed in a particular direction.  A change in velocity can mean  change in speed  change in direction
  94. 94. Centripetal Force Dynamics 94  When an object is moving in a circle, there must be a force acting on it to change its direction.  This force, which always act towards the centre of the circle, is centripetal force. It acts perpendicularly to the direction of motion of the object at any instant
  95. 95. Dynamics 95
  96. 96. Example 1 Dynamics 96  A stone on the end of a string, being whirled in a horizontal circle
  97. 97. Example 2 Dynamics 97  The Moon, orbiting the Earth
  98. 98. Example 3 Dynamics 98  A car turning a corner
  99. 99. Example 4 Dynamics 99  Roller coaster loop
  100. 100. Example 5 Dynamics 100  A banking aircraft uses the horizontal component of the lift force to provide the centripetal force for turning.
  101. 101. Satellites around the Earth Dynamics 101  A satellite travels round the Earth in a curved path called orbit.  Gravitational pull (satellite’s weight) provides the centripetal force needed.
  102. 102. Electrons around the nucleus Dynamics 102  In atom, the electrons are in orbit around a positively charged nucleus.  The attraction between the opposite charges (electrostatic force) provides the centripetal force
  103. 103. Geocentric Model An obsolete concept which held that the Earth was the centre of the universe and everything revolved around the Earth. 105 Dynamics
  104. 104. Dynamics 107  The planetary motion is a result of the gravitational attraction of the Sun at the centre of the Solar System. As the planets are trying to fly out into deep space, the gravity of the Sun is pulling them into a curved orbit.
  105. 105. Dynamics 108 1. A body is moving in a circle at a constant speed. Which of the following statements about the body is true? A. There is no acceleration. B. There is a force acting at a tangent to the circle. C. There is a force acting away from the centre of the circle. D. There is a force acting towards the centre of the circle.
  106. 106. Dynamics 109 2. A particle P is moving in a horizontal circle about O. It moves at constant speed V.
  107. 107. Dynamics 110 1. Which statement is true? A. A force of constant size is acting in the direction of V. B. A force of constant size is acting towards O. C. The force on P varies in size as it moves around the circle. D. There are no forces acting on P.
  108. 108. Dynamics 111 3. The diagram shows a cyclist leaning over in order to cycle around a corner. Which force is necessary to maintain the motion around the corner?
  109. 109. Dynamics 112 A
  110. 110. Dynamics 113 4. The diagram shows an aeroplane turning in a horizontal circle at constant speed. 5. In which direction is there a resultant force? D
  111. 111. Dynamics 114 5. A body P moves in a circle around a point S. A force F keeps it moving in the circle. What happens if the force F suddenly disappears? A. P moves directly towards S. B. P moves in a circle closer to S. C. P moves away from S in a curved path. D. P goes off in a straight line.
  112. 112. Dynamics 115 6. A car moves in a circle at a constant speed. 7. What is the direction of the resultant force acting on the car? B
  113. 113. Dynamics 116 7. The diagram represents the Moon in its orbit around the Earth. 8. Which arrow represents the direction of the resultant force acting on the Moon at the instant shown? A
  114. 114. Dynamics 117 8. What keeps an electron moving in a circle around the nucleus of an atom? A. a gravitational force away from the nucleus B. a gravitational force towards the nucleus C. an electrostatic force away from the nucleus D. an electrostatic force towards the nucleus
  115. 115. Dynamics 118 9. A turntable rotates at constant speed. A coin is placed on the turntable at P. The friction force between the coin and the turntable keeps the coin in the same position on the turntable.
  116. 116. Dynamics 119 1. In which direction does the friction force act? A