FORCE, WORK, ENERGY & POWER

Transcription

1 INAYA MEDICAL COLLEGE (IMC) PHYS LECTURE 5 FORCE, WORK, ENERGY & POWER DR. MOHAMMED MOSTAFA EMAM 1

2 What change the state of object is called force. We mean by saying state, shape or position of the object. 2

3 Newton's law of Motions (All objects save their state of motion) (Force causes acceleration) (Every actions cause reactions) 3

4 Newton's law of Motions (All objects save their state of motion) 1 st law of motion (Force causes acceleration) 2 ed law of motion (Every actions cause reactions) 3 rd law of motion 4

5 In summary, you observe and apply force every day. Force is a vector quantity having both magnitude and direction. We understand that cause of motion is force. The unit of force is Newton or kg.m/s². 5

6 You can kick the ball and apply a force then you give speed to the ball and changes its position or you can press the spring apply a force and give potential energy to it, we can increase the number of example. 6

7 Newton's First Laws of Motion (all objects save their state of motion) In his first law of motion Newton stated that all objects save their state of motion. In other words, if an object is at rest it continuous to be at rest and if it is moving it continuous to move unless a nonzero force is applied on it. What we mean by a word nonzero force? Look at the following picture to understand what we mean. 7

8 We called nonzero force net force also 8

9 Newton s Second Law of Motion 9

10 In the previous topics we said that force causes acceleration. Moreover, we also learned the net force concept in the last section. Now, we deal with the relation between force and acceleration. Newton s Second Law of Motion (Force causes acceleration) 10

11 As you remember, acceleration is the rate of change in the velocity of the object. This change occurs because of the net force. Thus, we can say that there is a linear relation between the net force acting on the object and acceleration. We show this relation like; 11

12 If we increase the amount of net force than the acceleration also increases in the same amount. If we decrease the net force than acceleration also decreases. Let s see it from the following pictures. 12

13 If we increase the amount of net force than the acceleration also increases in the same amount. If we decrease the net force than acceleration also decreases. Let s see it from the following pictures. 13

14 From this relation: 14

15 Example Find the acceleration (a) of the block given in the picture below. where, F is the force and its unit is Newton, m is mass and has the unit kg and a is the acceleration has unit m/s². 15

16 Example Find the acceleration (a) of the block given in the picture below. where, F is the force and its unit is Newton, m is mass in Kg, and has the unit kg and a is the acceleration has unit m/s². 16

17 Example Find the acceleration (a) of the block given in the picture below. where, F is the force and its unit is Newton, m is mass in Kg, and has the unit kg and a is the acceleration has unit m/s². 17

18 Example Given in the picture below, a horse is pulling the horsebox having 8 kg mass in it with a force of 40N; if the applied force has an angle of 37º to the horizontal; calculate the acceleration of the horsebox. 18

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23 Direction of the acceleration is in the direction of F x which is the net force in direction of motion. 23

24 Example Find the weight (W), of the object having mass 15 kg. 24

25 Example Find the weight of the object having mass 15 kg. 25

26 Equilibrium Conditions: Equilibrium in physics means, forces are in balance. The net force should be zero. In other words, forces acting downward and acting upward, and forces acting right and acting left should be equal in magnitude. Look at the illustration shape to understand that. 26

27 In the given picture there are five forces acting on our block. Let us analyze them, whether this block is in equilibrium or not. Forces acting downward are; 10N+5N=15N Force acting on upward is; 15N. Thus, we can say that, 27

28 In the given picture there are five forces acting on our block. Let us analyze them, whether this block is in equilibrium or not. Forces acting downward are; 10N+5N=15N Force acting on upward is; 15N. Thus, we can say that, our block is in equilibrium in +y, -y direction. 28

29 Now, look at the forces acting right and left. As you see, they are also equal in magnitude and opposite in direction. Thus, we can say that, 29

30 Now, look at the forces acting right and left. As you see, they are also equal in magnitude and opposite in direction. Thus, we can say that, our block is in equilibrium in +y, -y direction. our block is in equilibrium in +x, -x direction. 30

31 Example If the boy is in equilibrium, find the G from the given data in picture. 31

32 Example If the boy is in equilibrium, find the G from the given data in picture. As we see; T at each arm is = 30 N 32

33 Example If the boy is in equilibrium, find the G from the given data in picture. 33

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37 (sin 30º =0,5) 37

38 (sin 30º =0,5) 38

39 (sin 30º =0,5) 39

40 Newton s Third Law of Motion: (Every actions cause reactions / normal force) In this law Newton states that, when we apply a force on an object then it also apples force in same magnitude but opposite in direction. In general, all actions have reactions in the same magnitude but opposite direction. Suppose that, when you swim you push the water to the backward direction and water also pushes you to the forward direction.40

41 We can increase the examples; 1- When birds fly they push the air with their wings and the air also push the bird in the opposite direction, in this way birds can stay in the air and fly. In every where, we can see this couple of forces. Every actions cause reactions. 41

42 2- Force exerted by the rifle to the bullet gives acceleration to the bullet, moreover, the bullet also exert a force to the bullet which is the reaction of action force and as a result rifle is recoiled. 42

43 Look at the picture given above. 3- The book applies a force because of its weight, and the table shows a reaction to this action. These force pairs are same in magnitude but as you see their directions are opposite. In the next section we examine this example in detail. 43

44 Normal Force: We call this force as normal force which is same in magnitude and opposite in direction with the applied force (weight of the book). For different situations; in general, force is the reaction to the perpendicular force exerting on it. 44

45 Example Find the normal force that the inclined plane exerts on the box. (sin37º=0,6 cos37º=0,8), (m=4kg, g=10m/s²). 45

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51 CHECK YOUR UNDERSTANDING 1. Who was the scientist who gave us the Laws of Motion? 51

52 CHECK YOUR UNDERSTANDING 1. Who was the scientist who gave us the Laws of Motion? Answer: Sir Isaac Newton 52

53 CHECK YOUR UNDERSTANDING 2. How many Laws of Motion are there? 53

54 CHECK YOUR UNDERSTANDING 2. How many Laws of Motion are there? Answer: three 54

55 CHECK YOUR UNDERSTANDING 3. What is another name for the first law of motion? 55

56 CHECK YOUR UNDERSTANDING 3. What is another name for the first law of motion? Answer: Law of Inertia 56

57 CHECK YOUR UNDERSTANDING 4. Which law explains why we need to wear seat belts? 57

58 CHECK YOUR UNDERSTANDING 4. Which law explains why we need to wear seat belts? Answer: First Law of Motion 58

59 CHECK YOUR UNDERSTANDING 5. Which law says that force is equal to mass times acceleration (F=MA)? 59

60 CHECK YOUR UNDERSTANDING 5. Which law says that force is equal to mass times acceleration (F=MA)? Answer: Second Law of Motion 60

61 CHECK YOUR UNDERSTANDING 6. Which law says that heavier objects require more force than lighter objects to move or accelerate them? 61

62 CHECK YOUR UNDERSTANDING 6. Which law says that heavier objects require more force than lighter objects to move or accelerate them? Answer: Second Law of Motion 62

63 CHECK YOUR UNDERSTANDING 7. Which law explains how rockets are launched into space? 63

64 CHECK YOUR UNDERSTANDING 7. Which law explains how rockets are launched into space? Answer: Third Law of Motion 64

65 CHECK YOUR UNDERSTANDING 8. Which law says that for every action there is an equal and opposite reaction? 65

66 CHECK YOUR UNDERSTANDING 8. Which law says that for every action there is an equal and opposite reaction? Answer: Third Law of Motion 66

67 Work, Energy & Power 67

68 Work The work done by force is defined as the product of that force times the parallel distance over which it acts. W Fs cos The unit of work is the newton-meter, called a joule (J).

69 Energy The amount of energy transferred to the object is equal to the work done. Types of Energy Kinetic Energy = Motion Energy Potential Energy = Stored Energy

70 Kinetic Energy Kinetic Energy is the energy possessed by an object because it is in motion. KE 1 mv 2 2

71 Gravitational Potential Energy Gravitational Potential Energy is the energy possessed by an object because of a gravitational interaction. PE G mgh

72 Conservation of Energy Energy can neither be created nor destroyed, but only transformed from one kind to another. ( KE PE) W (KE PE inital ) final

73 Power Power is the time rate of doing work. AveragePower work done by a force time taken to do this work Force Speed

74 Power The unit of power is; a joule per second, called a Watt (W).