Chapter 3: Force and Force Equilibrium Chapter 3: Force, Work and Energy
Chapter 3: Force, Work and Energy 3.1 Mass and Weight 3.2 Newton's Law of Gravitation 3.3 Force and Newton's 3 Laws of Motion 3.4 Types of force 3.5 Work 3.6 Energy 3.7 Conservation of Energy 3.8 Power
Introduction According to previous chapter, we described about motion such as displacement, distance, velocity, speed, acceleration. In this chapter, we will discuss about force which is cause of motions. This study called Dynamics
Introduction Force Something that causes a object to move, changes its speed or direction, or distorts its shape. Force has its Magnitude and Direction. So it is Vector quantity.
Introduction Force can be divided into 2 types 1. Contact Force 2. Non-contact Force
3.1 Mass and Weight The quantities that define weight of object in physics are mass and weight. Definition of Mass Mass Quantity of inertia that resists motion. Ball and metal ball have the same size. Which object can move easier? Object that has lower mass can move easier than object that has greater mass.
3.1 Mass and Weight Normally, we can define that mass is quantity of matter composed in object. Therefore, mass can indicate only quantity of object Scalar quantity in kilogram unit. m m m Earth Moon Saturn Mass of object always has constant value anywhere in the universe, because mass depends on number of its atoms and molecules.
3.1 Mass and Weight Definition of Weight Weight Earth gravity reacts to object. The g value of the earth is 9.8 m/s 2 approximately at sea level. According to equation, unit of weight is kg.m/s 2 called Newton (N) Weight is vector quantity that defines quantity of gravitation force reacted to object. Direction of weight always directs to the center of the earth.
3.1 Mass and Weight Object weight is not always constant like mass. It depends on gravity that object is located. m m m Earth Moon Saturn g E = 9.8 m/s 2 g M = 1.62 m/s 2 g S = 11.2 m/s 2 For 1 kg of mass W E = 9.8 N W M = 1.62 N W S = 11.2 N
3.1 Mass and Weight Actually, g-value of earth is not constant. The g value at Cambridge is 9.80398 m/s 2 The g value at Lampang is 9.78591 m/s 2 The g-value around equator is lower than the g-value around the earth pole
3.1 Mass and Weight Ex A man has 490 N of his weight on the earth. What is his weight when he measure on the moon? (Answer: 81 N) Hint: gravity of the moon is 1.62 m/s 2
3.2 Newton's Law of Gravitation Newton's Law of Universal Gravitation An Object attracts every other object in the universe using a force that is directly proportional to the product of their masses but also inversely proportional to the square of the distance between them. M F G,M d F G,m m G-value obtained by experiment is 6.67 x 10-11 N.m 2 /kg 2 M and m are mass of each object (kg). d is displacement between objects.
3.2 Newton's Law of Gravitation Ex. A nurse standing at the earth surface has 45 kg of her mass. If the earth has 5.97x10 24 kg of its mass and 6,378 km of its radius, What is the gravity between a nurse and the earth? (Answer: 4.41x10 2 N)
3.2 Newton's Law of Gravitation Ex. Mr. A and Mr. B have the same mass which are 60 kg. the distance between them is 1 m. Find gravity between Mr. A and Mr. B. (Answer: 4.4x10-7 N)
3.2 Newton's Law of Gravitation Object falls above the earth surface m m d M
3.2 Newton's Law of Gravitation The earth radius at equator is 6378136 m The earth radius at earth pole is 6356751 m g-value of the earth is not constant. The g value at Cambridge is 9.80398 m/s 2 The g value at Lampang is 9.78591 m/s 2 If object is more far away from the earth surface, the gravity decreases continuously.
3.2 Newton's Law of Gravitation Gravity and Nursing Science Gravity reacts to body liquid such as blood. Varicose Veins Patient body sets after surgery.
3.3 Force and Newton's 3 Laws of Motion Force Any interaction that will change the motion of an object. In other words, a force can cause an object with mass to change its velocity. It can be described by intuitive concepts such as a push or a pull. It can make object changed their shape. Force has both magnitude and direction, making it a vector quantity.
3.3 Force and Newton's 3 Laws of Motion Newton's 3 Laws of Motion It describes the relation between force and motion. 1 st Law of Newton Object is still or moveing linearly with constant speed, when the resultant force equals to zero. Sometime, we call Law of inertia. Inertia A property of object to keep its motion condition. The more mass, The more inertia.
3.3 Force and Newton's 3 Laws of Motion Examples of Inertia
3.3 Force and Newton's 3 Laws of Motion 2 nd Law of Newton Object moves linearly with acceleration in the same direction of resultant force, when the resultant force is not equal to zero. Unit of force is kilogram x acceleration kg.m/s 2 N (Newton) Force and acceleration are vector quantity
3.3 Force and Newton's 3 Laws of Motion High acceleration Low acceleration Acceleration depends on mass.
3.3 Force and Newton's 3 Laws of Motion System Object or group of objects that we need to study. Internal Force Force occurs in system. External Force Force occur out of system.
3.3 Force and Newton's 3 Laws of Motion F Force F reacts to mass m 1 and m 2. Therefore, forces between m 1 and m 2 occur as shown in Figure. If we consider both m 1 and m 2 as system, F is external force. F 12 and F 21 are internal forces in this system. If we consider m 1 as system, F and F 21 are external force. F 12 is internal forces in this system. If we consider m 2 as system, F and F 12 are external force. F 21 is internal forces in this system.
3.3 Force and Newton's 3 Laws of Motion Find force that reacts to 0.5 gram object. The object move with 2 m/s 2 of acceleration. (Answer: 1 N)
3.3 Force and Newton's 3 Laws of Motion Ex. John pushes a 20 kg box. If he uses 5 N to push. Find the acceleration. (Answer: 0.25 N)
3.3 Force and Newton's 3 Laws of Motion Ex. Joe pulls 2 boxes shown by figure. If he pulls with 6 N, Find acceleration of each box. (No friction) (Answer: 0.25 N) system m system = 2 kg + 1 kg = 3 kg
3.3 Force and Newton's 3 Laws of Motion 3 rd Law of Newton A force is a push or a pull that acts upon an object as a results of its interaction with another object. Action force = Reaction force
3.4 Types of force Normal force Force reacts to object surface perpendicularly. Normally Sometime, N is not equal to W
3.4 Types of force Friction force Force reacts the object movement. It depends on normal force. It has coefficient of friction. The direction is opposite the object movement. Examples of friction
3.4 Types of force Type of Friction 1. Static friction 2. Kinetic friction Static Friction Object is still Kinetic Friction Object is Moving Static coefficient of friction Kinetic coefficient of friction
3.4 Types of force
3.4 Types of force Ex. A boy pushes metal box on wooden floor with 10 N. This metal box has 10 kg. Find friction of box and wooden floor. (µ s = 0.3 and µ k = 0.5)
3.4 Types of force Tension force Force occurs in rope, cable, string, etc. It occurs in the same line of rope. The direction is out of the system.
3.4 Types of force There are more about types of force Gravitational force Electromagnetic force Nuclear force
3.5 Work Work In physics, work relates to force and motion directly. Work = Force x Displacement in force direction. A. B. C.
3.5 Work How can we calculate work in this situation?
3.5 Work
3.5 Work Unit of work is N.m Joule, J. Work is scalar quantity (No direction), but It can be -, + or 0 depending on F direction. NEGATIVE ZERO POSITIVE F opposites to d. F perpendiculars to d. F has the same direction to d.
3.5 Work POSITIVE Work of woman pushes barbell. NEGATIVE Work of gravity reacts to barbell. Show the situation that work is zero.
3.5 Work Ex. If we push 1 N object moving for 1 m., Find work of pushing force. (Answer: 1 J) 1 N 1 m
3.6 Energy Energy Ability to do work Water energy uses to generate electricity. Spring energy can shoot a bullet. Bowling ball energy can impact pins.
3.6 Energy Relation between Work and Energy Work reacts to system Energy transfers to system System energy increase No work reacts to system Energy transfers out of the system System energy decrease We can say that work is energy transfer.
3.6 Energy Kinetic Energy (K.E.) Energy of motion Every moving object (v 0) always has kinetic energy. Kinetic energy can be calculated by Unit of K.E. is Joule, J.
3.6 Energy Ex. A 0.5 kg rock is thrown with 4 m/s of its speed. Find the kinetic energy of this rock. (Answer: 4 J)
3.6 Energy Potential Energy (P.E.) Energy depends on position and shape. P.E. depending on position. P.E. depending on shape. Gravitational potential energy Elastic potential energy Unit of P.E. is Joule, J.
3.6 Energy Gravitational potential energy We can choose referent point anywhere we want. P.E. of gravity Reference point P.E. of force ***P.E. grav depends on height from referent point
3.6 Energy P.E. grav is negative value **Consider potential energy of gravity Referent point At referent point P.E. grav = 0 J P.E. grav is positive value P.E. can be +, - or 0 depending on referent point.
3.6 Energy Ex. Hold a 0.4 kg book far from table with 50 cm. If the height of table is 50 cm, find P.E. when (Answer: A. 2 J A. Use table as referent point B. 5.2 J ) B. Use floor as referent point
3.6 Energy P.E. elastic depends on displacement from referent point. We can find elastic constant (k) from this equation.
3.6 Energy Ex. A spring balance can read 0 50 N. It can extend for 0.2 m when it read 50 N. If 3 kg mass is measured, find P.E. of this spring. (Answer: 1.8 J)
3.7 Conservation of Energy Conservation of Energy In physics, the law of conservation of energy states that the total energy of an isolated system remains constant. Energy can neither be created nor destroyed; rather, it transforms from one form to another. We can draw an equation of the conservation of energy using the following equation.
3.7 Conservation of Energy Consider when object falls. Referent point
3.7 Conservation of Energy How to calculate conservation of energy Initial Final
3.7 Conservation of Energy Ex. According to figure, 20 kg snow board starts sliding. Find the velocity of this board at the referent point, if the height of this mountain is 100 m. (Answer: 44.3 m/s) Referent point
3.8 Power Power is quantity of work in time unit or rate of energy using. Work Time Unit of power is joule/second (J/s) Watt (W) Some favorite unit of power is horse power 1 hp = 746 W
END OF CHAPTER 2 END OF CHAPTER 3 Force, Work and Energy