Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
8TH CLASSES PHYSICS DAILY PLAN SUBJECT: Work-Energy-Power GOALS: DURATION: IN PRACTICE: PRESENTATION: Work: The work is done if an object undergoes a displacement under the effect of a force. The magnitude of the work is the product of the displacement and the force. F Energy due to motion. If something is in motion it has a kinetic energy. It depends on the mass and velocity of the object. V K.E.= 1/2 . m . v2 Example : Find the kinetic energy of a 400-kg car moving with a speed of 10 m/s. Solution : K.E. = 1/2 . m . v2 = 1/2 . 400 . 100 = 20000j Work-Energy Theorem : Work is change I n kinetic energy. If work is done on an object its the amount of work. W = K.E. X W=F.X Unit of Work : The unit of work is joule. Newton x Meter = Joule ( J ) Example : A 20-N force pulls a wooden block for 6 m. What is the work done by the force? Solution : F Potential Energy : Energy due to the position Gravitational Potential Energy If an object has a distance (or a height) from the earth, it has s a potential energy. The amount of energy is determined by the mass of the object, its height and the gravitational constant. m P=mgh h X= 6 m W = F . X = 20 . 6 = 120 J Power: Power is the work done in a unit time. Work Power = Time P.E. = m . g . h Conservation of mechanical energy When an object falls under only the influence of gravity: (no friction): h is decreasing, v is increasing v 1 The unit of power is watt. OR h PE is decreasing, KE is increasing 1 Joule v = watt . second Example : A child pulls an object for 15 meters with a force of 20 N in 4 seconds. Find the work done by the child and his power. Solution : W = F . X = 20 . 15 = 300 J. W 300 P= = = 75 Watt. t 4 Energy : Energy is the ability to do work. So its unit same with work. There are many kinds of energy such as : 1. Mechanical Energy 2. Electrical Energy 3. Chemical Energy 4. Nuclear Energy 5. . . . . . . . . . . . . . .. In this chapter we will only study the mechanical energy There are two types of mechanical energy. Kinetic energy and potential energy. Kinetic Energy:: h2 2 Suppose the object falls from A to B. A g Since the object is falling down, potential energy decreases by (mgy). y h Since gravitational force does work kinetic energy increases by (mgy). 1 B Therefore: h 2 Increase in KE = Decrease in PE KE= - PE KE + PE = 0 (KEf – KEi) + (PEf – PEi) = 0 PEi + KEi = PEf + KEf. If external work on an object is zero, total mechanical energy of the object is conserved (=does not change): MEi=MEf . Ex: PE = 100J KE = 0 ME = 100 J PE = 75J KE = 25J ME = 100 J PE = 50J KE = 50J ME = 100 J PE = 0 KE = 100J ME = 100 J {*The following 6 examples are to be solved by using PEi + KEi = PEf + KEf method*} Ex: An object of mass m is released from 5 m above the ground. Find its velocity just before it touches the ground. {*Draw the picture*} HOMEWORK: MULTIMEDIA: serway1,akadem,serway11 DEMONSTRATION: EXPERIMENT: TEACHER: DIRECTOR: