Tiêu đề TOPIC 9 WORK ENERGY AND POWER - WazaElimu.com.pdf ✅

84 Example What is K.E of a 12g bullet travelling at 320m/s? Solution Data given M = 12g = 0.12kg V = 320m/s K.E =? From K.E = 1 2 ⁄ MV2 K.E = 1 2 ⁄ x o.12kg x (320 m/s)2 K.E = 6144J NOTE:  If the speed of an object is doubled its K.E is multiplied by 4. Question An object has a mass of 5kg. What is its kinetic energy if its speed is (a) 5m/s? (b) 10m/s? (ii) POTENTIAL ENERGY (P.E)  This is the energy due to a position of the body. SI-Unit of potential energy is Joule (J). Potential energy is given by the following formula: Where by M = Mass of an object g = Gravitational constant h = Height of an object being raised Also P.E is given by the following formula if the object being raised from initial height to final height The above formula is also used to calculate Gravitational Potential energy (G.P.E) Example 1 A stone of 2kg falls from a height of 25m above the ground. Calculate the P.E possessed by the stone. Solution Data given M = 2kg h = 25m g = 9.8 m/s2 P.E =? From P.E = Mgh P.E = 2kg x 9.8m/s2 x 25m P.E = 490J Potential energy of the stone = 490J Questions 1. Determine the P.E possessed by a particle of mass 0.2kg resting on a table 1.6 above the floor (use g = 10m/s 2 ) 2. A stone of mass 10 kg is thrown down to the ground 10m down. If it hits the ground with a velocity of 20m/s, determine its: (a) Kinetic energy (b) Potential energy P.E = Mgh P.E = Mg(hf − hi )
85 TRANSFORMATION OF ENERGY Kinetic energy and Potential energy can be transformed from one form of energy to another. Energy is transformed from one form to another form. It is never lost or used up. Example: a) Water at the top of a dam has potential energy. This is transformed into K.E as the water falls. P.E falling water K.E b) The potential energy of a bent bow can be transformed into K.E of an arrow. P.E of bow K.E of arrow PRINCIPLE OF CONSERVATION OF ENERGY It states that: “Energy cannot be created or destroyed but can be transformed from one form to another”. OR “Energy can neither be created nor destroyed but can be transformed from one form to another”. TOTAL MECHANICAL ENERGY (E)  This is the sum of its potential energy and kinetic energy. Thus: Where by: E = Total mechanical energy P.E = Potential energy K.E = Kinetic energy USES OF MECHANICAL ENERGY a) Gravitational force keeps objects on the ground. It facilitates stability of motion. b) Kinetic energy helps objects to move around. c) Elastic energy is the energy responsible for motion. d) Transfer of energy is accompanied by performance of work. For example, a bow does work on an arrow. Questions 1. (a) What happens when force and distance are in the same direction? (b) What happens when force and distance are in opposite direction? 2. A stone of mass 10kg is thrown down to the ground 10m down. IF it hits the ground with a velocity of 20m/s, Determine its: (a) Kinetic energy (b) Potential energy change. E = P.E + K.E