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

82 TOPIC 9: WORK, ENERGY AND POWER 1. WORK  This is defined as the product of force and distance moved in the direction of force. Thus; Work = Force (F) x Distance (d) SI-Unit of work is Newton meter (Nm) or Joule (J) or Kgm2 /s2 . JOULE  This is the work in which a force applied in a body tends to move a certain distance. REQUIREMENTS OF WORK TO BE DONE There are two requirements for work to be done. These are: a) There must be a force acting on an object. b) The object must move parallel to the force. That is in the same or opposite direction as the force. Therefore:  The greater the force and the longer the distance moved, the more the work done. Example 1 A force of 80N pulls a box along a smooth and level ground through a distance of 5m. Calculate the work done by the force. Solution Data given Force (F) = 80N Distance (d) = 5m Work done (W) =? From; W = F x d W = 80N x 5m W = 400Nm or 400J ∴Work done by the force = 400J Example 2 How much work is done to lift a 7kg object a distance of 2m and then hold it at that height for 10 seconds? Solution Data given Mass (m) = 7kg Distance (d) = 2m Work done (W) =? First find force From; F = mg since g = 10N/kg F = 7kg x 10 N/kg F = 70N Then, Work done = F x d Work done = 70N x 2m Work done = 140J To hold the object for 10 sec F = 70N D = 0 sec NOTE: To hold means to remain at rest W = F x d W = 70 N x 0 m W = 0 ∴140 J of work was required to lift the object 2m, but no additional work was required to hold the object at that height for 10 sec. W = F x d
83 Questions 1. A man lifts a load of 20kg through a height of 3m. Calculate the work done. 2. If a man pushes a van against a force of 3000N for a distance of 0.1 m, how much work does he do? 3. When does an elephant do work? How is this work measured? 2.ENERGY  This is the ability to do work SI-Unit of energy is Joule (J). NOTE: Work is therefore done with the availability of energy. FORMS OF ENERGY a) Thermal energy ( heat, light and solar energy) b) Electrical energy c) Chemical energy d) Nuclear energy e) Electromagnetic energy f) Sound energy g) Elastic potential energy h) Mechanical energy MECHANICAL ENERGY  This is the energy which posses both kinetic and potential energy. TYPES OF MECHANICAL ENERGY (i) Kinetic energy (ii) Potential energy (i) KINETIC ENERGY (K.E)  This is the energy due to motion of the body. SI-Unit of Kinetic energy is Joule (J). Kinetic energy of an object depends on its mass and velocity. K.E is expressed as: K.E = 1 2 ⁄ MV2 When a body is in motion its kinetic energy change Hence, OR Where by Vi = Initial velocity Vf = Final velocity K.E = 1 2 ⁄ MVf 2 − 1 2 ⁄ MVi 2 K.E = 1 2 ⁄ M(Vf 2 − Vi 2 )

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