Energy - Physics Chapter 4 PDF

C H 4 A Energy E=mc' THEME 4: Energy...

C H 4 A Energy E=mc' THEME 4: Energy example, a bottle Machines help us to do work. For opener is a machine. i The abilityAtoneedle, a doorknob do work is calledare. also machines. Some machines are more complex than others. Energy..A.simple machine changes the direction or the magnitude of the force applied. The six simple machines are the! screw. The factor by which a machine multink lever, pulley, wheel-and-axle, inclined plane, wedge and location of fulcrum (the pivot point), th.! the force applied is called 'mechanical advantage'. On the basis of or orders. The aim of this theme is to enahi load and the effort, levers may be classified into three types chldren to know and understand about different types of machines and levers. Syllabus Simple Machines: " Basic Concept Mechanical Advantage Types of Simple Machines: " Lever " Wheel and Axle " Pulley "Inclined Plane "Wedge " Screw " Diferent Orders of Levers " Numericals based on mechanical advantage or leverage Load x Load arm = Effort x Effort arm We need to do some kind of work every day. For example, washing clothes, school or office, all are kinds of work. Whatever work we do, energy is needed.cooking food, going to dothe work. However, we can make our work little easier by Energy is required to using some tools. By doing this, we can also reduce our energy expenditure, time and efforts. Let us learn that make our work convenient in this about work, energy, and the tools chapter. WORK Work is usually a physical or mental activity For playing football is a physical work. The household example, reading a book is a mental work whik activities also belong to work. In science, howvel, sitting at a place and reading a book is nowork. Similarly, no work is done while standing at a pla with a heavy load on the head. Work is said to be done when an object moves as a is done only when a force is applied on result of force acting on it. In other words, a k an object. Due to this, the object the direction of the applied force. moves a certain dist 60  Examples of Work Being Done Some examples of work being done are given below: "Pushing acar at rest. Walking or running on the road. Carrying aschool bag or heavy loa Acoolie carrying baggage at the railway station, Climbing on stairs. Bringing things from markct, Fig. 4.1: Some examples where work is being done Examples of No Work Done In some cases, we apply efforts but work is not being done.. Pushing something that does not move like a heavy box or a wall. Lifting a bag and holding it in same position. You sit on a chair for hours. Reading a book. Fig. 4.2:Some examples where work is not being done 61  ENERGY Energy is the ability to do work. It takes energy to cook food, to drive to school and to run on the playground. Theone transfornm from type towind., blowing the warm another, Sun and a falling leaf are examples of energy. Energy but it can never be destroyed or created. Some examples where energy is used are given below: Doing household work, such as sweeping, mopping floor, etc. Playing games in the playground. Morning walk and exercising. Going to school, offices and market. Labourers carrying load. Rickshaw puller pulling rickshaw. A Playing games Mopping floor Labourers carrying load Fig. 4.3: Activities where energy is required MACHINES Machine is an object or tool which helps to make work-easier With the help of machines, a small force can be used to overcome a large force. For example, a screw jack is used to lift an object as heavy as acar to change its tyres. Some common uses of machines are given below. FAST FACT Lift heavy loads with a small effort. Simple machines were first discovered Carry out unsafe and dangerous tasks. and described by a Greek philosopher Increase the speed of a moving object. Archimedes. Reduce the risk in performing hazardous tasks. Principle of a Machine Amachine produces force and controls the drection and motion of force, but it cannot create energy. It canonly transmit mechanical work from one part of a device to the other. When the effort is smaller than load. it has to move a greater distance in order to achieve the same work. The machine work on the principle that the work output of a machine is equal to the work input Theability of machine to do work can be measured by two factors. These are following: Mechanical advantage Efficiency 62 Mechanical Advantage The mechanical advantage(M.A.) of a machine tell us how well a machine works. Itisthe factor by which a machine multiplies lorce. The mechanical advantage gives the ratio of the force exerted by the machinetothe force that is applied to the machine, The more is the mechanical advantaye, lesser isthe amount offeffort to be applied. The machine is aforce multiplier, if its mechanical advantaye is greaterthan 1. The mechanical advantage formula is given by Load (L) M.A. Efort(E) M.A. Mechanical advantage L Load E = Etfort Efficiency The ratio between the work done by the machine to the work done on the machine is called the efficiency of amachine. Friction is produced in working machines which can be reduced by oiling its sliding or rotating parts. For example, a lever is highly efficient due to its low internal resistance. This is because the work done and effort applied is almost equal. Simple machines always haveefficiencies less than 1.0due to internal friction. Ideal Machine Anideal machine is one in which there is no loss of energy in any manner, such as through friction, 100% wear or otherinefficiencies. The work output is equal to the work input. In other words, there is efficiency. The mechanical eficiency of a simple machine is calculated by dividing the actual power output by the ideal power output. This is usually expressed as a percentage. SIMPLE MACHINES better. Complex machines Simple Machines are basic mechanical tools that help us to work faster and different simple machines every are the combination of one or more simple machines. We use many and staplers. All these tools are simple day. Some common examples are scissors, screwdrivers machines. The simple machines help us in following ways: convenient to pedal a bicycle instead of applying By Applying Force at Convenient Position: It is force directly to the bicycle. Convenient Direction: Drawing a bucket full of water from the well is easy by Applying Force at using a pulley. Objects: Ascrew jack can be used to lift a car, truck or other Dy Applying Less Efort to Lift Heavy too heavy objects. change the speed of the automobile. 10Change the Speed of a Body: Gears are used to 63 Important Terms: Some important lernms relatcd to simple machincs are given bclow: M.A.: The ratio of the force exerted by the machine to the force applicd to it is called advantage or M.A. It is also calledas leverage. mechanic Load: Aresistive force that is to Overcome by a machincis called load. Its S.I. unit is N Effort: An external force applicd to a simple nachinc to overcome a load is called an Newton (N). unit is Newton (N). effort. Its SJ. Types of Simple Machines There are six basictypcs of simple machincs, These are levcr, pulley, wheel and axle, inclined plan. wedge and screw. Lever Along rod and a fulcrum constitute a lever. It can be used tocut things, lift heavyobjects and open tins. A hammer, a bottle opener, a nut cracker, a pair of scissors and see-saw are some examples of lever that we use in our daily life. Levers use a bar to transfer an effort (applied force) through a fulcrum (focused point) to a load or resistance.The lever changes the magnitude and direction of the force applied tomove an object. It minimises the effort required to lift the object. Parts ofa Lever:Alever has four important parts. Efort Beam: A wooden plank or metal bar resting on the fulcrum. Fulcrum: The pivot or the turning point. Beam Load Force/Effort: The effort or input needed to move the beam and Fulcrum Load: The item or object being moved or lifted on the plank. Fig, 4.4: Parts of a lever Working of a Lever: If one end of the lever is pushed down, the force will lift other end has a load on top of it, it will move or lift easily. the other end. If the The lever makes the work easier. For example, if one child wantsto lift his friend using his can be done by sitting on a see-saw. On a hands, he will not be able to do this. But, it see-saw, a child (the load) sits on one end and the easily pushes down the other end of the see-saw to lift that other child two people use force to move each other up and child. This is how a see-saw works, where down. In this example, since the lever amplifies the applied force, the load gets balanced by the effort applied about the advantage of lever according to this can be calculated by the following fulcrum. Hence. the mechanical ways: Load x Load arm = Effort x Efort arm Load Load arm Thus, Mechanical advantage Effort Effort arm Types of Lever: Levers are classified into three types on Load and effort. the basis of the position of fulcrum, loa0 Class I Lever: In class I lever, the fulcrum is placed l as efort-fulcrum-load. This is the most basic type ofbetween effort and load. Its order is represented the lever. Aforce is applied (by pulling or pushing) 64 n the section of the bar, which causes the lever to swing about the fulcrum, overcoming the resistance force on the oppOsite side. Some common examples of class I lever are scissors, crow bars, can opener, bicycle hand brakes, see-saw and pliers. Load Efort Effort Fulcrum Fulcrum Load Effort Load Load Arm Effort Arm ‘Efort Fulcrum Class I lever Apair of pliers Scissors Fig. 4.5: Example of class I lever Class IILever: In Class II lever, the load lies between the effort and the fulcrum. In this type of lever, the movement of the load is in the same direction as that of the effort. The examples of class II lever are wheel barrow, nail clippers, bottle opener, paper cutter, bottle opener and nut cracker. Load Load Load Effort Fulcrum Load Arm Efort Effort Arm fulcrum Effort Fulcrumn Class II lever Wheel barrow Bottle opener Fig. 4.6: Example of Il class lever Class III Lever: A Class IIIlever has the effort between the load and the fulcrum. In this type of lever, length both the effort and load are in the same direction. The length of load arm is greater than the fulcrum, often a bearing or of the effort arm. Because of the location of the effort with respect to the effort will pull the arm off other device is needed to hold the beam in place as it pivot. Otherwise, the forceps, sugar tongs, the fulcrum. Some examples of class IIIlever are a pair of tongs, broom, knife hammer. fishing rod, and forearm of a person holding a load, spade for lifting sand, stapler, Effort Fulcrum Load -Effort Effort Fulcrumn LoadArm EffortArm Load Load Fulcrum Broom Stapler Class III lever Fig. 4.7: Example of class III lever 65  Activity To know the1position of load, effort andfulcrum in different kinds of levers that we use as simple Bring diferent machines in ourkinds daily oflife. levers in the class and sit in groupS. Identify the position of load, effort and fulcrum in each of these. Pulley A pulley is a device made up of a grooved wheel anda rope. The wheel can be of any s1Ze, and the length of the rope can vary as well. It is used to lift loads. In villages, people draw water out of the wells using pulleys. The machines such as cranes and sailboats also make use of pulleys. Some and elevators examples where pulleys are used are flagpole, lifting water from the well The two commonly used types of pulleys are fixed pulley and the movable pulley. Pulley FAST FACT Abicycle makes use of nearly every kind of simple machine in order tomake amore complex machine. Fig. 4.8: Pulley is used to draw water from well Fixed Pulley: A type of pulley which consists of a grooved wheel to a fixed and made of wood or metal with a rope attached passing through it is called fixed pulley. The pulley rotates about an axle passing through its load is tied to one end of the rope and the effort is centre. The -Puller applied at the other end. The fixed pulley makes our work easy by lifting up a load using a downward effort. It means our work becomes easy by changing the direction of Examples of fixed pulley are hoisting a flag and drawing water from theforce. well., Movable Pulley: A movable pulley has a block of two pulleys. In a block and tackle Pulley arrangement, the pulleys are assembled together to form Fig.4.9: Fixed then blocks are paired so that one is blocks and fixed pulley other moves with the load. The rope is and the threaded through the pulleys provide mechanical advantage that amplifies W The examples of movable pulleys theeffort applied to the rop. include Fig. 4.10: Movable pulley elevators, and some types of weight lifting construction cranes, moder machines at the gym. 66  ASSESS YOURSELE Answer the following questions.. Name the class of lever by the given picture. displayed COass IL Pee Give any two Load common examples of it. 2. Name any two devices using movable pulleys. C9hanes, 3. What is asimple machine? 4. What is theformula to In key calculate the w Force nfechanica advantage of lever? Wheel and Axle )Lo Wheel and Axle 1s an arrangement consisting of a whecl fixcd to a rod (axle), As the axle turns, the wheel attached to it also moves, It uses a Wheel uheel with arod attached in the middle callcd an axlc to hclp it to lift or mOVe loads. Such a combination helps objects to move and change their Axle direction. Some examples of wheel and axle arrangements are bicycle nedal, screw driver, door knob, steering wheel of a car and water tap. A wheel and axle can work together to help move things casily in two ways. Fig. 4.1 1: Wheel and Axle 1. When Force is Applied to theWheel: Ascrewdriver is an example of a wheel and axle. The handle of a screw driver is the wheel where Axle force is applied. It turns and increases the force of the axle which Wheel helps turn the screw. Screwdriver Rotation 2. When Force is Applied to the Axle: A Ferris Wheel is also an example of wheel and axle. When force is applied to the axle, it turns Axle the giant wheel. The wheel is much larger than the axle and covers more distance. Aceiling fan and gears work in the same way. Inclined Plane Door opener Wheel Fig. 4.12: Some examples An inclined plane is any sloping but flat surface along which a of wheel and axle load can be pushed or pulled with less effort. It is used to move heavy objects. When moving an object up a plane, it requires a Consistent force to keep it moving. This force, however, is much lower as compared to the force required to lift the same object. the Inclined plane Wnen moving an object down an inclined plane, like a slide, O0ject willcome down gradually than if simply dropped. This becomes a much safer way to nmove down heavy objects. Load pushed easily, Ahospital ramp on which a wheel chair can be a WOoden plank used to load heayy boxes into the rear Of a Fig. 4.13: An inclined plane are some roads arounda hill, winding staircases truck, winding examples of inclined planes. 67  Dclined plane Inclinedn lnclined plane Hospital ramp A slide Winding road around a hill Fig.4.14: Examples of incdined plane Wedge edge cas Awedge is made up of two inclined planes. These planesmeet and form a sharp edge. This work split things apart. The longer and sharper the wedge is, the lesser effort it will take for thethe tin A get done. A shorter wedge will require more force and effort because it hasa wider angle at wedge can be made of wood, metal, stone or plastic. It is thick at one end and usually thinto a shar edgeon the other end. The wedges come in all sizes and usually have handles attached to them. Knif plough, saw, needle, nail, door stopper and axe are some common examples ofa wedge. A wedge Axe Knife Nail Fig. 4.15: Some examples of wedge Screw A screw is an incined plane wound around a cylinder. It usually has a flat end called head atone end and a sharp-pointed tip at the other end. Screws are used to hold objects together. The winding edge of a screw is called a thread. The distance between Head two adjacent threads is called the pitch. The grooved part of the screw is an inclined plane. The head of the screw has a groove for the tip of ascrewdriver. When the screw is held against a wooden block and its head is turned using a screw driver, the tip of the screw moves into the Threads -Inclined wood. Due to the grooves, a screw holds the wood more firmly than a plane nail. Also, less force is required to drive a screw in to the wood than a nail because of the inclined edge. Tip Screws are used in key rings, bottle caps and fountain pen caps and Fig. 4.16: Ascrew many other things. They may also be used for lifting heavy objects and examples of screws are bolts, bottle tops, guitar tuners, light bulbsand cork to tighten things. Some openers. 68 CARE OF MACHINES Siachines are an eSsential part of our daily life. We use them to ast. Suppose you want to use your bicycle and it make our work easy and complete machines need care and is not working properly. This is maintenance. They will not because all regularly. Proper care of machines increases their work properly if we do not maintainthem covered when not in use. working efficiency. Machines should be kept Theyshould be protected from dust to prevent their wear and tear. Moisture causes rusting in iron parts of the machines. The should be painted. non-movable iron parts of a machine The moving parts of a machine should be regularly lubricated to reduce friction. NUMERICALSBASED ON MECHANICAL ADVANTAGE (MA) Erample 1: Calculate the mechanical advantage, if 500 N force is required to of 1200N. overcome a load Load Solution: MA Effort L=Load = 1200 N E =Effort to overcome the force of object, E = 500 N L MA = E 1200 MA = =2.4 S00 Example 2: The mechanical advantage (MA)of a machine is 4. Aperson wants to lift a load of 50 N. How much effort he/she should apply? Solution: MÀ =4, Load = 50 N Load MA = Effort 50 N Hence, 4 Effort 50 Effort = -= 12.5 N 4 load arm 8 cm. Calculate its mechanical DXampie 3: A lever has an effort arm measuring 80cm and the advantage. Solution: Effort arm =80cm, Load arm =8cm Effort arm Mechanical advantage Load arm 80 = 10 69  move a 40 Nload. What Example 4: Aramp with a mechanical advantage 5 is used to input Torceis needed to push the load up the ramp? Solution: MA =5, Load = 40 N Load MA Effort Load Effort MA 40 =8N Example 5: You are given a class IIIlever which has a rod of length 60 cm. It has fulcrum at one end and the load is at the other. If an effort of 20 Nis applied at a distance of 30 cm fulcrum, calculate the following. from the a. Length of load arm b. Length of effort arm c. Mechanical advantage of lever d. Load Solution: Effort = 20 N a. Load arm = 60 cm b. Effort arm = 30 cm C. MA = Effort arm 30 =0.5 Load arm 60 d. Load can be calculated as following: MA Load Effort L 0.5 = 20 Load =20 x 0.5 = 10N ASSESS YOURSELF Fill in the blanks with the correct answer. 1. Ascrewdriver is an example of a NheeAl 2. An.PS... is used to move heavy objects. 3. Awinding road along the hills is an example of an..a.. 4... is the narrow or pointed part of a screW. 70