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Work,Machines, and Energy Work and Power Simple Machines Mechanical Advantage Work Two conditions must be met in order for work to be done on an object: The object moves. A force must act on the object in the direction the object moves. Work= force * distance Problem Example: How much work is completed when a digger uses 26,000 newtons to dig 6 meters down in the dirt? Work= force* distance 26,000 newtons * 6 meters= 156000 joules Power=work/time Power is the rate at which work is done. Example: How much power was done if there was1253.4 joules of work completed in 3600 seconds? Power= work/time Power = 1253.4joules = 3600 seconds 0.34816 watts Example: If a receiver catches the ball at his 20-yard line. He then runs towards the goal. At the opponents 10 yard line he makes a U-turn and runs back to his 20yard line, where he started. Did he do any work? No, he did not move. His distance was zero. 10-yard line 50 yard line 20-yard line Machines A machine is a device that makes work easier by changing the direction or the amount of force needed to do the work. The force applied to the machine is referred to as the effort force. The force opposing the effort is called the resistance force or “load”. Mechanical energy Is the energy of movement or position of an object. It is also called Kinetic energy. Wind and moving water have Mechanical energy. Other examples: windmills, pencil sharpener, Chain-fall,Tire-jack Simple Machines Simple machines are devices that do one movement. There are six simple machines that make up all of the compound machines. Compound machines have two or more simple machines, such as a computer. Simple Machines The six simple machines are: Inclined plane Levers Pulleys Wheel and Axle Screw Wedge Inclined plane First class levers First class levers:multiply the effort force and change the direction. Other examples of 1st class levers Other examples: shears, seesaw,scissors, pliers, shovel Second class levers On a 2nd class levers the distance is increased. Force is multiplied without changing the direction. Other 2nd class levers Doors, bottle openers, nutcrackers, Third class levers In a 3rd class lever, the effort force is greater than the resistance force. The Mechanical advantage is less than one. A 3rd class lever is used to increase the distance moved, not to reduce the force. Other 3rd class lever examples Rake, broom, fishing pole Pulleys A pulley is a rope wrapped around a grooved wheel. There are fixed and movable pulley systems. The fixed pulley always has a Mech. Advantage of one. (It only changes the direction) A movable pulley has a Mech Advantage of greater than one. Fixed Pulley M.A.=1 Movable Pulley M.A. of 2 2 ropes supporting the load Movable Pulley M.A. = 3 3 ropes supporting the load Wheel and axle The M.A. is always greater than one. Other examples are a door knob, a Ferris wheel,a wheel chair. Screw A screw is an inclined plane wrapped around a shaft. Water faucets, lids, etc. Wedge Is an inclined plane used to push objects up or apart. Examples; knives, razors,chisels, axes, splitting logs (wedge), saws Mechanical Advantage Most machines multiply the force of your efforts. The number of times a machine multiplies an effort force is called Mechanical Advantage (M.A.). A machine with a M.A. of 2 doubles your effort force. Machines can also change the direction of the force. If a machine has a M.A. of less than 1, increases the distance or the speed of motion. (remember that it will still decrease the resistance force.) Mechanical Advantage Tells how much a machine multiplies force or increases distance Mechanical Advantage = output force/input force OR MA= Input Distance/Output Distance Mechanical efficiency Mechanical efficiency = work output * 100% work input Work output: amount of work done by the machine. Work input: amount of work you apply to the machine. Mechanical efficiency will always be less than 100% since the work output is always less then the work input. Mechanical efficiency Mechanical efficiency is always less than 100% due to the following factors: Heat loss, body, car engine, all motors, etc. Friction Sound & vibration Energy changes With all motion energy is required. When an object moves it has Kinetic energy (motion). When an object is standing still it has Potential energy. A burning match has Kinetic energy. The amount equals the Potential energy that was stored in the match before it was lighted. A battery before it is used has Potential energy. Food setting on a shelf has Potential energy. Kinetic energy= Mass* velocity2 2 Kinetic energy increases with an increase in mass and/or velocity. Law of Conservation of energy Energy cannot be created or destroyed. Energy can change forms. Such as mechanical energy of a hand changing to electrical when it turns a hand generator. Or mechanical energy changing to sound energy in a speaker. Energy Energy - The ability to cause changes. Forms of energy: Radiant Electrical Chemical Thermal Nuclear Potential energy, energy of position or condition A special type of potential energy is called gravitational potential energy. G.P.E. - dependent upon the height above the earth’s surface Conservation of Energy Energy can not be created or destroyed. Mechanical energy Total amount of kinetic and potential energy in a system. http://science.howstuffworks.com/ roller-coaster2.htm Work Define: The transfer of energy through motion. A force must be exerted through a distance in order for work to be done. Work = force x distance or W=Fxd Work Work is measured in joules. 1 joule equals a newton – meter (N• m) Joule - amount of work done when a force of one newton acts through a distance of one meter. When 1 joule of work is done on an object, 1 joule of energy has been transferred to the object. Work When describing work it is important to identify on what the work is being done. For work to be done, something must move and the movement is in the direction of the applied force.