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Chapter 13: Work and Machines Aim: How does height affect an object’s gravitational potential energy? Gravitational Potential Energy • Is any time an object is raised above the ground • The greater an object’s height and weight the more gravitational potential energy • You can calculate it by multiplying the object’s weight by its height • Unit is joules when the height is meters and weight is newtons • The higher an object is, the more speed it will pick up on its way down Aim: How does energy change? • Kinetic energy depends on both an objects speed and mass • Greater the mass, greater the speed, greater the kinetic energy • Potential energy changes to kinetic energy • When a roller coaster is coming over a hill it has potential energy • When the roller coaster begins moving down the hill its energy is changing to kinetic energy • It reaches its highest speed at the bottom of the hill because the potential energy has completely changed to kinetic energy • Food stores potential energy stored in chemicals • Our bodies change the food energy into other forms, such as kinetic energy, electrical energy and heat energy Aim: How do batteries produce electricity? Batteries • Change chemical energy into electricity • A chemical reaction inside the cell(battery) causes a flow of electrons • This flow is electricity 1. Dry-cell batteries • Called a primary battery b/c it can only be used once • Dead dry-cell batteries should be thrown away after being used • This will prevent toxins from leaking out 2. Secondary Battery • Can be recharged • In order to recharge the battery needs to be hooked up to an outside source of electricty • During recharging chemicals are turned back into their original form, enabling the battery to be used again • Lead-acid storage batteries found in cars • Nickel-cadmium battery found in laptops, camcorders, portable tools and space shuttles Aim: What is work? Work • Is applying a force to an object to move it through a distance • W= distance x force • Units is joules (d-meters, f-newtons) • Just holding a box is not work • Picking up a box you are doing work • When work is done energy is added to it • When throwing a ball your work is equal to the kinetic energy gained by the ball • Work can be changed into heat • Friction between the table and the moving books turns mechanical energy, used to do work, into heat energy Aim: How do we calculate work? Aim: What is the law of conservation of energy? Law of Conservation of Energy • Total amount of energy in the universe remains constant • Energy can change its form • Energy can never be created nor destroyed Aim: What are simple machines? Simple Machines • A machine is a device that makes it easier for us to do work • Simple machines are the most basic kinds • Have none or a few moving parts • Changes the direction of a force we apply to something • Can also increase the strength of an applied force • Ex: pulling a nail out with a hammer • Effort force is the force that you apply to a simple machine • Force the machine applies to an object in response to the effort force is called the output force • The force against which the machine acts is called the resistance force • 6 kinds of simple machines: • Lever, pulley, wheel and axle, inclined plane, screw and the wedge Levers • All levers have a rigid bar that rests on a fulcrum, which is the pivot point • Effort arm-side that applies an effort force • Resistance arm-side that produces an output force • 3 classes of levers: 1. First-class levers • Fulcrum lies between the effort force and the output force • Change the direction of the effort force • Ex: hammer pulling a nail out • Ex: seesaw 2. Second Class Lever • Output force is between the effort force and the fulcrum • Do not change the direction of the effort force • Output force is greater than the effort force b/c the effort arm is longer • Exs: wheelbarrow, nut cracker, bottle opener, paper cutter 3. Third Class Lever • Effort force is between the fulcrum and the output force • Do not change direction of effort force • Produce an output force that is less than the effort force • Distance of the force is multiplied • Exs: fishing pole, tweezers, human forearm and a broom Aim: How do levers multiply force? • Moving the position of the fulcrum changes the amount of the output force • First-class levers can produce an output force that is greater than the effort force when the fulcrum is placed closer to the output force than the effort force • Effort arm must be longer than the resistance arm b/c input work must equal output work Aim: What is mechanical advantage? Mechanical Advantage • Is the number that tells you how much a simple machine should multiply your effort • Lever’s mechanical advantage is found by dividing the distance the effort arm moves by the distance the resistance arm moves • When the output force is greater than the input force, the mechanical advantage is greater than one • Using a broom has a mechanical advantage less than one • The other way to find the mechanical advantage is dividing the length of the effort arm by the length of the resistance arm Aim: How do pulleys work? Pulleys • A rope is threaded through a wheel and axle • Pulling on the rope can lift an object which is tied to the other end of the rope • Can be either fixed or movable 1. Fixed pulley • The wheel is attached to a fixed support • It does not multiply the effort force • It changes the direction of the effort force 2. Movable Pulley • Pulley is attached to a movable object and moves with it • Multiplies the effort force by 2 • Has a mechanical advantage of 2 • Does not change the direction of the effort force Pulley System • Is made up of fixed and movable pulleys • The pulleys act together • Has the same mechanical advantage as a single movable pulley • It does change the direction of the effort force Aim: What is the mechanical advantage of a pulley? • By increasing the number of wheels and ropes, we can increase the mechanical advantage • While mechanical advantage increase, the effort force required decreases • 2 ways to find a pulley’s mechanical advantage 1. By dividing the distance the effort rope moves by the distance the object moves 2. By counting the number of strands of rope that feel a downward pull from the load Aim: How does a wheel and axle work? Wheel & Axle • Fulcrum lies between the effort arm and the resistance arm • The wheel applies the effort force • Small axle produces the output force • The mechanical advantage can be found by dividing the length of the effort arm by the length of the resistance arm • Effort arm is the radius of the wheel • Resistance arm is the radius of the axle • Has a large mechanical advantage Aim: What is an inclined plane? Inclined Plane • Is a straight, slanted surface, like a ramp • Have no moving parts • Makes it easier to do work because they multiply the effort force • By using an inclined plane to move a heavy object to a different height, you need less force • When using an inclined plane the force has to be exerted over a longer distance • Lifting heavy objects can hurt your back • To find the mechanical advantage divide the output force by input force • The output force is the weight of the object • The work put into a machine must equal the work produced by the machine, therefore the effort force must act over a greater distance than the output force Aim: What is the mechanical advantage of a ramp? Ramp’s Mechanical Advantage • The steeper the inclined plane, the more force is needed to move an object up the incline • Both girls are doing the same amount of work, even though the effort force is different • You can find the MA of a ramp by dividing the output force by the input force or by dividing the length of the inclined plane by its height • The longer the length of the ramp, the less the effort Aim: How does a screw work? Screws • They multiply effort force • They have a high mechanical advantage • Are created by wrapping an inclined plane around a central bar, which we called threads • The head of the screw is the part we turn • The distance from thread to thread is called the pitch • To find the mechanical advantage we divide the distance around the head by the pitch • Screws with a larger pitch have a lower mechanical advantage • Screws with large heads and very close pitch have a very high mechanical advantage • Friction keeps screws in place • Without friction, the screw would unscrew and the object would fall Aim: What is a wedge? Wedges • A wedge is a single inclined plane or 2 inclined planes joined back to back • A wedge must be moved by an effort force • Wedges that are thin have a high mechanical advantage • Ex: knife blades, ax heads, and chisels • These work best when they are sharpened • A wedge changes the effort force and increases it’s strength • The effort force is downward • The output force is horizontal Aim: What is a compound machine? Compound machines • A compound machine is a combination of 2 or more simple machines • Ex: scissors (2 first-class levers and wedges) • Ex: screwdriver and screw (screwdriver is a wheel and axle, screw is an inclined plane) Aim: What is efficiency? Efficiency • No machine runs without friction • When there is friction some of the input work is changed into heat energy, therefore not all of the input work becomes output work • To find efficiency write a ratio: the work done by a machine over the work put into a machine and multiply by 100 to get a percentage • The closer the efficiency is to 100% , the less energy the machine wastes • The more friction there is, the lower the efficiency of a machine Aim: How do machines help us? • Simple machines make tasks easier • Machines, whether compound or simple, allow us to do things that we could not do with our bare hands