Work, Power & Simple Machines Work Work- A force acting through a distance. The distance that the object moves must be in the same direction as the force applied to the object. Weight What force is having to be overcome by the Lifting Force lifting force? Weight! Movement Distance Work Work- A force acting through a distance. The distance that the object moves must be in the same direction as the force applied to the object. What force is having to be overcome by the pushing force? Pushing Force Friction! Distance Moved Work is not done every time a force is applied. Work is done only when a force moves an object in the same direction as the applied force. Is the man doing work when he lifts the box? Lifting Force Yes. Is the man doing Movement work when he Distance holds the box? Is the man doing No! work if he carries No! (But he IS applying a force) the box? (But he IS applying a force) Work Think about pushing on wall that does not move. A force is applied but the wall has not moved a distance. Is work done? No! But I did expend energy applying a force! Calculating Work Work = force (N) x distance (m) W=FxD The unit for work is a Joule (J). If you lifted an object weighing 1N through a distance of 1m, you did 1 Joule of work: W = FD = 1N x 1m = 1 J Think about how much force a Newton is…about ¼ of a pound. Is a Joule of work…very much work? No! Calculating Work Ifyou lifted an object weighing 200 N through a distance of 0.5m, how much work would you do? Work = force x distance = 200 N x 0.5m = 100 J. Power Power is the rate at which work is done , or the amount of work per unit of time. Two men can move a lot of sand using shovels… …but a front-end loader can do it in less time… …because the front-end loader has more POWER. Power Power is the rate at which work is done , or the amount of work per unit of time. Power = work / time or Power = force x distance / time Because work = force x distance Power Theunit for power is watt (W). One watt is equal to 1 joule per second (1 J/sec). Largequantities of power are measured in kilowatts (kW). One 1000 kilowatt equals _____ watts. Machines : How do machines affect A machine is a device work? that makes work easier. A machine is ANY device that helps you to do something. Machines : How do machines affect work? What are other examples of machines? Machines : How do machines affect work? Machines : How do machines affect work? Machines : How do machines affect work? Machine Efficiency: Input and Output There are always two types of work involved in using a machine. Input work is the work that goes into the machine (like turning a pencil sharpener). Output work is the work that comes out of the machine (like the grinding of the pencil). How hard was that?? Machine Efficiency: Input and Output The efficiency of a machine can be calculated: Efficiency = (work output / work input) x 100 This is easy to remember…think about it… If you put 100 Joules of work into a pencil sharpener, but only got 80 Joules of work out, the pencil sharpener is 80% efficient: (80 Joules / 100 Joules) x 100 = 80% efficiency Machine efficiency can never be greater than or equal to 100% because the work output can never be greater than the work input. Inother words, there is no machine that has a 100% efficiency! Why not? Think…it’s a force that opposes motion… Friction! Friction makes every machine <100% efficient Machine Efficiency The friction in a machine “wastes” energy in the form of heat Machines with the smallest amount of friction are the most efficient. Machine Efficiency The closer the work output is to work input, the more efficient the machine. Toyota hybrid “concept car” Many household appliances have energy guides that tell the consumer how efficient the appliance is. The more efficient the appliance the more money the consumer will save. You can also look for the “energy star” label on more efficient machines. Machines Machines make work easier because they change the size or the direction of the force put into the machine. More force Machines Most machines make work easier by multiplying either force or distance. Force! Distance! Which does which? Machines What is multiplied, force or distance? Force Distance Force when “prying” the dirt free, distance when lifting it up out of the hole or into a truck or wheelbarrow Determining How Helpful a Machine Is Besidesthe efficiency of a machine we also can determine how helpful a machine is. Determining How Helpful a Machine Is What we mean by how helpful is how many times the machine multiples the effort force to overcome the resistance force Effort Force Resistance Force Determining How Helpful a Machine Is ? Effort Force Resistance Force ? Effort Force or Resistance Force? Effort ? Force Determining How Helpful a Machine Is The number of times a machine multiplies the effort force is called the mechanical advantage. Thistells you how much force is gained by using the machine. The more times the machine multiples the effort force, the easier it is to do the job. Quick quiz –open notes 1. What is a machine? 2. Describe relationship between friction and the efficiency of a machine. 6 Kinds of Simple Machines Inclined Plane Wedge Lever Wheel & Axle Screw Pulley Inclined Plane Inclinedplane: A ramp is an example of an inclined plane. It is simply a flat slanted surface. It has no moving parts. Inclined Plane Aninclined plane decreases the size of the effort force needed to move an object. Wedge Wedge: An inclined plane that moves. In a wedge, instead of an object moving along the inclined plane, the inclined plane itself moves to raise the object. Wedge A wedge is usually a piece of wood or metal that is thinner at one end. A knife and axe are two examples. Wedge Wedge : the longer and thinner a wedge is, the less the effort force required to overcome resistance. This also is true for an inclined plane. Wedge When you sharpen a wedge, you are increasing its mechanical advantage by decreasing the effort force that must be applied in using it. A sharpened ax requires less force because the edge is thinner Wedge A lock is another device that depends on the principle of the wedge. The edges of the key are a series of wedges. Wedge A zipper is also a device that depends on the principle of a wedge. Lever Lever : A lever is a rigid bar that is free to pivot , or move about a fixed point. The fixed point is called the fulcrum. Lever When a force is applied on a part of the bar by pushing or pulling it, the lever swings about the fulcrum and overcomes a resistance force. Levers Leversare divided into three groups, or classes, depending on the location of the fulcrum and the forces. Levers Examples of first class levers are crowbars, seesaw and pliers. Levers The fulcrum of a lever is not always between the effort force and the resistance force like the example of a crowbar. Sometimes it is at the end of the lever. A wheelbarrow is a second class lever. Levers Where is the effort, resistance, and fulcrum? Because distance is decreased by the wheelbarrow, force must be increased. Levers In second class levers, you multiply force but decrease distance. Also with second class levers, the direction in which you lift is the same as the direction in which the load moves. A 2nd class lever does not change the direction of the force applied to it. Levers Ina third class lever, the fulcrum or fixed point, is at the end of the pole where you are holding it like a fishing pole. Levers The effort force is applied by your other hand as you pull back on the pole. At the top of the pole is the resistance force. In this class, you need to move your effort force only a short distance to make the end of the pole move a greater distance. Levers A 3rd class lever, reduces the effort required but multiplies the distance through which the output force moves. Wheel and Axle A wheel and axle is a simple machine made up of two circular objects of different sizes. The wheel is the larger object. It turns around a smaller object called the axle. Wheel and Axle A force applied to the wheel is multiplied when it is transferred to the axle, which travels a shorter distance than the wheel Wheel and Axle The mechanical advantage depends on the radius of the wheel and of the axle. Wheel and Axle Examples of wheels and axles are bicycles, Ferris wheels, gears, wrenches, door knobs, steering wheels, windmills and screwdrivers. Screw Screw : A screw is inclined plane wrapped around a central bar or cylinder to form a spiral. Screw A screw rotates and with each turn moves a certain distance up or down. A screw multiplies an effort force by acting through a long distance. The closer together the threads, or ridges, of a screw, the longer the distance over which the force is exerted and the more the force is multiplied. Thus the mechanical advantage of a screw increases when the threads are closer together. Screw Examples of a screw are the wood screw, corkscrew, nut and bolt, faucets, jar lids, and spark plugs. Pulley A pulley is a rope, belt, or chain wrapped around a grooved wheel. A pulley can function in two ways. It can change the direction of a force or the amount of force. Pulley A pulley that is attached to a structure is called a fixed pulley. A fixed pulley does not multiply an effort force it only changes the direction of the effort force. Pulley Pulleyscan be made to multiply the force with which you pull on them. This is done by attaching a pulley to the object you are moving. This type of pulley is called a moveable pulley. Pulley A greater mechanical advantage can be obtained by combining fixed and movable pulleys into a pulley system. Compound Machines A compound machine is a combination of two or more simple machines. Most of the machines you use everyday are compound machines: cars, bicycles, washing machine, VCR, blender,watches, etc. Compound Machines Simple or compound machines cannot multiply work. You can get no more work out of a machine than you put into it. True or False (If false correct) 1. Work equals force times time. 2. The unit of work in the metric system is the newton-meter or joule. 3. Power is work divided by force. 4. The unit for power is newton. 5. A pulley is an inclined plane that moves. 6. An ax is an example of a wheel and axle.
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