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Work, Energy, and Machines What is work? W=F*D Force = 200 N Distance = 15 m work W=F*D W = (200 N)(15 m) Scientific work = force used to move an object; familiar work = something you do, doesn’t require motion W = 3,000 N * m N * m joule 3,000 J lifting your backpack that’s full of books because you’re so excited to do your homework Backpack moves in the direction of the force Work occurs when the blade of the wind turbine turn. The force comes from the wind and occurs when the blades accelerate. The distance is the amount that the blades move Power = energy / time P = E/t P = 156 J / 6s P = 26 W The energy it uses and the time it uses that energy in CFL uses less power = less money in electric bill. work distance Same direction energy watts work energy power No, you haven’t done any work obj has not moved. Force and distance No, box has not moved a distance Power can be calculated from energy and time Moved a distance of 1.5 m W= f*d W = 150 N * 1.5 m W = 225 J P = e/t P =2400 J/ 2 s P = 1200 W Moving a desk; work transfers from my arm’s energy into the desk, transfer energy to the legs to move desk. On the Move Energy: ability to cause change Kinetic Energy the energy of MOTION Every moving object has kinetic energy (i.e. fast movement = more kinetic energy; slow = less) Kinetic Energy Calculated The kinetic energy of an object equals ONE HALF the object’s mass (m) times the square of its velocity (v) KE = ½ mv2 When mass is expressed in kilograms (kg) and speed is expressed as meters per second (m/s) KINETIC ENERGY is expressed in JOULES (J) The foal has a mass of 100 kg and is moving at 8 m/s along the beach. What is the kinetic energy (KE) of the foal? What do you know? m = 100 kg; v = 8 m/s What are we looking for? Kinetic energy What is the formula? KE = ½ mv2 KE = ½ (100 kg) * (8 m/s) 2 KE = 3,200 J 100 m2/s2 5,000 J 100 m2/s2 40,000 J 225 m2/s2 90,000 J Potential Energy An object that is NOT moving CAN still have energy Potential energy the energy an object has in regards to its position, condition, or chemical composition The ability to do WORK The use of force to move an object at a distance Types of Potential Energy Elastic potential energy: an object that has the potential to be stretched or compressed Rubber bands | springs Gravitational potential energy: an object’s position ABOVE THE GROUND An object held above ground has POTENTIAL to fall Higher object = higher potential energy Mechanical potential energy: potential energy that depends on an object’s position Chemical potential energy: result of chemical bonds Potential energy that can be released during chemical reactions Chemical PE Gravitational PE Elastic PE Calculating Gravitational PE The gravitational potential energy of an object is equal to its mass (m) times its height above the ground (h) times the acceleration due to Earth’s gravity (g) GPE = mgh An objects mass is expressed in KILOGRAMS An objects height is expressed in meters The acceleration due to gravity is expressed as 9.8 m/s2 Potential energy expressed in JOULES The cat has a mass of 4 kg and is 1.5 m above the ground. What is the gravitational potential energy of the cat? What do you know? m = 4 kg; h = 1.5 m; acceleration due to gravity = 9.8 m/s2 What are we looking for? Gravitational Potential Energy What is the formula? GPE = mgh PE = (4 kg) (1.5 m) (9.8 m/s2) PE = 58.8 J 3.92 J 5.88 J 58.8 J Mechanical Energy Mechanical energy: energy possessed by an object due to its motion and position Sum of kinetic energy and potential energy ME = KE + PE ME = (½ mv2) + mgh 0 18 18 18 0 18 Energy an obj has due to motion Energy an obj has due to its position/chem comp. Yes; if an obj is not moving it has zero kinetic energy. The sum of KE and PE would be the same as the GPE Sum of an obj’s PE and KE Greater the mass of an obj. at a given speed, the greater the PE Object’s mass and the object’s height KE due to mass and speed & GPE due to its mass and height above the ground Helicopter travels up at a steady rate for 5 sec and then downwards at same rate for 5 sec. 40 J Machines A machine is any device that helps people do WORK by changing the way work is done Machines that make up other machines simple machines Levers Wheel and axles Pulleys Inclined planes Wedges Screws Simple Machines Work is done when a force is applied to an object and MAKES IT MOVE Work done to a machine = work input Fore you apply to a machine through a distance = input force Work done by a machine on an object = work output Force a machine exerts on an object = output force What do simple machines do? Work = f * d Apply less force longer distance = work the same Some machines increase the amount of force needed (short distance) Some machines decrease the magnitude or size of the force needed to move an object (longer distance) Mechanical Advantage A machine’s mechanical advantage is the number of times the machine multiples the input force Input force vs. output force Mechanical Advantage (MA) divide output force by input force MA = input/output Mechanical Advantage (cont.) Mechanical advantage > 1 easier task Output force greater than the input force Mechanical advantage = 1 change direction of input force Mechanical advantage < 1 greater force over shorter distance Output / input Output / input 5N/5N 2 N / 6N .334 1 Output / input Output / input 5N/5N 2 N / 6N 1 .334 Mechanical Efficiency Mechanical efficiency: comparison of a machine’s work output with the work input Mechanical efficiency (ME) = work output divided by work input EXPRESSED AS A PERCENTAGE ME = work output/work input * 100% Ideally work a machine does is the same as the work you put into it Reality work input is greater than the work output because some work is done to OVERCOME FRICTION ME = work output/work input * 100% ME = 475 J / 500 J * 100% ME = .95 * 100% ME = 95% Gaining Leverage A lever is a simple machine that has a bar that pivots at a fixed point Fulcrum: fixed point Levers are used to apply a force to move an object Force of object = LOAD Ideal Mechanical Advantage distance from input force to a fulcrum (dinput) divided by the distance from the output force to a fulcrum (doutput). The mechanical advantage of a simple machine that does not take friction into account Mechanical advantage is 100% efficient Three classes of levers There are three classes of levers that differ based on the positions of the fulcrum, the load, and the input force First-class Second-class Third-class First-Class Lever First-class lever: the fulcrum is between the input force and the load Scissors, pliers, crowbar, hammer Second-Class Lever Second-Class Lever: the load is between the fulcrum and the input force Wheelbarrow, nut cracker, bottle opener, paper cutter Third-Class Lever Third-Class Lever: the input force is between the fulcrum and the load Tweezers, tongs, fishing rod Third-Class Lever First-Class Lever Second-Class Lever Wheel and Axle Wheel and axle: machine that is made of a wheel connected to a smaller cylindrical object THE AXLE The ideal mechanical advantage of a wheel and axle equals the radius corresponding to the input force (radiusinput) divided by the radius corresponding to the output force (radiusoutput) Ideal Mechanical Advantage = radiusinput /radiusoutput Wheel vs. Axle The radius of the wheel is ALWAYS larger than the radius of the axle Mechanical advantage > 1 input force applied to the wheel Mechanical advantage < 1 input force applied to the axle 1 meter 20 meters Mechanical Advantage = 1m/ 20m Mechanical advantage = .05 Mechanical advantage input force applied to the axle Pulley Pulley: simple machine that has a grooved wheel that holds a rope or cable A load is attached to one end of the rope, and an input force is applied to the other end Three types fixed, movable, and block and tackle pulley Fixed Pulley A fixed pulley is attached to something that does not move Allows you to pull down on the rope to lift the load up Movable pulley The wheel of a moveable pulley is attached to the object being moved One end of the rope is fixed The other end of the rope can pull the wheel and load to move along the rope Block and Tackle Pulley A block and tackle pulley is a pulley system made by combining a fixed pulley and a movable pulley Inclined Planes Inclined Plane = simple machine that is a straight, slanted surface A smaller input is needed to move an object using an inclined plane Force applied a longer distance SAME AMOUNT OF WORK DONE easier task than lifting Ideal Mechanical Advantage: dividing the length of the incline by the height that the load is lifted Ideal Mechanical Advantage = length / height Wedges A wedge is a pair of inclined planes that MOVE One thick end; one thin end Used to cut and split objects Output force of the wedge > input force force applied over a shorter distance Longer & thinner the wedge is = greater its ideal mechanical advantage Screw A screw is an inclined plane that is wrapped in a spiral cylinder When a screw is turned; a small force is applied through the distance along the inclined plane of the screw The screw applied a large force through the short distance it is pushed The longer the spiral on the screw & closer the threads are greater the mechanical advantage 3rd-class levers are useful when the output force is to be applied over a greater distance baseball bats & rakes Mechanical efficiency = (work output/work input) *100% 42/50*100 = 84% Machines can change the way work is done by changing the size and distance of the force used. They can also change the direction of a force. A first-class lever; the fulcrum is between the input and the load MA = input force/output force = 245 N/245N = 1 Ideal mechanical advantage = radius of input / radius of output radius of axle / radius of wheel Ideal mechanical advantage = length/ heigh 120 m / 6 m = 6 MA = distance input force/distance output foce = 1.5 m/ 2m = .75