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Work and Power

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Work and Power. Simple machine change the amount of force or the direction of the force or both. Force is measured in newtons (N) or pounds (lb) 4.48 N = 1 lb. Power: how fast you do work; P=W/t P in watts (or horsepower) W in joules T in seconds. Work- force X distance W= F(d)

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Work and Power

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  1. Work and Power • Simple machine change the amount of force or the direction of the force or both. • Force is measured in newtons (N) or pounds (lb) • 4.48 N = 1 lb • Power: how fast you do work; • P=W/t • P in watts (or horsepower) • W in joules • T in seconds • Work- force X distance • W= F(d) • W in joules • F in Newtons • d in meters

  2. Mechanical systems and machines • A machine is a device with moving parts that work together to accomplish a task. • They do something useful. • Input-what YOU do • Output-what the MACHINE does. This was the bicycle manufactured by the Wright brothers in 1897.

  3. Mechanical Advantage and Efficiency • MA=Fo/Fi • MA > 1, output force is larger. • MA < 1, output force is smaller. • If a machine is efficient most of the work input becomes output • An ideal machine would be 100% efficient • Friction always lowers efficiency • Efficiency = useful work output X 100 = % • Total work input • If the work output<work input then the machine is not efficient! input Output

  4. A simple machine is an unpowered mechanical device. Lever, Wheel and axle, block and tackle, gear, ramp Simple machines work by manipulating forces. Most machines we use today are compound machines-they have many simple machines combined. Simple Machines

  5. Levers • A lever has several different parts. • Fulcrum-fixed point. • Input arm-between fulcrum and force you apply. • Output arm -the side where output force is applied.

  6. Block and Tackle • Uses ropes and pulleys to multiply forces. • Input force-100 lbs • Output force-200 lbs (the weight of the object being lifted) • # of ropes = # of times the force is multiplied. Tension force

  7. # Ropes = MA 100 = 2 50 Two Ropes Input Force=50 lb Output Force=100 lb

  8. C. Wheel and Axle • Wheel and Axle • two wheels of different sizes that rotate together • a pair of “rotating levers” Wheel Axle

  9. h l D. Inclined Plane • Inclined Plane • sloping surface used to raise objects

  10. E. Screw • Screw • inclined plane wrapped in a spiral around a cylinder

  11. F. Wedge • Wedge • a moving inclined plane with 1 or 2 sloping sides

  12. F. Wedge • Zipper • 2 lower wedges push teeth together • 1 upper wedge pushes teeth apart

  13. A. Compound Machines • Compound Machine • combination of 2 or more simple machines

  14. Mechanical Energy • Two most common forms: • Energy due to MOTION = KE • Energy due to POSITION = PE • Potential Energy has the potential to do WORK. • Ex. Fossil Fuels, Food • Gravitational PE: The PE due to ELEVATED positions. • Ex. Water in an elevated reservoir • GPE is = to the WORK done in lifting it.

  15. Potential Energy • Potential Energy is stored energy. • PE: Has the potential to do work • Stored chemically in fuel, the nucleus of atom, and in foods. • PE: mgh • Or stored because of the work done on it: • Stretching a rubber band. • Winding a watch. • Pulling back on a bow’s arrow. • Lifting a brick high in the air.

  16. Kinetic Energy • KE = ½ mv2 relationships or Fd = ½ mv2 • a. double the speed and the KE quadruples • b. it takes 4x the work to double the speed • c. objects moving twice as fast takes 4x as much work to stop • d. Ex: a car going 100 km/hr has 4x the KE it would have at 50 km/hr • e. Ex: a car going 100 km/hr will skid 4x as far when brakes are locked-as it will at 50 km/hr because speed is squared for kinetic energy.

  17. Energy • Energy- ability to do work; in Joules • Heat Energy • Radiant Energy (sun/light) • Nuclear Energy • Electrical • Mechanical • Potential (p=mgh) and Kinetic (KE= 1/2mv2) • Law of Conservation of Energy • Energy cannot be created or destroyed; it can only be transformed to another type

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