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Work and Power

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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