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Chapter 13

Chapter 13. Work and Power. 13.1: Work is the use of force to move an object. We have learned about: Acceleration Newton Force Speed Velocity Vectors Now we will learn: How are force and work related? How moving objects do work. Force is necessary to do work. Work:

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Chapter 13

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  1. Chapter 13 Work and Power

  2. 13.1: Work is the use of force to move an object • We have learned about: • Acceleration • Newton • Force • Speed • Velocity • Vectors • Now we will learn: • How are force and work related? • How moving objects do work

  3. Force is necessary to do work • Work: • The use of force to move an object some distance • You do work only when you exert a force on an object and move it • If you want to do work, you have to use force to move something

  4. Force, Motion, and Work • Work: • Must involve a push or pull resulting in motion • Work done by force is related to the size of the force and the distance over which the force is applied: • Longer distance = more work • More force = more work • Work is done only by the part of the applied force that acts in the same direction as the motion of the object. (pg 420)

  5. Calculating work: • Work = force times distance • W = fd • A measure of how much force is applied over a certain distance. • The distance involved is the distance the object moved in the direction of the force • Units for work: • Force = Newtons Distance = meters • Work = Newton-meter or the joule (J)

  6. Objects that are moving can do work: • Moving objects such as bowling balls, air particles, and water do work • Early inventions / machines used moving water or air to do work • Water wheel • windmills

  7. Work transfers energy • Energy: • The ability of a person or an object to do work or to cause a change • By doing work on an object, you are transferring some of your energy to the object.

  8. Work Changes potential and kinetic energy • Kinetic energy: • The energy of motion • Any moving object has some kinetic energy • The faster an object moves, the more kinetic energy it has. • Potential energy: • Stored energy • Energy of position or shape • When you think of “shape” think about a spring being compressed

  9. Calculating gravitational potential energy • Potential energy caused by gravity • GPE = mgh • g = acceleration due to gravity (9.8m/s2) • Units are still J (joules) because it is a measurement of energy

  10. Calculating Kinetic Energy • Kinetic energy = mass x velocity2 ÷ 2 • KE = ½ mv2 • Order of operations: • 1st: square the velocity • 2nd: multiply mass x the squared velocity • 3rd: divide by 2 • Units are still J (joules) because you are calculating energy

  11. Calculating Mechanical Energy • Mechanical energy: • The energy possessed by an object due to its motion or position. • The combination of an object’s potential and kinetic energy • Any object that has mechanical energy can do work on another object. • ME = PE + KE

  12. The total amount of energy is constant • Law of conservation of energy: • No matter how energy is transferred or transformed, all of the energy is still present somewhere in one form or another • Pg 431: • Skater has potential energy where? • As she rolls down the ramp, what happens to the potential energy? Kinetic energy? • What energy does she have at the bottom of the ramp?

  13. Forms of energy: • Thermal: • Energy an object has due to the motion of its molecules • Chemical: • Energy stored in chemical bonds that hold chemical compounds together • Nuclear: • Potential energy stored in the nucleus of an atom • Electromagnetic: • Energy associated with electrical and magnetic interactions

  14. 13.3: Power is the rate at which work is done • ME = KE + PE • Calculations for energy • Work transfers energy • How power is related to work and time • How power is related to energy and time • Common uses of power

  15. Power can be calculated from work and time • Power: • The rate at which you do work • Calculating power: • Power = Work / time • P = W / t • Power is measured in watts (W) • 1 watt = one joule of work done in one second

  16. Horsepower • The amount of work a horse can do in a minute • The average horse could move 150 pounds 220 feet in 1 minute • 1 horsepower is = 745 watts which means that the horsepower is a much larger unit of measurement than the watt

  17. Calculating power from energy • Power = energy / time • P = E / t • Power is also the amount of energy transferred over a period of time • Energy = power x time • Unit is still the watt (J / s)

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