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Published byLynn Wilkinson Modified over 6 years ago

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Work and Power

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We have learned about: We have learned about: Acceleration Acceleration Newton Newton Force Force Speed Speed Velocity Velocity Vectors Vectors Now we will learn: Now we will learn: How are force and work related? How are force and work related? How moving objects do work How moving objects do work

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Work: Work: The use of force to move an object some distance 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 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 If you want to do work, you have to use force to move something

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Work: Work: Must involve a push or pull resulting in motion 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: 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 Longer distance = more work More force = 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) 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)

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

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Moving objects such as bowling balls, air particles, and water 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 Early inventions / machines used moving water or air to do work Water wheel Water wheel windmills windmills

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

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

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Potential energy caused by gravity Potential energy caused by gravity GPE = mgh GPE = mgh g = acceleration due to gravity (9.8m/s 2 ) g = acceleration due to gravity (9.8m/s 2 ) Units are still J (joules) because it is a measurement of energy Units are still J (joules) because it is a measurement of energy

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

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

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

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

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ME = KE + PE ME = KE + PE Calculations for energy Calculations for energy Work transfers energy Work transfers energy How power is related to work and time How power is related to work and time How power is related to energy and time How power is related to energy and time Common uses of power Common uses of power

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Power: Power: The rate at which you do work The rate at which you do work Calculating power: Calculating power: Power = Work / time Power = Work / time P = W / t P = W / t Power is measured in watts (W) Power is measured in watts (W) 1 watt = one joule of work done in one second 1 watt = one joule of work done in one second

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

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

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