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Understanding Work, Power, and Machines

Explore the principles of work and energy transfer, power calculations, and the mechanics of machines. Learn about effort and resistance forces, mechanical advantage, efficiency, and the conservation of energy.

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Understanding Work, Power, and Machines

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  1. Chpt. 5—Work and Machines Work: is the transfer of energy that occurs when a force makes an object move For work to be done, a force must make something move ex: pushing on something that doesn’t move is not doing work

  2. Two conditions have to be met to do work on an object • 1. object has to move • 2. motion of the object must be in same direction as force applied When work is done, a transfer of energy always occurs So: Energy is also the ability to do work (not just the ability to cause change)

  3. Calculating Work • Work = force X distance • OR • W = F X d • *Units for work are Joules • Practice Calculating Work—pg. 128

  4. Power • Power: the amount of work done in a certain amount of time ( it is the RATE at which work is done) • Calculating Power • Power = Work / time • OR • P = W/t • Units used with power are watts—W (1 J/s) • Practice calculating Power—pg. 130

  5. Power and Energy • Power is produced or used any time energy is transferred from one object to another • SO: • Power = energy /time • ORP = E/t • Anytime you do work on an object, you cause its energy to increase • Section 1 Assessment questions

  6. Section 2—Using Machines • Machines: a device that makes doing work easier • Machines increase the force applied to an object OR increase the distance over which a force can be applied Machines can also change the direction of an applied force Ex: car jacks, ramps, wedges

  7. Effort and Resistance Forces • Two forces are involved when a machine is used to do work • Effort force: the force applied to the machine • Resistance force: the force applied by the machine to overcome resistance • Ex: when you pull a nail out w/a hammer, effort force is applied to the handle, resistance force is applied to the nail by the claw

  8. Two kinds of work are involved when using a machine • The work done by you on a machine is called the INPUT work • The work done by the machine is called the OUTPUT work • Remember: Energy is always conserved • A machine cannot create energy, so work OUTPUT is never greater than work INPUT

  9. Ideal machines create no friction • So for an ideal machine, Work INPUT = Work OUTPUT, • But, this is not so in normally operating machines

  10. Mechanical Advantage: the number of times a machine multiplies the effort force • To calculate mechanical advantage, you divide resistance force by effort force • Some machines simply change direction of effort force (ex: window blinds) • Therefore, the effort force and resistance force are equal, so mechanical advantage is 1.

  11. Efficiency: a measure of how much of the work put into a machine is changed into useful output work by the machine • High efficiency means less heat produced from friction • Efficiency = W output/W input X 100% • Friction causes output work to always be less than the input work • SO: Efficiency of a real machine is always less than 100%

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