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Chpt. 5—Work and Machines

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. Two conditions have to be met to do work on an object

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Chpt. 5—Work 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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