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Work and Simple Machines

Work and Simple Machines. What is work?. The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction .). Which ones are work?. A scientist delivers a speech to an audience of his peers. NO

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Work and Simple Machines

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  1. Work and Simple Machines

  2. What is work? • The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)

  3. Which ones are work? • A scientist delivers a speech to an audience of his peers. NO • A body builder lifts 350 pounds above his head. YES • A mother carries her baby from room to room. NO • A father pushes a baby in a carriage. YES • A woman carries a 20 kg grocery bag to her car? NO

  4. Formula for work Work = Force x Distance • The unit of force is newtons • The unit of distance is meters • The unit of work is newton-meter • 1 newton-meter = 1 joule • So, the unit of work is a joule

  5. Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? 200 joules (W = 20N x 10m)

  6. Power • The rate at which work is done. • Power = Work*/Time *(force x distance) • The unit of power is the watt.

  7. 1.Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. a. Which student does the most work? b. Which student delivers the most power?

  8. a. Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads. b.Yet, Ben is the most powerful since he does the same work in less time.

  9. 2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s2a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Force=10 x 2 Force=20 N Work=Force x Distance Work = 20 x 10 Work = 200 Joules Power = Work/Time Power = 200/5 Power = 40 watts

  10. Machines Make work easier by changing at least one of the following: • Amount of force exerted. • Distance over which force is exerted. • Direction in which force is exerted.

  11. Input and Output • Force • Input force – the force that is exerted on themachine. • Output force – the force the machine exerts on the object. • Work (W = f x d) • Input work = Input force x Input distance • Output work = Output force x output distance

  12. Mechanical Advantage (MA) • The number of times a machine increases a force exerted on it. • MA = Output force Input force • Machine that: • increases force – MA > 1. • increases distance – MA < 1 • changes the direction of force – MA = 1.

  13. Efficiency X 100% Input work • Efficiency = Output work • The higher the percent, the more efficient the machine is. • Work is usually wasted overcoming the force of FRICTION!! • Ideal machine – Efficiency of 100%

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