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TAKS Objective 5

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TAKS Objective 5

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    1. TAKS Objective 5 Work, Power, Efficiency and Simple Machines

    2. Work Work: using a force for a distance W = F x d The work done by forces on an object is equal to the changes in energy for that object. Work and Energy are measured in Joules 1 Joule=1 Newton • meter

    3. A accelerated rapidly B remained motionless C decreased its velocity D gained momentum

    5. 42 How much work is performed when a 50 kg crate is pushed 15 m with a force of 20 N? F 300 J G 750 J H 1,000 J J 15,000 J

    6. Why use a machine? In an ideal (perfect) machine the work put into the machine (Win) = the work put out by that machine (Wout) That would be 100% efficiency. However, all machines produce friction so they are not 100% efficient.

    7. 6 Types of simple machines Some Simple Machines: Inclined planes Screws Pulleys Wheel and axle Levers Wedge

    8. Machines make work easier The ideal mechanical advantage of a machine (IMA) is the number of times the output force is larger than the input force IMA= Fout/Fin A machine can only make this happen by using the input force for a farther distance than the output force Fin • din= Fout • dout

    9. 50 Which lever requires the least effort to lift the load?

    10. 48 The diagram shows an electric motor lifting a 6 N block a distance of 3 m. The total amount of electrical energy used by the motor is 30 J. How much energy does the motor convert to heat? F 9 J G 12 J H 18 J J 21 J

    12. Real Machines use Energy No real machine is 100 % efficient. i.e. none put out more work than is put in Efficiency of a machine is work output/work input X 100 % Eff = Wout X 100% Win

    13. Machines use power Power: the rate at which energy is used (work is done) P = Work/time Power is measured in H.P. or watts 1 watt = 1 Joule 1 sec

    14. Let’s apply the power definition: 12 An advertisement claims that a certain truck has the most powerful engine in its class. If the engine has more power, which of the following can the truck’s engine do, compared to every other engine in its class? F Produce fewer emissions G Operate more efficiently H Perform work faster J Accelerate longer

    15. Your turn! 1. Which of these represents a properly balanced system?

    17. 3. A horizontal force of 600 N is used to push a box 8 m across a room. Which of these variables must be known to determine the power used in moving the box? A The weight of the box B The potential energy of the box C The time it takes to move the box D The length of the box

    18. 4. A mechanic used a hydraulic lift to raise a 12,054 N car 1.89 m above the floor of a garage. It took 4.75 s to raise the car. What was the power output of the lift? A 489 W B 1815 W C 4796 W D 30,294 W

    19. 5. What is the efficiency of an air conditioner if there is a work input of 320 J and a work output of 80 J? F 4% G 25% H 240% J 400%

    20. And the answers are: A; to be balanced the force x distance from the fulcrum must be equal on both sides. H: turning the larger wheel once will spin the smaller pulley several times. C: to calculate power, you need work/time, force and distance gives you work so you need time. C; power = (force x distance)/time G: work out/work in x 100%

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