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Chapter 11

Work is the product of force and the componenet of displacement in the direction of the force without motion there is no work. Power is the rate of doing work. If work is done on an object the work maybe used to change the object's kinetic energy, its potential energy and it internal energy. Energy is the ability to produce a change in itself or it surroundings. By doing work, you have to transferred energy from your body to the work that you are doing.

Key Equations

Work: W=Fd

W is the work, F is the magnitude of force, and d is the magnitude of displacement in the direction of force Even thought force and displacement are vector quantities and work is a scalar quantity. The unit of work is the newton times meter, which is called a joule (J).

  • Problem:

    • How much work is done on an object if a force of 30newtons (south)displaces the object 200 meters?
  • Solution: W=F s =(30N(S)(200M/S) =6000J

    • The magnitude of the component of the force F acting in the direction of the motin is found by multiplying the force F by the cosine of the angle between F and the direction of motion. W=FcosO(d) =fdcos0

  • Problem : As Alex pulls his red wagon down the sidewalk, the handle of the wagon makes an angle of 60 degree with the pavement. If Alex exerts a force of 100 newtons along the direction of the handle , how much work is done when the displacement of the wagon is 20 meters?

  • Solution: W=(fcosx)times (s) (100)(cos 60)(20m) (50N(20N) =1000J

    • Power

    P=W/T

    Power: is work done divided by the time it takes. Power is also a scalar quantity and its measured in watts (W). One watt is one joule of energy transferred in one second.

  • Problem: If 3000 joules of work is performed on an object in 1.0 mintute, what is the power expended on the object.

  • Solution: P=W/t = 3000J/60s =50W

    • Simple Machines:Key Ideas

  • A machine whether powered by engines or humans, make work easier. A machine eases the load either by changing the magnitude or the direction of the force exerted to do work.

  • Effort force, Fe is the force you exert on a machaine

  • The force exerted by the machine is the resistance force Ff

  • Mechanical advantage is the ratio of resistance force to effort force, Fr/Fe (Ma) of the machine.

  • Ideal mechanical advantage is the relationship given above assumes that no friction is present in the machine and the mechanical advantage. (IMA)

  • The efficienty of a machine is defined as the ratio of output work to input work.

  • A compound machine consists of two or more simple machines linked so that the resistance force of one machine becomes the effort force of the second.

    • Key Equations

    Mechanical advantage(MA): MA=Fr/Fe

    The input work is the product of the effort force you exert, Fe, and the output work is the product of the resistance force,Fr.

    An ideal machine transfers all the energy, so the output work equals the input work. W0=wi,or Frdr=Fede

    Ideal Machanical advantage(IMA):

    IMA=de/dr

  • You measure distance moved to calculate the ideal mechanical advantage IMA and measure the forces exerted to find the actual mechanical adantage, MA

    • Efficiency: Wo/Wi times 100%

  • The ratio of the output work into input work=1 and the value is always less than 100%

    • efficiency=MA/IMA times 100%

  • IMA of most machines is fixed by the machine's design. An efficient machine has an MA almost equal to it IMA.A less efficient machine has a smaller MA.

    • Compound Machine:

  • MA= F on chain/F on pedal times F on road/F by chain = F on road/ F on pedal

    • Summary of the Chapter

    WORK AND Energy:

  • Work is the product of the force exerted on an object and the distance the object moves in the direction of the force.

  • Work is the transfer of energy by mechanical means.

  • Power is the rate of doing work. That is, power is the rate at which energy is transferred. It is measured in watts.

    • Simple Machines:

  • Machines, whether powered by engines or humans,make work easier. A machine eases the load either by changing the themagnitude or the direction of the force exerted to do work.

  • The mechanical advantage, MA, is the ratio of resistance force to effort force.

  • The ideal mechanical advantage, IMA, is the ratio of the displacements. In all real machines, MA is less than IMA.

    • LINKS
    seorf sfhs sln necc/a> home.earthlink.net/a>

    References

  • Force and Work: Energy in Action Copyright 1999 AIMS Multimedia
  • Machines-Wikipedia Free encyclopedia
  • Merrill Phsics:Principles and Problems Authors Paul W. Zitzewitz and Robert F. Neff