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Work Power Energy. Work Concepts. Work ( W ) = Force x distance Work is the transfer of energy by mechanical means. Work is done on an object only if it moves in the direction of the force Only the component of the force in the direction of the motion does work. Work Equation.
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Work Concepts • Work (W) = Force x distance • Work is the transfer of energy by mechanical means. • Work is done on an object only if it moves in the direction of the force • Only the component of the force in the direction of the motion does work.
Work Equation Work= F(cos Q)d Unit: work = newton * meter or Nm This is also known as a “joule” or j which is commonly used for energy Here, the force is parallel to d and provides the forward force so W = Fd Here, the Fx is the component in the direction of the motionW = F cos Q d F d F d
The Sign of Work • Positive work is done when the motion is in the same direction as the force of motion. For a moving object (+) net work results in increased velocity. • Negative work results from a friction force applied in the direction opposite the motion. For a moving object (-) net work results in decreased velocity. F F does (+) work f does (-) work f d
Power • Power (P) is the rate at which work is done or • rate at which energy is transferred. • Measured in watts. • Watt (W) ~ one joule of energy transferred in one second. Power = Work / time 1 watt = 1 joule / second (J/s)
Practice Power Problems • Cody, a weightlifter, raises 240 kg of weights a distance of 2.35 m. • How much work is done by Cody in lifting the weights? • How much work is done by Cody in holding the weights above his head? • Does he do work if he lets them fall? • If Cody completes the lift in 2.5 sec, how much power is used?
A force of 300 N is used to push a 145 kg mass 30 m across the floor in 3 sec. Calculate the work done on the mass. What is the power generated? • If the floor generates 50 N of friction, how much work does the floor do on the mass? • What is the net work done on the mass?
Energy Energy (the ability to do work) Mechanical Non-mechanical chemical Kinetic Potential heat/light KE = ½ mv2 nuclear Gravitational Elastic PEg = mgh PEel = ½ kx2
Kinetic Energy KE = ½ * mass * velocity² KE = ½ mv2 Kinetic energy is the energy an object has because of its motion
Work-Energy Theorem According to the Work-Energy theorem, the work done on an object by the net force is equal to the change in kinetic energy of the body Work = D KE F cosQ d = KEf - KEi
Mechanical Energy Mechanical energy is the energy which is possessed by an object due to its motion or its stored energy of position, shape, or form M Etotal = Ktotal + PEtotal
Potential Energy Potential energy (U) is the energy an object has because of its position, shape, or form
Gravitational P E Ug = mass * gravity * height Gravitational potential energy depends on the weight of the body and its position in a gravity field
Conservation of Energy According to the law of conservation of energy,the total energy of a closed, isolated system is constant MEinitial = MEfinal Ki + Ugi + Ueli = Kf + Ugf + Uelf
Ideal Machines • Ideal machines exist only in a frictionless, air resistance-less world. • No energy or work is lost to the system through outside forces • For ideal machines: Work input = Work output so: Findin = Fout dout
Efficiency • Efficiency ~ ratio of work output to work input expressed in percent. • Ratio of final energy to initial energy efficiency = (Wout / Win)* 100 Or efficiency = (Ef / Ei)*100
