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ENERGY

ENERGY. Different forms; Chemical, Electrical, Heat, Electromagnetic, Nuclear and Mechanical Energy can be transformed from one type to another but the total amount never changes If one form of energy decreases then another must increase

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ENERGY

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  1. ENERGY • Different forms; Chemical, Electrical, Heat, Electromagnetic, Nuclear and Mechanical • Energy can be transformed from one type to another but the total amount never changes • If one form of energy decreases then another must increase • Mechanical Energy = the sum of kinetic and potential energies ( motion & position)

  2. WORK • Work is done if an object is moved through a displacement while a force is being applied to it • If either the force or displacement is doubled so is the work • W=F delta x • Force and displacement are in the same direction • Joule= newton .meter = kg.m2/s2

  3. Facts about Work • Work is a scalar quantity (no direction) • Work does not depend on time • Units are joule or foot-pound • When force is not in the same direction as displacement only the component parallel to displacement does work on the object • W= (F cos a) x where a is the angle to the horizontal

  4. Frictional Work • Frictional Work is the work done due to friction during motion. Energy dissipates (lost) • W fric = -fk x = -uk n.x = -uk mg.x • W net = W applied - W fric

  5. Kinetic Energy • K.E is the energy of movement • K.E. = ½ mv2 • As W = F .S = m.a.s ; s= displacement • From v2 = vo2 + 2as; then as =(v2 – vo2 )/2 • Then W = m(v2 – vo2 )/2 • Therefore Work = K.E. final – K.E. initial

  6. Potential Energy • P.E is the energy an object has due to its position in space (units Joules) • Gravitational P.E. = m. g. y where y = vertical height and g = gravity Gravitational P.E is the energy of an object based on its position in space Work = P.E initial – P.E final

  7. Conservative Forces • The work done on an object moving between two points, is independent of the path taken. It depends only on the initial and final positions. Gravitational force is conservative • If the work done by a force on an object moving through a closed path is zero • Potential energy functions can be defined only for conservative forces.

  8. Non-conservative Forces • The work done on an object is dependent on the path taken . Ex. Work due to friction will differ for different paths. a b Path 1 W= F.S Path 2 W= F.(22/7d)/2 If the force leads to the dissipation of mechanical energy

  9. Conservation • A physical quantity is conserved when the value of the quantity remains constant. The form of the quantity may change but the final value is the same as the initial value. For example the gravitational potential energy of a falling object will change to kinetic energy, but the total energy will remain the same.

  10. Mechanical Energy • The total mechanical energy in any isolated system remains constant if the objects within the system interact only through conservative forces. • Total mechanical energy in a system is the sum of the total kinetic and potential energies in the system • Therefore 1/2mvi2+mgyi=1/2mvf2+mgyf

  11. Spring Potential Energy • A spring neither stretched or compressed is in equilibrium • When compressed or stretched by a force there is stored potential energy in the spring • Stored spring energy is called elastic P.E. • Hooke’s law F = k.x Where k is the spring constant particular to a spring • (heavy spring = a large k value)

  12. Work done to store spring energy • The force require is proportional to the distance the spring is change from equilibrium • Therefore Fave.= Fo + Fx/2 = 0 + kx/2=1/2kx • As work is F.x then W = 1/2kx2 =P.Es • The potential energy is maximum when the spring has reached its max. compression or stretch P.Es is always positive

  13. New Mechanical energy Formula • Incorporating the value of spring potential energy in the previous formula • (KE + PEg + PEs)I = (KE + PEg + PEs)f • Where g is for gravity and s is for spring

  14. Non-conservative forces and work • In the real world non-conservative forces such as friction are present. • Therefore; Wnc = (KEf – KEi) + (PEf- PEi) • The work done by all non-conservative forces equals the change in kinetic energy plus the change in potential energy.

  15. Power • Power is the rate of energy transfer with time • P = W/t SI units (watt W = J/s ) kg.m2/s3 • Also P = F.x/t as x/t = v then P = F.v Power therefore is a constant force times the average speed The component of force is in the direction of the average velocity

  16. Power cont. • U.S. customary system unit of power is the horsepower (hp) =746W • Power consumption is referred to in kilowatt-hours 1kWh = (103W) (3600s) = 3.60 x 106 J • A kilowatt-hour is a unit of energy not power • A 100 watt bulb consumes 3.6 x 105 J in 1h

  17. Mechanical Advantage • MA. Is the advantage granted to us by the use of machines • MA. = Fr/Fe where Fr is the resistance force Fe is the effective force • MA. Is usually greater than 1

  18. Ideal Mechanical Advantage • IMA. Is mechanical advantage based on a distance. Ex. The rope one pulls in a pulley system to move an object. • IMA.= de/dr where de is the distance of effort dr is the distance of resistance • IMA. Is usually greater than 1

  19. Efficiency • e is the extent that a machine is efficient • e = Work out/Work in • e = Fr.dr/Fe.de • e =( MA./IMA ) .100

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