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Energy • According to Einstein, a counterpart to mass • An enormously important but abstract concept • Energy can be stored (coal, oil, a watch spring) • Energy is something moving objects have • How to deal with this idea??? Work • Easiest to start with the notion of work • Work = Force X Distance • Lift a box from the floor, you apply a force to overcome gravity • Multiply that force by the distance through which you apply the force and you calculate the amount of work accomplished Is this Work? Work • Unit is the JOULE • A Joule is a newton-meter Power • The rate at which work is done • Takes more power to run up the stairs than to walk up the stairs, but the energy consumed is the same in either case work done power time interval Power • • • • • • Unit is the WATT A Watt is a newton--meter per second Think of 100-Watt light bulb Bigger units are kilowatts and megawatts Utility sells energy in kilowatt-hours 1 KWh = 1000 Joules/second times 3600 Seconds = 3.6 X 106 Joule Potential Energy • If we lift an object from the floor into the air, it has the potential to do work for us • This ability to do work is called POTENTIAL ENERGY • Other forms of potential energy include the compression of a spring, the stored energy in coal or oil, the stored energy in a uranium nucleus Potential Energy • Gravitational potential energy is simple to calculate • Gravitational Potential Energy = weight X height PE mgh Gravitational Potential Energy • Independent of Path to get there Kinetic Energy • The energy of moving objects • Kinetic Energy = 1/2 Mass X Speed2 1 2 KE mv 2 Energy Conversion Energy Conversion Work-Energy Theorem • Work done on an object can give the object either potential or kinetic energy or both • If we do work on an object to lift it into the air, we give it potential energy • If we do work on an object and set it into motion, we give it kinetic energy • The work-energy theorem relates to the second case Work-Energy Theorem • If we do work on an object and set it into motion without changing the object’s potential energy, the work done appears as kinetic energy of the object Work KE Conservation of Energy • Perhaps the most important discovery of the past two centuries • In the absence of external work input or output, the energy of a system remains unchanged. Energy cannot be created or destroyed. • Remember from Einstein, that mass is a 2 form of energy E mc Collisions • Elastic Collisions conserve both momentum and kinetic energy • Inelastic Collisions conserve momentum by energy is lost to heat Machines • A device that multiplies forces by taking advantage of the definition or work and the conservation of energy • Work input = Work output • Levers Machines Machines Efficiency work done Efficiency energy used In many machines, some energy is lost due to friction. This may be metal-on-metal (oil the parts to reduce friction) or air resistance (energy loss moves molecules in the air faster giving them kinetic energy). Energy Sources • For the earth, there are two energy sources, the sun and radioactive decay in the earth’s interior • The earth receives about 1400 Joules/meter2 each second • This is 1.4 kW per square meter • Recover for use in plants (burn wood) • Recover from wind Man’s Need for Power • Man can generate about 75 Watts to do work • Domesticated Animal about 750 Watts • Machines limited by size • Power plants generate electricity in the hundreds of megawatt range Universal Gravitation (Newton) • Every mass attracts every other mass with a force that is proportional to the product of the two masses divided by the square of the distance between the masses • For distances, calculate from the CENTER OF MASS • For the earth, that is at the center of the earth Universal Gravitation m1 m2 FG 2 d Acceleration Due to Gravity FG m earthm ob ject 2 Rearth F m ob ject g mearth gG 2 Rearth g 6.67 10 11 g 9.8 m/sec 2 6 10 24 6.4 10 6 2 Inverse Square Law Inverse Square Law Weight and Weightlessness Tides Stretch is about one meter high.