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
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 10 6 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
Gravitational Potential Energy Independent of Path to get there
Kinetic Energy The energy of moving objects Kinetic Energy = 1/2 Mass X Speed 2
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
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 form of energy
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
Efficiency 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/meter 2 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
Acceleration Due to Gravity
Inverse Square Law
Weight and Weightlessness
Tides Stretch is about one meter high.