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 work done

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

KE 12

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 form of energy E mc 2

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

Efficiency work done

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

F Gm1m2

d 2

Acceleration Due to GravityF G

mearthmobject

Rearth2

F mobject g

g Gmearth

Rearth2

g 6.67 10 11 6 1024

6.4 10 6 2

g 9.8 m/sec2

Inverse Square Law

Inverse Square Law

Weight and Weightlessness

Tides

Stretch is about one meter high.