Chapter 9 - EnergyEnergy can change from one form to another without a net loss or gain.

9.1 WorkWork is the product of the net force

on an object and the distance through which the object is moved.

(If/when the force causes the object to move.)

Work is done when a force acts on an object and the object moves in the direction of the force.

Work = net force x distanceW = Fd

9.1 WorkWork falls into two categories:1. Work done against a force

such as gravity, friction, or to stretch a spring.

2. Work done to change the speed of an object such as speeding up or slowing a car.

9.1 WorkUnitsFd = W

(Force- Newton, Distance-meter,Nm = Joule (rhymes with cool.)

One Joule of work is done when one Newton of force is exerted over the distance of one meter (- like raising an apple over your head.

9.2 PowerPower is the rate at which (or how fast) work

is done.Power equals the amount of work done

divided by the time interval during which the work is done.

With more power, work is done faster. With twice the power, twice as much work

can be done in the same time, or the same amount of work can be done in half the time.

time

WorkPower

9.2 PowerUnits

One Watt (W) of power is expended when one Joule of work is done in one second.

Kilowatt=1,000 Watts, Megawatt=1,000,000Watts

The space shuttle uses 33,000 MW of power.

One horse-power(hp) = 765W

Wattsecond

Joule

time

WorkPower

9.3 Mechanical EnergyEnergy is the property of an object

or system that enables it to do work.Energy is measured in Joules (like

work.)Mechanical Energy is the energy due

to the position or movement of something.

The two forms of mechanical energy are potential energy and kinetic energy.

9.4 Potential EnergyPotential Energy (PE) is energy that

is stored and held in readiness.Some Example include:Elastic Potential Energy – stretched

or compressed.Chemical Potential Energy – Energy

stored in the molecules of matter.Gravitational Potential Energy –

Energy due to elevated position.

9.4 Potential EnergyGravitational potential energy

(PEg)= weight x height.PEg=mgh(Remember mg = weight.)Note that the height is the distance

above some arbitrary chosen reference level. The gravitational potential energy is relative to that level and depends only on mg and h.

9.5 Kinetic EnergyAnything in motion is capable of

doing work. It has kinetic energy.Kinetic energy is the energy of

motion.The kinetic energy of an object

depends on its mass and speed.Kinetic Energy = ½ mass x speed2

2speed X mass 2

1KE

9.5 Kinetic EnergyThe kinetic energy of a moving object is

equal to the work required to bring it to its speed from rest, or the work the object can do while being brought to rest.

net force x distance = kinetic energy.Fd = ½ mv2.Note that the speed is squared, so if the

speed is doubled, its kinetic energy is quadrupled.

A car moving at twice the speed has four time the energy.

9.6 Work-Energy TheoremThe work-energy theorem describes the

relationship between work and energy.The work-energy theorem states that

whenever work is done, energy changes.Work equals the change in energy.WorkNet = ΔKEIf you push a box at constant speed (with

a force equal to friction) no net work is done.

Kinetic energy is involved in heat, sound, light, and electricity.

9.7 Conservation of Energy

The law of conservation of energy states that energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes.

Our energy comes from the sun.

9.8 Machines

A machine is a device used to multiply forces or simply to change the direction of forces.

A machine transfers energy from one place to another or transforms it from one form to another.

A lever is a simple machine made of a bar that turns about a fixed point.

Work input = work outputSince work = Fd(force x distance)input = force x distance)output

No machine can multiply work or energy.

9.8 Machines

Mechanical advantage of a machine is its ratio of output force to input force.

Without friction, MA is also the ratio of input distance to output distance.

Simple Machines:◦ Levers◦ Pulleys◦ Inclined Plane◦ Wedge◦ Screw◦ Wheel and Axle

9.8 Machines

Levers Type 1 lever – the fulcrum is between the (input)

force and the load (output force.)◦ Ex. Seesaw◦ Changes direction◦ Can multiply force or distance

Type 2 lever – the load is between the fulcrum and the force.◦ Ex. Wheelbarrow◦ Multiplies Force

Type 3 lever – the force is between the fulcrum and the load.◦ Ex. Hockey/Lax stick◦ Multiplies distance (Speed?)

9.8 Machines

Pulley (pulley system) – a kind of lever that can change the direction of the force or multiply the force.

Like a lever, Fulcrum, Input Force and Output Force apply.

Mechanical advantage is equal to the number of ropes that support the load.

Mechanical Advantage can also be determined by the ratio of distances.

tanceOutput Dis

anceInput DistMA

9.9 Efficiency

An Ideal Machine is 100% efficient. In any machine, some energy is transformed into

atomic or molecular kinetic energy – making the machine warmer.

Efficiency of a machine is the ratio of useful energy output to total energy input, or the percentage of the work input that is converted to work output. (Energy converted is work.)

The lower the efficiency of the machine, the greater is the amount of energy wasted as heat.

100% X input work Total

output work UsefulEffeciency

9.9 Efficiency

Inclined plane is simply a sloped surface.

MA is equal to the length divided by the height.

100% X advantage mechanical lTheoretica

advantage mechanical ActualEffeciency

height

lengthMA

9.9 Efficiency

Complex Machines. A complex machine is a combination of two or more simple

machines. Scissors – What simple machines make a pair of scissors?

◦ Two wedges and a lever. A car jack is an example of a complex machine combining a

screw and a wheel and axle. An automobile engine is called a heat engine. It converts

thermal energy to mechanical energy (actually often chemical potential to thermal to mechanical).◦ Roughly 35% efficient◦ Energy expended through the cooling system◦ Exhaust gasses◦ Friction - Almost half heat the engine parts through friction.◦ Incomplete combustion

Thermal energy is the graveyard for useful energy.

9.10 Energy for LifeEvery living cell in every organism is a

machine.They, like every machine, need energy to

run.There is more energy stored in the molecules

of food than there is in the reaction products after the food is metabolized. This energy difference sustains life.

Cells metabolize food like engines burn fossil fuels.

In metabolism carbon joins with oxygen to form carbon dioxide.

9.10 Energy for LifeThe reverse process is more

difficult.Through photosynthesis, plants

combine carbon dioxide with water to produce hydrocarbon compounds like sugars. Sugar is the simplest food.

Green plants capture the sun’s energy to provide all other life on earth (almost.)

9.11 Sources of EnergyThe sun is the source of practically all

our energy on Earth.◦Exceptions are nuclear and geothermal.

Fossil fuels are the byproducts of photosynthesis hundreds of millions of years ago.

Solar Power – Sunlight is directly transformed into electricity by photovoltaic cells

Sunlight evaporates water which falls in the mountains.

9.11 Sources of EnergyUneven warming of the Earth

produces wind.Fuel Cells – (like a battery)

◦Electrolysis uses electricity to separate hydrogen and oxygen from water H2O.

◦Fuel cells combine hydrogen and oxygen to make water and give off electricity.

9.11 Sources of EnergyNuclear and Geothermal Energy.The most concentrated form of useable energy

is stored in uranium and plutonium, which are nuclear fuels.

A byproduct of the radioactivity at the Earth’s core is geothermal energy.

“Energy sources such as nuclear, geothermal, wind, solar, and hydro power are environmentally friendly.”

“The combustion of fossil fuels , on the other hand, leads to increased atmospheric concentrations of carbon dioxide, sulfur dioxide, and other pollutants.”