Notes: Chapter 9 Energy

Objectives1. Define and describe work (9.1)2. Define and describe power (9.2)3. Define mechanical energy. (9.3)4. Define potential energy. (9.4)5. Define kinetic energy and describe work-energy

theorem. (9.5)6. State the law of conservation of energy. (9.6)7. Describe simple machines and mechanical

advantage. (9.7)8. Explain why no machine can have an efficiency of

100%. (9.8)9. Describe the role of energy in living organisms. (9.9)

9.1 Work

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

• Work is the product of the net force working on an object and the distance through which the object moves.

FdW What are the units for force?

• Work falls into two categories:– To move an object against a force.– To change the speed of an object.

• Work changes the energy state of an object.

9.2 Power

• Power is the rate at which work is done.

• Power equals to the amount of work done divided by the time interval during which the work is done.

tWP

What is the unit for power?

Question

• If a forklift is replaced with a new forklift that has twice the power, how much greater a load can it lift in the same amount of time? If it lifts the same load, how much faster can it operate?

9.3 Mechanical Energy

• Energy is the property of an object or system that measures its ability to do work.

• Energy due to the position or movement of something is referred to as mechanical energy.– Two forms:• Potential energy• Kinetic energy

9.4 Potential Energy

• Potential energy is stored energy related to an object’s relative position that has the ability to do work.

• Three types of potential energy:– Elastic potential energy– Chemical potential energy– Gravitational potential energy

• The amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity to lift it.

• Height is the distance above some arbitrarily chosen reference level.– A position below this level gives the PEg that is

negative with respect to the reference point.

mghheightweightPEGPE g

Questions

1. How much work is done on a 100-N boulder that you carry horizontally across a 10 m room? How much potential energy does it gain?

2. A.) How much work is done on a 100 N boulder when you lift it 1 m?

B.) What a power is expended if you lift the boulder a distance of 1 m in a time of 1 s?

C.) What is the GPE of the boulder in the lifted position?

9.5 Kinetic Energy

• A moving object has the ability to do work.• Kinetic energy is the energy of motion.

• The KE of an object is equal to the work required to bring to its speed from rest, or the work the object can do while being brought to rest.

2

21 mvKE

2

21 mvFd

Question

• When the brakes of a motorcycle traveling 60 km/h become locked, how much farther will the motorcycle skid than if it travels at 20 km/h?

9.6 Work-Energy Theorem

• Work-energy theorem – whenever work is done energy changes.

22

21

21

ifif

ifif

mvmvKEKEKEW

mghmghGPEGPEGPEWEW

FdW

A car that has TWICE the KE than another car would require twice the work to stop it. If the braking friction were the same in both cases, how much more stopping distance would the car with twice the KE need?

22

21

21

imvmvFd

KEW

f

• Kinetic energy can take several forms:– Heat– Light– Sound– 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 of energy to another but the total energy never changes.

• When it appears that some energy is lost from the system, it was not destroyed. The unaccounted energy was given off as heat.

• Conservation of mechanical energy– The total mechanical energy of a system remains

the same before during and after energy is transformed.

2212

21

afterafterbeforebefore

afterafterbeforebefore

afterbefore

mvmghmvmgh

KEPEKEPE

MEMEKEPEME

Questions

1. A 45 kg mass is dropped from a height of 10.0 m. How fast is it going when it gets to a height of 5.0 m? How much work did gravity have to do to get this mass up to this speed?

2. How fast would you have to jump in order to reach a height of 0.6096 m (2 ft)?

9.8 Machines

• Machine – a device used to multiply forces or simply change the direction of forces, or both.– Transfers energy from one place to another or

transforms it from one form to another.– Cannot do more work than what is put into it.

That is, it cannot create energy.– Basic idea: Work in = Work out

• Lever – a simple machine made of a bar that pivots about a fixed point called the fulcrum.

outoutinin

outin

dFdFWW

• Mechanical advantage (MA) – ratio of output force to input force for a machine. It’s the number of time the machine multiplies force.

• Three kinds of levers:Type I/First Cass Type II/Second Class Type III/Third Class

Fulcrum between effort and load

Load between fulcrum and effort

Effort between fulcrum and load

• Pulley – a type of lever that is a wheel with a groove in its rim, which can be used to change the direction of a force exerted on a rope. A pulley or system of pulleys can also multiply force.– General rule for pulleys: the number of ropes

supporting the load equals to the mechanical advantage (ME).

9.9 Efficiency

• Ratio of useful energy output to total energy input. Or, also, the percentage of work input that is converted to work output.

– Lower the efficiency the, the greater is the energy wasted to heat loss.

100

in

out

in

out

WWefficiency %

WWefficiency

• Inclined plane – a simple machine sometimes called a ramp.

9.10 Energy for Life

• There is more energy stored in the molecules of food than in the reaction products (CO2 and H2O) after the food has been metabolized.– Process is called respiration.

• Energy enters an ecosystem when photoautotrophs capture energy from the sun to convert carbon dioxide and water into sugars.– Process is called photosynthesis.

9.11 Sources of Energy

• The sun is the source of practically all our energy here on earth.– Fossil fuels: petroleum, coal, and natural gas– Exceptions: nuclear energy and geothermal energy

• Solar power– Photovoltaic (solar) cells: direct transformation to

electricity– Wind: Indirect. Caused by uneven heating of earths surface.– Running water: Indirect. Caused by evaporation,

condensation, and precipitation at higher elevations

• Fuels cells: the opposite reaction of electrolysis. Hydrogen and oxygen are recombined to form water. The process can be used to generate an electric current.

• Nuclear and geothermal energy– Most concentrated form in

uranium and plutonium.– Earth’s interior is kept hot by

nuclear reaction.Basic set up for geothermal power generation.

• Power demand for an exponentially growing human population.– Reached 7 billion in October 2011– Pros and cons of fossil fuels– Pros and cons of nuclear power– Pros and cons of wind and hydropower– Pros and cons of solar