Work, Energy, & Power (Chapter 4).

Work, Energy, & Power (Chapter 4)

Student Learning Objectives
Compare and contrast mechanical work, energy, and power Name examples of simple machines Describe renewable energy sources. (30 seconds)

What is mechanical work?
Mechanical work is the use of energy to move an object a distance. Depends on total displacement, not path Scalar quantity Energy is transferred from one object to another when work is done. W = Fd

No work is done because there is no movement (d = 0)
Section 4.1

Work Being Done Applied force (F) through a distance (d)
Section 4.1

Work in horizontal component No Work in vertical component (d = 0) (even if part of your effort pushes the mower down) Section 4.1

Working against Something
When work is done, we generally feel the other part of Newton’s third-law force acting against us Gravity and friction are common agents working against work In the case of gravity, we must apply force to overcome the force of gravity (weight = w = mg) Work = W= Fd = wh = mgh (h is the height lifted) Section 4.1

Practice These Practice Problems are presented in class

What is mechanical energy?
Mechanical energy is the energy an object has that is in motion or may become in motion. Ability to do mechanical work Kinetic energy is the energy an object has when it is in motion. The work done to change the motion of a moving object = DKE KE = (½)mv2

GPE = mgh These Practice Problems are presented in class
Gravitational potential energy is the energy an object has that may fall or travel downward from a height. The work done by or against gravity = DGPE GPE = mgh These Practice Problems are presented in class

Potential Energy Depends only on the initial and final positions (difference in height - Dh) and is independent of path If we disregard any frictional loss, it takes the same amount of work (W) to lift the mass (m), no matter the path Section 4.2

An object can have both GPE and KE at the same time!
More than one type of energy may be present in a system. The type of energy may change. Image Credit: Physics Classroom

How is conservation of mechanical energy used?
Energy cannot be created or destroyed. The total amount of energy in the universe is conserved. Mechanical energy is conserved when no energy is lost or gained by the system. For Systems on a Hill Mechanical energy and work are interchangeable. Energy  Work GPEtop = KEbottom

These Practice Problems are presented in class
Elliptical orbits are maintained by conservation of energy. Planets move in elliptical orbits with the Sun at one focus of the ellipse, sweeping out the same area of space in equal time intervals. These Practice Problems are presented in class

P2 = a3 These Practice Problems are presented in class
The orbital periods of the planets depend on the semi-major axis of the orbit. P2 = a3 These Practice Problems are presented in class

Practice (Always assume mechanical energy is conserved)
These Practice Problems are presented in class

More Practice These Practice Problems are presented in class

What is Mechanical Power?
Power is the rate at which work is done or energy is used. Examples: Car Engines & Light bulbs P = W t P = E

Forms of Energy Thermal (heat) Energy – related to the kinetic and potential energies on a molecular level Gravitational potential energy – from an object’s position, stored gravitational energy Electrical energy – associated with the motion of electric charges Chemical energy – molecular bonds Radiant energy – sun Nuclear energy – rearrangement of nuclei Fission – breaking apart of larger nuclei Fusion - smaller nuclei are put together Section 4.5

Fossil Fuels Fossil Fuels – oil, gas, coal - from once living organisms; basically stored solar and chemical energy Oil – from marine organisms (U.S. imports more than 50% of our needs) Gas – from marine organisms (most is produced domestically) Coal – from terrestrial (land) plants (the U.S. has large reserves, but environmental problems in coal mining) Methane hydrate – crystalline form of natural gas and water (research on possible usage) Section 4.5

Pump Jacks pumping crude oil – Fossil Fuel
Photo Source: Copyright © Bobby H. Bammel. All rights reserved. Section 4.5

“Christmas Tree” – Gas Well – Fossil Fuel

Approximate Relative Fuel Consumption in the U.S.
Section 4.5 Approximate Relative Fuel Consumption in the U.S.

Fuels for Electrical Generation
Section 4.5

Energy Consumption by Sector
Section 4.5

What is a renewable energy source?
Renewable energy sources can be replenished in a short period of time. Wind Water Solar Geothermal Biofuel These Practice Problems are presented in class

Alternate and Renewable Energy Sources
Alternate energy sources – energy sources not based on fossil fuels or nuclear processes. Renewable energy sources – energy sources that cannot be exhausted. In large part these types of energies overlap. Section 4.6

Hydropower Uses the gravitational potential energy of water flowing downhill due to gravity Best dam sites (where large amounts of water can be stored in narrow deep canyons) have been taken Environmental/ecological damage Loss of agricultural land Section 4.6

Bonneville Dam, Columbia River

Wind Power Not particularly aesthetic
Must be located where the wind is sufficiently constant and fast enough Minor environmental effects Has been used for thousands of years to grind grains and pump water Section 4.6

Wind Farm near Mojave, California

Wind Farm near South Point, Hawaii

Solar Power Very promising future source of reliable and economic energy. Although some solar power is in use now, many more applications are possible. Section 4.6

Geothermal Energy Very site specific
Depends on a large natural change in temperature being present and accessible Extensively used in Iceland Section 4.6

Puna Geothermal Plant, Hawaii

Active volcanic flank of Big Island, Hawaii

Tides Tidal energy is steady, reliable, and predictable
Must be designed to handle bidirectional tidal currents Tidal currents have been generating electricity along the Rance River in France since the 1960’s. Section 4.6

Electrical Generation with planned Under Water Turbines, using Tidal Currents
Section 4.6

Biofuels Corn, sugarcane, and other plants can be used to make ethanol (an alcohol). Ethanol can be mixed with gasoline and used as fuel for cars (gasohol). Depending on the plant processed, more energy may be used in the production of ethanol than will be supplied by burning the ethanol itself. Section 4.6

Biomass Any organic matter available on a renewable basis is considered biomass. Biomass includes agricultural crops, wood, animal wastes, municipal wastes, aquatic plants, etc. Biomass is capable of being burned and therefore can generate energy. Section 4.6

Work, Energy, & Power (Chapter 4).

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Work, Energy, & Power (Chapter 4).

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