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Energy. Adapted From. Exploring Engineering. Chapter 4, Part 1 Energy. Energy. Energy is the capability to do work Work = force x distance Distance over which the force is applied Energy Units: SI: joules Mixed SI units: Watt-hours (= 3.6 kJ) English: ft-lbf “foot pound force”.

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Exploring engineering

Adapted From

Exploring Engineering

Chapter 4, Part 1



  • Energy is the capability to do work

  • Work = force x distance

    • Distance over which the force is applied

  • Energy Units:

    • SI: joules

    • Mixed SI units: Watt-hours (= 3.6 kJ)

    • English: ft-lbf “foot pound force”


  • Mixed SI units: Watt-hours (= 3.6 kJ)


  • How fast work is done or how rapidly the amount of energy possessed by an object changed

  • “Power is defined as time rate of doing work or time rate of change of energy”

  • Power = work/time

  • Power Units:

  • SI: watts (joules/sec)

  • English: Horsepower

Kinds of energy
Kinds of Energy

  • Kinetic Energy

  • Potential Energy

  • Some other forms of energy:

    • Magnetic energy

    • Electrical energy

    • Surface energy

    • Chemical energy (a form of potential energy)

    • Internal energy etc.

Often mechanical energy

Kinetic energy
Kinetic Energy

  • Also known as “Translational Kinetic Energy” (TKE)

    TKE = ½ mv2 (SI units)

    = ½mv2/gc (English units)

    m = mass, v = speed, gc = 32.2 lbm.ft/lbf.s2

    Units: ???

Kinetic energy example
Kinetic Energy: Example

  • What is the translational kinetic energy of an automobile with a mass of 1X103 kg traveling at a speed of 65 miles per hour (29 m/sec)?

  • Need: TKE of the vehicle

  • Know: Mass: 1X103 kg, speed: 29 m/sec

  • How: TKE= ½mv2

  • SOLVE: TKE = 4.2 x 105 J

Anything that has mass and is moving in a line has TKE.

Gravitational potential energy
Gravitational Potential Energy

  • GPE is the energy acquired by an object by virtue of its position in a gravitational field-- typically by being raised above the surface of the Earth.

    • In SI, GPE = mgh in units of joules

  • In Engineering English units,

    • GPE = mgh/gc  in units of ft.lbf

Gpe power example
GPE & Power: Example

  • A person takes 2.0 seconds to lift a 1. kg book a height of 1. meter above the surface of Earth. Calculate the power expended by that person or calculate the energy spent by the person per unit time.

    • Work done =Force x distance = mgx h = 1. x 1. x 9.81 [kg][m/s2][m] = 9.81 [J][m] = 1. x 101 J

    • Power expended = Work done/time = 1. x 101/2.0 [J/s] = 5 Watts

Gravitational potential energy1
Gravitational Potential Energy

  • Mt. Everest is 29, 035 ft high. If a climber has to haul him/herself weighing 200. lbm (including equipment) to the top, what is his/her potential energy above sea level when on the summit. Give your answer in both in joules and in ft.lbf.

Gravitational potential energy2
Gravitational Potential Energy

  • Need: GPE in English and SI units

  • Know:

    • m = 200. lbm = 90.7 kg (“Convert”); h = 29, 035 ft. = 8850. m (“Convert”); g = 32.2 ft/s2 = 9.81 m/s2 & gc = 32.2 lbm ft/s2 lbf (English) and gc = 1 [0] in SI

  • How: GPE = mgh/gc  English

    GPE = mgh SI

Gravitational potential energy3
Gravitational Potential Energy

  • Solve: English … GPE = mgh/gc

    = 200.  32.2  29,035/32.2 [lbm][ft/s2][ft][lbf.s2 /lbm.ft]

    = 5.81  106 ft.lbf (3 significant figures)

  • SI … GPE = mgh

    = 90.7  9.81  8850. = 7.87  106 J

  • A check direct from the units converter: 5.81  106 ft.lbf = 7.88  106 J …OK

Potential energy
Potential Energy

  • GPE is NOT the only form of PE.

    • Chemical, nuclear and electromagnetic are other forms of PE

    • For us, chemical and electrical energy are so important that we will reserve extra chapters and lectures to them for later presentation.

Thermal energy
Thermal Energy

  • Thermal energy, often referred to as heat,is a very special form of kinetic energy because it is the random motion of trillions and trillions of atoms and molecules that leads to the perception of temperature

    • All higher forms of energy dissipate to thermal energy, the ultimate energy sink.

    • The laws of thermodynamics state 1) all energy is conserved and 2) that the thermal energy in the universe, corrected for temperature, always increases.


  • We have defined energy is the capability to do work

    • But energy comes in different guises

      • Potential, translational kinetic, rotational kinetic, thermal and others

    • Energy can be converted from one form to another

      • The energy in the Universe is conserved

      • A “control volume” is a subset of the Universe you construct to isolate the problem of interest. It exchanges energy with the rest of the Universe

Energy conservation

: Energy exchanges

: Energy exchanges

“The Universe”

“The Universe”





System energy changes

System energy changes



Universe energy changes = 0

Universe energy changes = 0

Energy Conservation

  • Energy = F distance is generic equation for energy

  • Energy is conserved (although it may change form)

Example of a book lying on a table and then falling on ground

Energy conservation1

C.V. boundary

C.V. boundary

This class room

This class room

Insulated walls

Insulated walls



Control volume

Control volume



Energy Conservation

  • Example of a control volume

  • The energy in the room is constant unless we allow exchange with the Universe

    • E.g., a person could walk through the door and add energy

    • A heating duct could also add thermal energy

    • On a winter day, a window could break and the c.v. would lose thermal energy

Application of control volumes
Application of Control Volumes

  • The TKE of the vehicle, RKE of the wheels, electrical energy in the lights, thermal energy lost from the radiator, etc.

    • We deduce that the source of all these energies is exactly equal to the loss in chemical (potential) energy in the fuel.

Summary energy
Summary: Energy

  • We specifically identified gravitational, potential, and thermal energy

  • We learned that energy is conserved in the Universe, but not necessarily in a control volume.

    • Deficiencies within a control volume mean that energy in leaking in or out of the control volume at an exactly compensating amount.