A.3 Work, energy and power

Objective notes

14 learning objectives

—A.3.1—Conservation of energy

• Principle of the conservation of energy.

—A.3.2—Work as energy transfer

• Work done by a force is equivalent to a transfer of energy.

—A.3.3—Sankey diagrams

• Energy transfers can be represented on a Sankey diagram.

—A.3.4—Work by constant force

• Work by constant force along displacement: W = Fs cos θ.

—A.3.5—Work-energy change

• Work done by the resultant force on a system is equal to the change in the energy of the system.

—A.3.6—Mechanical energy

• Mechanical energy includes kinetic, gravitational potential and elastic potential energy.

—A.3.7—Mechanical energy conservation

• Mechanical energy is conserved when friction/resistive forces are absent.

—A.3.8—Mechanical energy transformations

• If mechanical energy is conserved, work transforms energy between mechanical forms.

• Relevant forms: translational kinetic, near-Earth gravitational potential and elastic potential.

—A.3.9—Kinetic energy

• Translational kinetic energy: Ek = 1/2mv^2 = p^2/2m.

—A.3.10—Gravitational potential energy

• Near Earth, gravitational potential energy change: ΔEp = mgΔh.

—A.3.11—Elastic potential energy

• Elastic potential energy: EH = 1/2k(Δx)^2.

—A.3.12—Power

• Power is rate of work or energy transfer: P=ΔW/Δt=Fv.

—A.3.13—Efficiency

• Efficiency: η=Eoutput/Einput=Poutput/Pinput.

—A.3.14—Fuel energy density

• Energy density of the fuel sources.