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8th grade · Science · Unit 4 of 9

Work, Energy & Efficiency

Force, distance and wasted energy

What your child practices

Using the scientific terms force, displacement, work, energy and efficiency, calculating work as force times displacement, finding efficiency, and how friction and design affect the energy lost from machines.

Arizona Academic Standards learning standard: 8.P4U1.3 · how energy moves from one store to another

How we explain it

Children can open this short lesson before they practice.

  1. Work is done when a force moves an object through a distance. Work = force × displacement, in joules.
  2. Efficiency = useful energy out ÷ energy in × 100%.
  3. Friction turns some energy into heat, so no machine is 100% efficient.

Example: A motor takes in 400 J and gives 300 J of useful energy. What is its efficiency? 300 ÷ 400 = 0.75. 0.75 × 100% = 75%. Answer: 75%

Work, Energy & Efficiency in Arizona

In the Arizona Academic Standards, this is unit 4 of 9 in 8th grade Science, after Particles & Mixtures and before Light & Radiation.

Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.

Source: Arizona Department of Education

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    A force of 15 N moves a crate 5 m in the direction of the force. How much work is done, in J?

    answer boxJ

  2. Question 2

    When we say energy dissipates from a machine, we mean it…

    • is stored forever
    • turns into mass
    • is destroyed
    • spreads out as less useful energy, such as heat and sound

  3. Question 3

    A streamlined shape helps a cyclist go farther on the same energy because it…

    • adds friction
    • removes gravity
    • reduces air resistance
    • adds weight

  4. Question 4

    A machine does 50 J of work moving a load 5 m. What force did it apply, in N?

    answer boxN

  5. Question 5

    Where does much of the “lost” energy go in a bicycle chain?

    • to heat from friction
    • to a new type of matter
    • nowhere; energy is created
    • to the ground as mass

  6. Question 6

    Efficiency compares…

    • force with distance
    • mass with volume
    • speed with time
    • useful energy output with total energy input

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