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8th grade · Science · Unit 13 of 16

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.

Alabama Course of Study content standard: 8.15 · work, energy transformations and efficiency

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 Alabama

In the Alabama Course of Study, this is unit 13 of 16 in 8th grade Science, after Kinetic & Potential Energy and before Circuits & Static.

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: Alabama State Department of Education (ALSDE)

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 motor takes in 200 J of energy and gives 150 J of useful energy. What is its efficiency, in %?

    answer box%

  2. Question 2

    Which unit is used to measure work and energy?

    • meter (m)
    • kilogram (kg)
    • joule (J)
    • newton (N)

  3. Question 3

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

    answer boxN

  4. Question 4

    Displacement is…

    • how heavy it is
    • the distance an object moves in a certain direction
    • how hot it is
    • how fast it spins

  5. Question 5

    Efficiency compares…

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

  6. Question 6

    The force on a cart doubles and the distance stays the same. What happens to the work?

    • It becomes zero.
    • It halves.
    • It doubles.
    • It stays the same.

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