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6th grade · Science · Unit 6 of 6

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.

Alaska content standards learning standard: MS-PS3-5 · when an object's kinetic energy changes, energy is transferred to or from it, as in work

How we explain it

Children can open this short lesson before they practice.

  1. In science, work means a force moves something a distance: work = force × distance.
  2. Work and energy are measured in joules (J).
  3. No machine is perfect: some energy is wasted, often as heat.
  4. Efficiency = useful energy out ÷ energy in × 100%.

Example: A motor takes in 500 J and gives 350 J of useful energy. What is its efficiency? Efficiency = useful ÷ total × 100%. 350 ÷ 500 = 0.7. 0.7 × 100% = 70%. Answer: 70%

Work, Energy & Efficiency in Alaska

In the Alaska content standards, this is unit 6 of 6 in 6th grade Science, after Engineering Design.

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: Alaska Department of Education and Early Development

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

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

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

  2. Question 2

    A machine does 175 J of work moving a load 7 m. What force did it apply, in N?

    answer boxN

  3. Question 3

    A motor takes in 250 J of energy and gives 150 J of useful energy. What is its efficiency, in %?

    answer box%

  4. Question 4

    A force of 20 N moves a crate 4 m in the direction of the force. How much work is done, in J?

    answer boxJ

  5. Question 5

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

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

  6. Question 6

    How does oiling a bicycle chain help efficiency?

    • The chain gets heavier.
    • Less friction means less energy lost as heat.
    • More friction means more energy is useful.
    • The energy disappears.

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