Grade 8 · Science · Unit 10 of 13
Work, Energy & Efficiency
Force, distance and wasted energy
What your child practises
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.
Ontario Curriculum expectation: D2.4, D2.5, D2.7 · displacement, force, work, energy and efficiency, and how energy dissipates from mechanical systems
Sample questions
A taste of this unit. In the app, questions change every time and get easier or harder to match your child.
Question 1
In science, work is done when a force…
- is thought about
- is applied but nothing moves
- is read in a book
- moves an object through a distance
Show answer
moves an object through a distance
Hint: Work needs both a force and movement.
Question 2
An LED bulb is more efficient than an incandescent bulb because it…
- produces more heat
- turns more of the electrical energy into light
- is made of glass
- uses no electricity
Show answer
turns more of the electrical energy into light
Hint: An incandescent bulb gives off most of its energy as heat.
Question 3
Where does much of the “lost” energy go in a bicycle chain?
- to the ground as mass
- to a new type of matter
- to heat from friction
- nowhere; energy is created
Show answer
to heat from friction
Hint: Friction changes some energy into heat.
Question 4
A motor takes in 200 J of energy and gives 80 J of useful energy. What is its efficiency, in %?
answer box%
Show answer
40 %
Hint: Efficiency = useful output ÷ total input × 100.
Question 5
A force of 10 N moves a crate 8 m in the direction of the force. How much work is done, in J?
answer boxJ
Show answer
80 J
Hint: Work = force × displacement.
Question 6
When we say energy dissipates from a machine, we mean it…
- turns into mass
- is destroyed
- is stored forever
- spreads out as less useful energy, such as heat and sound
Show answer
spreads out as less useful energy, such as heat and sound
Hint: Energy is conserved, but it can spread out in forms that are hard to use.