Conservation of Energy — Drills · Grade 8 Science

Chapter: Energy & Heat Transfer

What these drills covers

By the end of Conservation of Energy you should be able to:

8 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: law of conservation of energy, mechanical energy, energy dissipation, model, evidence, system boundary, assumption.

Problem 1

Conservation of energy states that in a closed system the total energy
  1. increases
  2. decreases
  3. stays the same
  4. becomes zero
Answer and explanation

stays the same

Energy changes form and moves, but the total is unchanged.

Problem 2

A ball has 60 J of gravitational potential energy at the top of a drop. Ignoring air resistance, how many joules of kinetic energy does it have just before landing?
Answer and explanation

60 J

All the potential energy converts to kinetic, so 60 J.

Problem 3

A pendulum has 40 J at its highest point. Halfway down it has 25 J of kinetic energy. How many joules of potential energy remain?
Answer and explanation

15 J

40 − 25 = 15 J, assuming no losses.

Problem 4

A bouncing ball returns lower each time. This shows that
  1. energy was destroyed
  2. energy transferred to the surroundings as heat and sound
  3. the ball gained mass
  4. gravity weakened
Answer and explanation

energy transferred to the surroundings as heat and sound

Energy left the ball but still exists, dispersed in the floor, the air and the ball itself.

Problem 5

A system starts with 500 J. After running, 350 J is accounted for usefully and the rest is heat. How many joules became heat?
Answer and explanation

150 J

500 − 350 = 150 J; the total is still 500 J.

Problem 6

A perpetual motion machine is impossible mainly because
  1. energy cannot be transformed
  2. some energy always disperses to the surroundings
  3. gravity is too strong
  4. machines cannot be built precisely
Answer and explanation

some energy always disperses to the surroundings

Friction and resistance always transfer some energy away, so a machine cannot keep returning all of it.

Problem 7

Explain why 'energy is conserved' and 'energy is wasted' are both true statements.
Answer and explanation

Conserved means the total quantity never changes — it is only transferred or transformed. Wasted means some of it ends up in a form or place where it cannot do the job intended, usually as heat spread through the surroundings. Both are true at once: the energy still exists, but it has become dispersed and no longer useful, which is why devices need a continuing supply.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 8

Describe how you would test conservation of energy with a pendulum, and what would count as a fair result.
Answer and explanation

Release the pendulum from a measured height and record the height it reaches on the opposite side, repeating several times. If energy were perfectly conserved within the pendulum, the heights would match. In practice the return is slightly lower each swing, and a fair result accounts for that by identifying where the missing energy went — air resistance and friction at the pivot — rather than concluding energy disappeared.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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