Calculating work done and power · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Push along, and count the joules

Science · Energy · ages 15-16
Name ______________________   Date ____________

  1. A constant force of 50 N pushes a box 4 m along the force. How much work does this force do in J?

    Answer: ______________

  2. For a constant force along an object's motion, how is work done calculated?

    • Force times distance along the force
    • Force divided by distance
    • Mass times speed

  3. The upward supporting force does work on a bag carried horizontally at constant height.

    Circle one:   True   False

  4. Which pairing gives the correct units?

    • Watts for work, joules for power
    • Joules for work, watts for power
    • Newtons for work, joules for power

  5. A motor does 1200 J of work in 5 s. What is its power in W?

    Answer: ______________

  6. You push a wall with no displacement. How much work does your push do on the wall?

    • Some, effort counts
    • None, nothing moved
    • Double, time counts

  7. A student calculates power for 900 J of work in 3 s as 900 J times 3 s. Which correction is needed?

    • Work should be in watts
    • Seconds should be minutes
    • Time should divide, not multiply

  8. A crane does 1500 J of work in 8 s, then another 1500 J in 12 s. What is its average mechanical power over both stages in W?

    Answer: ______________

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Push along, and count the joules W1-mt_udulQxseg1-s1

  1. 200 · Work by this force is 50 × 4 = 200 J.
  2. Force times distance along the force · Work = force times distance moved along that force, in joules.
  3. False · The displacement is perpendicular to the upward force. Work by that force on the bag is zero.
  4. Joules for work, watts for power · Work is transferred energy measured in joules. Power is work per second measured in watts.
  5. 240 · Power = work divided by time: 1200 divided by 5 = 240 W.
  6. None, nothing moved · The wall's displacement is zero, so work by your push on the wall is zero. Your body can still use energy.
  7. Time should divide, not multiply · Average power is work divided by time: 900 ÷ 3 = 300 W. Multiplication gives J s, not watts.
  8. 150 · Total work is 3000 J and total time is 20 s. Average power is 3000 ÷ 20 = 150 W.
Worksheet · LightMySky