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Lesson: Chapter 18 — Work, Energy and Power

18.3. Power 5 of 5

Putting the chapter together

Three problems that use more than one section at once, then everything the chapter asked you to keep.

Mixed exercise

Question 1

  1. A child lifts a bag of mass 4 kg to a height of 1.5 m. How much work is done? (g = 10 m s-2)
  2. If the time taken to do that work was 3 seconds, what is the rate of doing work, that is, the power?

Question 2

An object of mass 800 g is thrown vertically upwards with a velocity of 20 m s-1.

  1. What is its kinetic energy at the moment it begins to rise from the ground?
  2. How long does the object take to reach its maximum height?
  3. What is the maximum height it rises to?
  4. What is its potential energy at that maximum height?

Question 3

A child of mass 35 kg climbs a flight of stairs to a vertical height of 4 m.

  1. How much work does the child do?
  2. If the child took one minute to climb the stairs, what is their rate of doing work, that is, their power?

Figure to be added

Figure for question 3 — a child climbing a flight of stairs, seen from the side.

Summary

  • The work done by a force is equal to the product of the magnitude of the force and the displacement along the direction of that force — that is, work = force × displacement in the direction of the force.
  • If there is no loss of energy, the amount of work done is equal to the energy spent.
  • The unit that measures work and energy is the joule (J).
  • The two kinds of mechanical energy are potential energy and kinetic energy.
  • The kinetic energy of an object depends on its mass and its velocity, and is calculated from Ek = ½ m v2.
  • Three factors affect gravitational potential energy — mass (m), the acceleration due to gravity (g) and height (h) — and it is calculated from Ep = m g h.
  • When an object rises under gravity its kinetic energy is lost, and the energy lost becomes potential energy.

Technical terms

Sinhala English
කාර්යය Work
ශක්තිය Energy
යාන්ත්‍රික ශක්තිය Mechanical energy
චාලක ශක්තිය Kinetic energy
විභව ශක්තිය Potential energy
ජවය Power

Three quantities, three units

Work and energy are both measured in joules; power is measured in watts. If an answer comes out in the wrong one of those, the formula was arranged wrongly.