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Grade 9 Science — Work, energy and power

Work out work done, kinetic energy, potential energy and power, say when work is positive, negative or zero, and explain what a pulley, a ramp and a lever actually change.

Name: ________________________

Before you start

The parts of a lever, each one named.

1234
  1. 1. The load, the weight to be lifted
  2. 2. The fulcrum, the fixed point the bar turns about
  3. 3. The effort, the push you apply
  4. 4. The load arm, from the load to the fulcrum

Put the fulcrum close to the load and the effort arm becomes long. A small push over a long distance then lifts a large load through a short one.

  1. 1.A ball of mass 0.2 kg moves at 30 m/s. What is its kinetic energy, in J? Answer with a number.

  2. 2.A lever balances with an effort of 20 N acting 60 cm from the fulcrum and the load 15 cm from it on the other side. How large is the load, in N? Answer with a number.

  3. 3.A book of mass 2 kg is raised 1.5 m above the floor. Taking g as 10 m/s², what is its potential energy, in J? Answer with a number.

  4. 4.A pump does 600 J of work in 4 s. What is its power, in W? Answer with a number.

  5. 5.A student pushes as hard as she can against a rigid wall for a minute and gets tired. How much work has she done on the wall?

    • a) None, because the wall did not move
    • b) A large amount, because she applied a large force
    • c) It cannot be worked out without knowing her mass
    • d) A small amount, because the wall moved a tiny distance

  6. 6.A force of 25 N moves a crate 4 m along the direction of the force. How much work is done on the crate, in J? Answer with a number.

  7. 7.A machine raises a load of 300 N when an effort of 100 N is applied. What is its mechanical advantage? Answer with a number.

  8. 8.A stone is dropped from a height and falls freely. What happens to its mechanical energy on the way down?

    • a) It rises, because the stone speeds up
    • b) It becomes zero the moment the stone starts moving
    • c) It stays the same, as potential energy turns into kinetic energy
    • d) It falls, because the stone loses height

Answer key — Grade 9 Science — Work, energy and power

  1. 1. 90
  2. 2. 80
  3. 3. 30
  4. 4. 150
  5. 5. a) None, because the wall did not move
  6. 6. 100
  7. 7. 3
  8. 8. c) It stays the same, as potential energy turns into kinetic energy