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Year 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.Sort each case by the work the named force does on the named object.

    Groups: Positive work done on the object · Negative work done on the object · No work done on the object

    • A goalkeeper stopping a moving ball
    • Friction acting on a sliding stack of coins
    • A girl slowly lowering a dumbbell
    • A person pushing hard on a rigid wall
    • A boy pushing a wheelchair forward

  2. 2.Three simple machines are sketched below. Tap the one that changes the direction of the force you apply without reducing how large it has to be.

    Write the number of the part.

    123

  3. 3.A vehicle's speed doubles. What happens to its kinetic energy?

    • a) It becomes four times as large
    • b) It becomes twice as large
    • c) It stays the same
    • d) It becomes eight times as large

  4. 4.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.

  5. 5.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.

  6. 6.Match each form of energy to what it is.

    • Mechanical energy
    • Thermal energy
    • Chemical energy
    • Nuclear energy
    • Electrical energy
    • Energy an object has from its motion or its position
    • Energy stored in the nuclei of atoms
    • Energy that makes things warm or hot
    • Energy stored in fuels and food in the bonds between atoms
    • Energy to do with the position or the motion of charges

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

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

  1. 1. A boy pushing a wheelchair forward → Positive work done on the object; A goalkeeper stopping a moving ball → Negative work done on the object; Friction acting on a sliding stack of coins → Negative work done on the object; A girl slowly lowering a dumbbell → Negative work done on the object; A person pushing hard on a rigid wall → No work done on the object
  2. 2. 1 — A wheel with a rope over it
  3. 3. a) It becomes four times as large
  4. 4. 30
  5. 5. 100
  6. 6. Mechanical energy → Energy an object has from its motion or its position; Thermal energy → Energy that makes things warm or hot; Chemical energy → Energy stored in fuels and food in the bonds between atoms; Nuclear energy → Energy stored in the nuclei of atoms; Electrical energy → Energy to do with the position or the motion of charges
  7. 7. 3
  8. 8. b) It stays the same, as potential energy turns into kinetic energy