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Class 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 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.

  2. 2.A goalkeeper applies 200 N to a ball while her hands move back 0.15 m, so the ball's displacement opposes her force. How much work does she do on the ball, in J? Include the minus sign.

  3. 3.Put the working of a slingshot in order, starting from The elastic band is pulled back, and work is done on it.

    • The band returns to the shape it started in
    • The stone gains kinetic energy and shoots forward
    • The elastic band is pulled back, and work is done on it
    • The band is let go and pushes on the stone
    • The stretched band holds that work as stored energy

  4. 4.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.

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

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

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

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

  8. 8.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.

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

  1. 1. 100
  2. 2. -30
  3. 3. 1. The elastic band is pulled back, and work is done on it 2. The stretched band holds that work as stored energy 3. The band is let go and pushes on the stone 4. The stone gains kinetic energy and shoots forward 5. The band returns to the shape it started in
  4. 4. 3
  5. 5. 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
  6. 6. b) None, because the wall did not move
  7. 7. b) It becomes four times as large
  8. 8. 30