Work Done (Cambridge International IGCSE Physics)

Work done & energy transfers

Spec point — 1.7.5 · Understand that mechanical or electrical work done is equal to the energy transferred Recall and use the equation for mechanical working W = Fd = ∆E

Work done & energy transfers

• Mechanical work is done when an object is moved over a distance by a force applied in the direction of its displacement

  • It is said that the force does work on the object
  • If a force is applied to an object but does not result in any movement, no work is done

• When work is done, energy is transferred

• Work done and energy transferred are equivalent quantities

Work is done when a force is used to move an object over a distance, and energy is transferred from the person to the box

Work done equation

• The formula for work done is:

$W=Fd=\Delta E$

• Where:

  • $W$ = work done, measured in joules (J)
  • $F$ = force applied, measured in newtons (N)
  • $d$ = distance moved in the direction of the force, measured in metres (m)
  • $\Delta E$ = energy transferred, measured in joules (J)

• Mechanical work done and electrical work done are equivalent to energy transferred

$\mathrm{work} \mathrm{done}=\mathrm{energy} \mathrm{transferred}$

• Work done can also have units of newton-metres since:

$1Nm=1J$

Examples of work done

• Work is done on a ball when it is lifted to a height:

  • A force is required to lift the ball
  • Work is done against the weight force to lift the ball through the gravitational field
  • Energy is transferred as work done

Work done lifting a ball

*Work is done against the ball's weight as it is lifted, transferring energy to its gravitational potential store *

• Work is done when a bird flies through the air

  • A force is required to overcome the drag force
  • Work is done against the drag force as the bird flies over a distance
  • Energy is transferred as work is done

Work done by a bird

The bird does work against air resistance (drag) as it flies through the air

Worked Example A car moving at speed begins to apply the brakes. The brakes of the car apply a force of 500 N, which brings it to a stop after 23 m.

Calculate the work done by the brakes in stopping the car.

[2]

Answer:

Step 1: List the known quantities

• Distance, $d=23m$
• Force, $F=500N$

Step 2: Write out the work done equation

$W=Fd$

Step 3: Calculate the work done on the car by the brakes

$W=500\times23$* [1 mark]

$W=11500J$* [1 mark]

Exam Hint Remember to always convert the distance into metres and force into newtons so that the work done, which is equal to force multiplied by the distance moved in the direction of the force, is in joules.

In addition, remember that work done is equivalent to energy transferred. For example, the work done by a force will usually describe a change in kinetic energy or gravitational potential energy of an object.

Energy, Work & Power Video

Spec point — 1.7.5 · nan