Grade 12 · Physics
Apply the work–energy theorem
The work–energy theorem states that the net work on an object equals its change in kinetic energy: Wnet = ½mv² − ½mu². In Grade 12 Physical Sciences, work must include every force doing work along the displacement. A force perpendicular to displacement does zero work. Use the net work rather than assuming all applied work becomes kinetic energy when friction acts.
Original explanation prepared with AI assistance.
Lees hierdie verduideliking in AfrikaansWhat you will practise
Relate signed net work to a change in kinetic energy.
The method
Calculate net work or the change in kinetic energy using consistent SI units.
Worked example
A 2 kg object speeds up from 3 m/s to 5 m/s. Find the net work done on it.
- Initial kinetic energy is ½(2)(3²) = 9 J.
- Final kinetic energy is ½(2)(5²) = 25 J.
- Wnet = 25 − 9 = 16 J.
- Positive net work agrees with the increase in speed.
Common mistake
Square each speed separately. v² − u² is not (v − u)².
Try it yourself
A 4 kg object slows from 6 m/s to 2 m/s. Find the net work.
Show the explained solution
Wnet = ½(4)(2² − 6²) = 2(4 − 36) = −64 J. The negative sign indicates a loss of kinetic energy.
Curriculum and sources
Use the official CAPS subject document and your teacher’s annual plan to confirm the sequence and scope for your class. The link below is an official reference, not evidence of DBE approval of this explanation.
DBE: CAPS for the FET phaseFind your next lesson
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