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WAEC Physics 2013 Paper

Question : 2

Total: 46

Which of the following units is equivalent to the watt?

Solution:

Concept:

Power is the rate of doing work, and the watt is its SI unit.

Explanation:

Power is given by the formula:

\text{Power} = \frac{\text{Work done}}{\text{time}}

Work done is equal to force multiplied by distance:

\text{Work} = \text{Force} \times \text{distance}

Force is mass times acceleration, so we can write:

\text{Power} = \frac{\text{mass} \times \text{acceleration} \times \text{distance}}{\text{time}}

Substitute SI base units for each quantity:

\text{Power} = \frac{\mathrm{kg} \times \mathrm{m} \times \mathrm{m}}{\mathrm{s}^{2} \times \mathrm{s}}

Simplify the expression:

1\ \text{watt} = 1\ \mathrm{kg}\,\mathrm{m}^{2}\,\mathrm{s}^{-3}

This matches option B.

Answer:

\mathrm{kg}\,\mathrm{m}^{2}\,\mathrm{s}^{-3}

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