Discussion
An object of certain mass (in kg) is raised at a 4 m height above the ground. If the potential energy of the object is 520 J, find the mass of the object (Given \(g = 10\ \mathrm{ms^{-2}}\)).
Answer: D
The mass is 13 kg.
Gravitational potential energy is \(E_{p} = mgh\). Rearranging for the mass:
\[ \begin{aligned} m &= \frac{E_{p}}{gh} \\ &= \frac{520}{10 \times 4} = \frac{520}{40} \\ &= 13\ \text{kg} \end{aligned} \]
Check: \(13 \times 10 \times 4 = 520\) J. 10, 11 and 12 kg give 400, 440 and 480 J, which are all short of 520 J.
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