WAEC Physics 2020 Paper

Concept:When an elevator accelerates upwards, the apparent weight of a person increases because the scale must provide both the support against gravity and the extra upward acceleration.Explanation:Let $m = 60\ \mathrm{kg}$ be the mass of the man.The acceleration due to gravity is $g = 10\ \mathrm{m\,s}^{-2}$.The upward acceleration of the elevator is $a = 5\ \mathrm{m\,s}^{-2}$.The resultant upward force on the man is given by $F = m(g + a)$.Substitute the values: $F = 60(10 + 5)$.This gives $F = 60 \times 15 = 900\ \mathrm{N}$.This value of $F$ is the normal reaction of the scale on the man, which is equal to the reading of the weighing balance in newtons.If the scale is calibrated to read mass, divide by $g$: $\frac{900}{10} = 90\ \mathrm{kg}$.Answer:The reading of the scale is $900\ \mathrm{N}$, equivalent to an apparent mass of $90\ \mathrm{kg}$.