WAEC Physics 2012 Paper

Concept:The apparent weight of a person in a lift changes only when the lift has acceleration.Explanation:The scale reading is the normal reaction $R$ exerted by the scale on the girl.Her actual weight is $mg$, where $m$ is her mass and $g$ is the acceleration due to gravity.If the lift accelerates downward with acceleration $a$, the equation of motion is:$$mg - R = ma$$So, the scale reading is:$$R = m(g - a)$$Since $a$ is positive, $R < mg$.Thus, the reading on the scale is less than her actual weight.This condition occurs when the lift moves downward with uniform acceleration.If the lift moves at uniform speed, acceleration is zero, so the reading equals her weight.Answer:D. downward with uniform acceleration