A) The lift was in constant motion upwards
B) The lift was in constant motion downwards
C) The lift while in motion upwards, suddenly stopped
D) The lift while in motion downwards, suddenly stopped
Correct Answer: C
Solution :
If the reading of the spring scale i.e., apparent weight of the person moving in a lift decreases, the formula for reaction, R must be \[m(g-a)\] there are only two possibilities of this, (i) the normal situation in which this equation is valid is the accelerated motion of the lift downwards. (ii) During upward accelerated motion of the lift the equation of reaction i.e. apparent weight is\[m(g+a)\]. If the upwards motion is retarded \[i.e.,\] acceleration is equal to\[-a\], the equation for upward motion also becomes\[m(g-a)\]. Now, it is given that the scale reading suddenly changes from \[60\,\,kg\] to \[50\,\,kg\] for a moment only and then comes back to the \[60\,\,kg\] mark. It means that for most part of the journey, since the reading is constant, the lift must have been in motion with uniform velocity either upwards or downwards. If in the downward motion, a sudden retardation is created, the equation of reaction will become\[m[g-(-a)]\,\,i.e.,\,\,m(g+a)\]. In that case the reading should momentarily increase. In the upward motion with constant velocity, for a sudden retardation, the equation of reaction will become \[m(g-a)\]and the reading of the scale will consequently decrease. Once the lift comes to rest in either of the case, the reaction will be same as that in the case of constant velocity motion\[i.e.,\,\,mg\].
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