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4-1.Newton's Laws of Motion
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A coin is dropped in a lift. It takes time $t_1$ to reach the floor when lift is stationary. It takes time $t_2$ when lift is moving up with constant acceleration, then
A
$t_1 = t_2$
B
$t_1 > t_2$
C
$t_2 < t_1$
D
$t_1 > t_2$
Solution
Time taken by coin to reach the floor is given by$:$
$h=1 / 2 \mathrm{gt}^{2}$ $(\because u=0)$
or $t=\sqrt{\frac{2 h}{g}}$
In stationary lift, $\mathrm{t}_{1}=\sqrt{\frac{2 \mathrm{h}}{\mathrm{g}}}$
In upward moving lift with constant acceleration
a,
$\mathrm{g}^{\prime}=\mathrm{g}+\mathrm{a}$
$\therefore t_{2}=\sqrt{\frac{2 h}{(g+a)}}$
Clearly, $\quad \mathrm{g}^{\prime}>\mathrm{g}$
$\text { Thus, } \quad \mathrm{t}_{2}<\mathrm{t}_{1}$
Standard 11
Physics
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