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6.System of Particles and Rotational Motion
normal
A student of mass $M$ is $1.5 \,m$ tall and has her centre of mass $1 \,m$ above ground when standing straight. She wants to jump up vertically. To do so. she bends her knees so that her centre of mass is lowered by $0.2 \,m$ and then pushes the ground by a constant force F. As a result, she jumps up such that the maximum height of her feet is $0.3 \,m$ above ground. The ratio $F / Mg$ is
A
$1.5$
B
$2.5$
C
$3.5$
D
$4.5$
(KVPY-2021)
Solution
(B)
Applying work energy theorem from just before jump to final height
$F \times 0.2- Mg (0.2+0.3)=0-0$
$F \times 0.2= Mg \times 0.5$
$\frac{ F }{ mg }=\frac{5}{2}=2.5$
Standard 11
Physics