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2.Motion in Straight Line
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A ball is thrown vertically upwards with a veloctty of $20 \;\mathrm{m} \mathrm{s}^{-1}$ from the top of a multistorey building. The helght of the point from where the ball is thrown is $25.0\; \mathrm{m}$ from the ground. How high will the ball rise? Take $g=10 \;\mathrm{m} \mathrm{s}^{-2}$
A
$25\;m$
B
$20\;m$
C
$5\;m$
D
$35\;m$
Solution
Let us take the $y$ -axis in the vertically upward direction with zero at the ground
$\text { Now } v_{o} =+20 \mathrm{m} \mathrm{s}^{-1}$
$a =-g=-10 \mathrm{m} \mathrm{s}^{-2}$
$v =0 \mathrm{m} \mathrm{s}^{-1}$
If the ball rises to height $y$ from the point of launch, then using the equation
$v^{2}=v_{0}^{2}+2 a\left(y-y_{0}\right)$
we get
$0=(20)^{2}+2(-10)\left(y-y_{0}\right)$
Solving, we get, $\left(y-y_{0}\right)=20 \mathrm{m}$
Standard 11
Physics
