Question

From the top of a 40 m tall multi-storeyed building, a boy projects a stone vertically upwards with an initial velocity 10 m $s^{-2}$ such that, it eventually falls to the ground. After how long will the stone strike the ground? (Take g = 10 m $s^{-2}$)(in sec)

Answer 1

R.E.F image

$V_1=0m/s$ at max height

$V_g=0ms$ at ground level

height reached

to find above the building

$V^2-U^2=2gh$

$U^2-100=2\times 10\times h$

$h=5m$

time taken to reach 5 m is

$V=U+gt$

$0=10+10\times t$ $t=1\sec$

$-10=10\times t$

(b) stone reaches same point after 2 seconds

$h=45m$

$h=ut+1/2g{t}^2$

$45=0\times t+\frac{1}{2}\times 10\times t^2$

$\frac{45}{5}=t^2$ $t=3secs\left(a\right)$