Question

A object is thrown straight down from the top of a building at a speed of 20m/s it hits the ground with a speed of 40/s
[g=9.8m/s2]
A-How high is the building?
B-How long was the object in the air? ​

Answer 1

Answer

A . 61.2 m

B . 2.04 s

Given

• A object is thrown straight down from the top of a building at a speed of 20 m/s it hits the ground with a speed of 40/s  [g=9.8 m/s²]

To Find

A - How high is the building?

B - How long was the object in the air? ​

Solution

$\rm Initial\ velocity\ ,u=20\ m/s\\\\\rm Final\ velocity\ ,v=40\ m/s \\\\\rm Acceleration\ due\ to\ gravity\ ,g=9.8\ m/s^2\\\\\rm A)\;\;Height,S=?\ m\\\\\rm B)\;\; Time,t=?\ s\\\\\rm Use\ 3rd\ equation\ of\ motion,\\\\\implies \rm v^2-u^2=2gs\\\\\implies \rm 40^2-20^2=2(9.8)s\\\\\implies \rm 1600-400=19.6 \; s\\\\\implies \rm 1200=19.6\ s\\\\\implies \rm s=61.2\ m$

$\rm Use\ 1st\ equation\ of\ motion\ ,\\\\\implies \rm v=u+gt\\\\\implies \rm 40=20+(9.8)t\\\\\implies \rm 9.8\ t=20\\\\\implies \rm t=2.04\ sec$