A ball is dropped from the top of a building 100 ,m high. At the … - Vidyadip

MCQ

A ball is dropped from the top of a building $100\,m$ high. At the same instant another ball is thrown upwards with a velocity of $40\,m / s$ from the bottom of the building. The two balls will meet after $..........\,s$

A
$3$
B
$2$
✓
$2.5$
D
$5$

✓

Answer

Correct option: C.

$2.5$

c
(c)

Let balls meet after $t s$. The distance travelled by the ball coming down is

$s_1=\frac{1}{2} g t^2$

Distance travelled by the other ball

$s_2=40 t-\frac{1}{2} g t^2$

$\because \quad s_1+s_2=100\,m$

$\therefore \quad \frac{1}{2} g t^2+40 t-\frac{1}{2}\,g t^2=100\,m$

$t=\frac{100}{40}=2.5\,s$

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