First slide

Rectilinear Motion

Question

An object falls from a bridge that is 45m above the water. It falls directly into a small row-boat moving with constant velocity that was 12m from the point of impact when the object was released. What was the speed of the boat?

Moderate

A

1 m/s
B

2 m/s
C

3 m/s
D

4 m/s

Solution

Boat velocity $v = \frac{s}{t}$
$\Rightarrow t = \frac{s}{v} = \frac{12}{v} s ........ (i)$
In the same time the ball has to reach the boat
$\begin{array}{l} ∴ t = \sqrt{\frac{2 h}{g}} = \sqrt{\frac{2 \times 45}{10}} = \sqrt{9} = 3 s \\ ∴ From (i) v = \frac{s}{t} = \frac{12}{3} = 4 m / s \end{array}$

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