First slide

Second law

Question

A house directs a horizontal jet of water moving with a velocity of 20 m/s on to a vertical wall. The cross-sectional area of the jet is 5 x 10^-4 m². If the density of water is 1000 kg / m³, the force on the wall assuming that the water comes to rest after striking the wall, is

Moderate

A

200 N
B

100 N
C

75 N
D

50 N

Solution

See fig.
Force exerted on the wall, $F = \frac{mv - mu}{t}$
Here u = 0, $∴ F = \frac{mv}{t}$
The mass of water coming out from the jet per second is
$\frac{m}{l} = \frac{volume \times density}{1} = \frac{(AV) \times d}{1}$
where A= area of cross section of the tube
and d = density of water.
$\begin{aligned} ∴ F & = {Adv}^{2} \\ = (5 \times 10^{- 4}) \times (1000) \times (20)^{2} \\ = 200 N . \end{aligned}$

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