[2 Mark Question Answer]

Question 12 Marks

A jeweller claims to make ornaments of pure gold of relative density 19.3. A customer buys from him a bangle of weight 25.25 gf . The customer then weighs the bangle under water and finds its weight 23.075 g with the help of suitable calculations explain whether the bangle is of pure gold or not.
[R.D. of gold is 11.61 and bangle is not of pure gold] Ans. R.D of pure gold $=19.3$...given

Answer

R.D of pure gold $=19.3$
$
\text { Also R.D }=\frac{\text { weight of bangle in air }}{w t . \text { in air }-w t . \text { of bangle in water }}
$
R.D of gold $=\frac{25.25}{25.25-23.075}=\frac{25.25}{2.175}$
R.D of gold bangle $=11.75$ which is not as 19.3
Hence, gold is not PURE.

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Question 22 Marks

A wooden block is weighed with iron, such that combination just floats in water at room temperature. State your observations when :
(1) water is heated above room temperature
(2) water is cooled below $4^{\circ} C$. Give reasons to your answers in (1) and (2).

Answer

(1) We know density of water decreases with rise in temperature and hence upthrust decreases.
(2) Density of water is maximum at $4^{\circ} C$. When water cooled below $4^{\circ} C$, then its density and hence upthrust acting on it decreases. So, wooden block weighed with iron, sinks more than earlier.

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Question 32 Marks

A solid weighs 105 kgf in air. When completely immersed in water, it displaces 30,000 cm of water, calculate relative density of solid.

Answer

$\begin{array}{l}
\text { Weight of solid in air }=105 kgf \\
\text { Volume of solid }=\text { Volume of } \text { water displaced }=30000 cm^2 \\
=30000 \times 10^4 m^2=0.03 m^2 \\
\rho_w=\text { Density of water }=1000 kgm \text { : } \\
\text { Wt. of water displaced by solid. } \\
=V \rho_w g=0.03 \times 1000 \times g=30 kgf \\
\text { R.D. of solid }=\frac{\text { Weight of solid in air }}{\text { Weigh of water displaced by solid }} \\
=\frac{105}{30}=3.5
\end{array}$

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Question 42 Marks

Why does a ship made of iron not sink in water, while an iron nail sinks in it?

Answer

Density of iron is more than density of water, therefore weight of iron nail is more than wt. of water displaced by it and nail SINKS. While shape of iron ship is made in such a way that it displaces MORE WEIGHT OF WATER than its own weight. Secondly the ship is HOLLOW and THE EMPTY SPACE contains AIR which makes the AVERAGE DENSITY OF SHIP LESS THAN THAT OF WATER and hence ship floats on water.

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PHYSICS [2 Mark Question Answer]

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