[2 Mark Questions]

Question 12 Marks

Numerical Problem.

Calculate the apparent weight of wood floating on water if it weighs 100g in air.

Answer

Apparent weight = Weight of the body – Weight of liquid
Since the body is floating the two are equal. So, apparent weight is zero.
100 – 100 = 0

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Question 22 Marks

Numerical Problem.

Density of mercury is 13600 kg m^–3. Calculate the relative density.

Answer

Relative Density $=\frac{\text { Density of Mercury }}{\text { Density of water }}$ at $4^{\circ} C$
R.D. = 13.6

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Question 32 Marks

Numerical Problem.

A block of wood of weight 200 g floats on the surface of water. If the volume of block is 300 cm³ , calculate the upthrust due to water.

Answer

Upthrust of floating object = weight of the water displaced
Weight = mg
$=0.200 Kg \times \frac{10 m }{ s ^2}$
= 2N

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Question 42 Marks

State the laws of flotation.

Answer

Laws of flotation are,

The weight of a floating body in a fluid is equal to the weight of the fluid displaced by the body.
The centre of gravity of the floating body and the centre of buoyancy are in the same vertical line.

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Question 52 Marks

How does an object’s density determine whether the object will sink or float in water?

Answer

Whether an object will sink or float in a liquid is determined by the density of the object compared to the density of the liquid. If the density of a substance is less than the density of the liquid it will float. For example, a piece of wood which is less dense than water will float on it. Any substance having more density than water (for example, a stone), will sink into water.

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Question 62 Marks

With an appropriate illustration prove that the force acting on a smaller area exerts a greater pressure.

Answer

Consider standing on loose sand. Your feet go deep into the sand. Now, when you lie down on the sand, you will find that your body will not go that deep into the sand. In both the cases the force exerted on the sand is the weight of your body which is the same. This force acting perpendicular to the surface is called thrust. When you stand on loose sand, the force is acting on an area equal to the area of your feet.

When you lie down, the same force acts on an area of your whole body, which is larger than the area of your feet. Therefore the effect of thrust, that is, pressure depends on the area on which it acts. The effect of thrust on sand is larger while ‘ standing than lying.

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Question 72 Marks

Why does a boat with a hole in the bottom would eventually sink?

Answer

A boat with a hole at the bottom allows water to enter it, thus increasing its weight and hence it sinks. As water starts entering the boat through the hole, the boat starts to get heavier, so it starts to sink, trying to displace more water. But the water keeps coming as the hydrostatic pressure at the hole is always higher than the atmospheric pressure pushing down on the surface of the water in the boat.

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Question 82 Marks

If you put one ice cube in a glass of water and another in a glass of alcohol, what would you observe? Explain your observations.

Answer

Ice floats in water and not in alcohol. This is because the density of ice is $\frac{0.917 g }{ cc }$ which is lower than that of water which is 1. Whereas, the density of ethanol (alcohol) is only $\frac{0.789 g }{ cc }$ which is lesser than ice, hence it floats in alcohol.

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Question 92 Marks

Higher Order Thinking Skill.

How does a fish manage to rise up and move down in water?

Answer

Fish has an internal swim bladder which is filled with gas. When it needs to rise or descend, it changes the volume and its density by filling this bladder with oxygen collected from the surrounding water through gills. When the bladder is filled with oxygen gas, the fish has a greater volume, with minimal increase in weight. When the bladder is expanded, it displaces more water and so experiences a greater force of buoyancy.

When the bladder is completely inflated, the fish has maximum volume and is pushed to the surface. When the bladder is completely deflated, the fish has minimum volume and sinks to the ocean floor.

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Question 102 Marks

Higher Order Thinking Skill.

How high does the mercury barometer stand on a day when atmospheric pressure is 98.6 kPa?

Answer

$\begin{aligned} & H =\frac{ P _{\max }}{ P _{ Hg }}=98.6 \times 10 \times 10 \times 10 \\ & =\frac{( N / m )^2}{13.6 \times 10 \times 10 \times 10 Kg / m ^3} \\ & =\frac{9.8 m }{ s ^2} \\ & =740 mm \end{aligned}$

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Question 112 Marks

You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than the other. Can you say which one is heavier and why?

Answer

The bag containing the iron bar is heavier than cotton. Reason: Although both of them have the same weight, the bag of the iron bar has less volume so more dense compared to the bag of cotton which has more volume and less dense.

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Question 122 Marks

Differentiate positive & negative buoyant.

Answer

Positive Buoyant

Weight of the object is less than the amount of water displaced.
More buoyant force
Object will float

Negative Buoyant :

Weight of the object is more than the amount of water displaced.
Less Buoyant force.
Object will sink.

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Question 132 Marks

How do submarines sink and float in water?

Answer

Submarines change the level of floating by pumping in and pumping out water into its compartments.

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Question 142 Marks

What do you understand by the term “Buoyancy”.

Answer

When a body partially or completely immersed in a liquid (fluid), the pressure is more at the bottom and less at the surface in the liquid. This Pressure difference causes an upward force called “Buoyant force”. The phenomenon is called ‘Buoyancy’.

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Question 152 Marks

Name different types of Hydrometers with their applications.

Answer

Name of Hydrometer	Application (measuring)
1. Lactometer	Density of milk
2. Saccharometer	Density of sugar in a liquid
3. Alcoholometer	Higher levels of alcohols in Spirits

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Question 162 Marks

Define Relative Density.

Answer

Relative density of a substance is defined as ratio of density of substance to density of water at 4°C.
$\underset{\text}{\text { Relative density }}=\frac{\text { Density of substance }}{\text { Density of water at } 4^{\circ} C }$.

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Question 172 Marks

Why do Scuba divers wear special suits and equipment?

Answer

Deep-sea has pressure twice that of atmospheric pressure.
At high pressure, parts of our body including blood vessels & soft tissues cannot withstand it.
Hence they use special suits & equipment for protection.

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Question 182 Marks

How will you find the absolute pressure?

Answer

For pressures higher than atmospheric pressure:
Absolute pressure = Atmospheric pressure + Gauge pressure.
For pressures lower than atmospheric pressure:
Absolute pressure = Atmospheric pressure – Gauge pressure

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Question 192 Marks

How will you calculate fluid pressure?

Answer

$\underset{\text}{\text { Fluid pressure }}=\frac{\text { Total Force exerted by the fluid }}{\text { Area over which the force is exerted }}=\frac{F}{A}$

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Question 202 Marks

What happens when pressure is increased in solids?

Answer

If the pressure is increased in solids

it experiences tension
it ultimately deforms (or) breaks.

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