Science chapter3

Answer in brief. 

1. On what factors the pressure exerted by the liquid depends on? 

Pressure exerted by a liquid at a point is determined by

1.Depth(h)

2. Density of the liquid(ρ)

3. Acceleration due to gravity(g)

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2. Why does a helium balloon float in air?

1.Helium is denser than ordinary air and this gives Helium buoyancy

2. Hence helium balloon floats in air

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3. Why it is easy to swim in sea water than in river water?

 

1. The density of seawater is greater than river water due to its high salt content

2. Saltwater provides more buoyancy force than freshwater because buoyancy force depends on the density

3.Hence it is easy to swim in sea water than in river water.

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4. What is meant by atmospheric pressure? 

1.Earth is surrounded by a layer of air up to certain height and this layer is called atmosphere

2. Since air occupies space and has weight it also exerts pressure

3. This pressure is called atmospheric pressure

4. The atmospheric pressure is the air pressure at sea level

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5. State Pascal’s law

The law states that the external pressure applied on an incompressible liquid is transmitted uniformly throughout the liquid.

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Answer in detail. 

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

1. Stand on loose sand. Your feet go deep into the sand

2. Now lie down on the sand. What happens?

You will find that your body will not go that deep into the sand.

3. In both the cases the force exerted on the sand is the weight of your body which is the same

4.When you stand on loose sand the force is acting on an area equal to the area of your feet

5.When you lie down the same force acts on an area of your whole body which is larger than the area of your feet

6.When area of contact decreases pressure will increase

Result: pressure is larger while standing than while lying

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2. Describe the construction and working of mercury barometer. 

Compulsory diagram

Barometer:

The instrument used to measure atmospheric pressure is called barometer. 

Construction:

1.A mercury barometer, consists of a long glass tube (closed at one end, open at the other) filled with mercury and turned upside down into a container of mercury.

2.This is done by closing the open end of the mercury filled tube with the thumb and then opening it after immersing it in to a trough of mercury

Working:

1.The barometer works by balancing the mercury in the glass tube against the outside air pressure. 

2.If the air pressure increases, it pushes more of the mercury up into the tub and if the air pressure decreases, more of the mercury drains from the tube. 

3.As there is no air trapped in the space between mercury and the closed end, there is vacuum in that space. 

4.Vacuum cannot exert any pressure. 

5.So the level of mercury in the tube provides a precise measure of air pressure which is called atmospheric pressure.

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

1.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. 

2.If the density of a substance is less than the density of the liquid it will float. 

3.For example a piece of wood which is less dense than water will float on it. 

4.Any substance having more density than water

(for example, a stone), will sink into it.

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4. Explain the construction and working of a hydrometer with diagram.

 

Compulsory diagram

Hydrometer:

A direct-reading instrument used for measuring the density or relative density of the liquid is called hydrometer. 

Principle:

Hydrometer is based on the principle of flotation, i.e., the weight of the liquid displaced by the immersed portion of the hydrometer is equal to the weight of the 

hydrometer.

Construction:

1.Hydrometer consists of a cylindrical stem having a spherical bulb at its lower end and a narrow tube at its upper end. 

2.The lower spherical bulb is partially filled with lead shots or mercury. 

3.This helps hydrometer to float or stand vertically in liquids. 

4.The narrow tube has markings so that relative density of a liquid can be read directly. 

Working:

1.The liquid to be tested is poured into the glass jar. 2.The hydrometer is gently lowered in to the liquid until it floats freely. 

3.The reading against the level of liquid touching the tube gives the relative density of the liquid.

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5. State the laws of flotation

1.The weight of a floating body in a fluid is equal to the weight of the fluid displaced by the body.

2. The centre of gravity of the floating body and the centre of buoyancy are in the same vertical line.

3.The point through which the force of buoyancy is supposed to act is known as centre of buoyancy.

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