PHYSICS [5 Mark Question Answer]

Air exerts pressure and this can be demonstrated by this experiment.

Take a glass tumbler. Fill it to the brim with water, with water overflowing the rim of the tumbler. Cover it with a piece of cardboard. Press the cardboard so that it touches the rim of the tumbler at all points. This will ensure that no air is left inside.
With the hand on the cardboard, invert the tumbler gently and remove the hand. Both cardboard and water shall not fall. This shows that air exerts pressure on the lower surface of the cardboard in the upward as shown in the figure. The upward thrust acting on the cardboard due to air pressure supports the combined weight of cardboard and water in the tumbler.
So, this shows that air exerts pressure.

Aneroid barometer is a portable type of barometer.


In this, no liquid is used. It consists of a metal box corrugated to make it flexible. The lower side of the box is fixed to the base of the instrument and the upper side is supported by a stout spring S. The box is partially evacuated. So it tends to collapse under the pressure of air. The stout spring balances the thrust on the metal box due to normal air pressure and prevents the box from collapsing.

When atmospheric pressure changes, the surface supported by the spring S moves in if pressure increases or out if the pressure decreases slightly. This small movement is increased considerably by a system of levers one end of which is connected to the spring S. The other end of the lever system is connected to a chain which is wrapped round a spindle carrying a pointer. The pointer moves over a scale which is graduated to read the pressure in centimeters or inches.

A barometer is an instrument which is used to measure the atmospheric pressure.

Construction of a simple barometer:

A simple mercury barometer can be made with a clear, dry, thick-walled glass tube about 1 metre ling. The glass tube is sealed at one end and is filled with mercury completely. While filling the tube with mercury care has to be taken so that there are no air bubbles present in the mercury column. Close the open end with thumb and turn the tube upside down carefully over a trough containing mercury. Dip the open end under the mercury level in the trough and remove the thumb.

The mercury level in the tube falls until it is about 76 cm (h =760 mm) vertically above the mercury level. It is the atmospheric pressure acting on the surface of the mercury in the trough that supports the vertical mercury column. The empty space above the mercury column is called the ‘Torricellian vacuum’.

The principle of Archimedes states that when a body is totally or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced. We can verify Archimedes' principle experimentally by doing this experiment.

The stone weighed 0.67 N in air and 0.40 N when immersed in water. The displaced water weighed is 0.27 N (= 0.67 - 0.40)

Pour water into eureka can till the water starts overflowing through the spout.

When the water stops dripping replace the beaker by another one of known weight. Suspend a stone with the help of a string from the hook of a spring balance and record the weight of the stone.

Now, gradually lower the body into eureka can containing water and record its new weight in water when it is fully submerged in water. When no more water drips from the spout, weigh the beaker containing water.

After observation, we can conclude that apparent loss of weight of stone (calculated by differences in weight measured by spring balance) = weight of water displaced (weight of water in beaker ).

This proves the Archimedes' principle that when a body is totally or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid

We can find the relative density of a solid which floats on water by following the experiment.
Measuring the relative density of a cork

Choose a sinker and find its weight in water by suspending it in water.

Tie the solid to the string attached to the sinker and find its weight in air but sinker in water.

Remove the solid and tie it together with the sinker and suspend it in water and find the weight of the solid together with the sinker in water.

Record your observation as shown below: Weight of sinker in water = x gf

Weight of sinker in water + solid in air = y gf Weight of solid in air = (y - x) gf

Weight of solid + sinker in water = z gf

upthrust on solid in water = (y - z) gf.

The upthrust on in water also represents the weight of the water displaced by the solid. Relative density of solid = (weight of cork in air )/(weight of equal volume of water) Relative density = (y - x)/(y - z).

The fractional part submerged equals the ratio of the density of the material of the block to the density of the liquid.
(Density of floating body / Density of liquid) = fraction submerged.
Height of wooden piece = 15 cm.
Height of wooden piece sinks in water = 10 cm.
Fraction of wooden piece submerged in water = 10/15 = 0.67.
As the liquid is water so the ratio of Density of wooden by the density of water gives a relative density of floating wooden pieces. So, the relative density of the wooden block is 0.67.
Height of wooden piece = 15 cm.
Height of wooden piece sinks in spirit = 12 cm.
Fraction of wooden piece submerged in water = 12/15 = 0.8.
We know the density of wooden piece = 0.67
(Density of floating body / Density of liquid) = fraction submerged.
Density of liquid/spirit = (Density of floating body /fraction submerged)
Density of liquid/spirit = 0.67/0.8 = 0.83.
The relative density of the spirit is 0.83.

A manometer is a simple pressure gauge that measures differences in pressure at the two ends of the apparatus.
The manometer is a U shaped tube containing water whose one limb is dipped in vessel and vessel is tightly covered with a plastic sheet. U shaped tube has two limbs one towards the vessel and the other is opened to the atmosphere.
Now if the level of water toward atmospheric open limb is more than the level of water in limb towards apparatus end then the liquid is said to be at a higher pressure than the atmosphere. And if the level of water toward atmospheric open limb is less than the level of water in limb towards apparatus end then the liquid is said to be at a lower pressure than atmospheric pressure.

This diagram illustrates that pressure of liquid increases with depth. The hole at greater depth has more pressure and hence the flow of liquid from there is more. Other hole is at less depth so the pressure of liquid there is less and hence the flow of liquid from there is less. So the diagram shows that pressure increases with an increase in depth.

Laws of liquid pressure:
(i) Pressure at a point inside the liquid increases with the depth from its free surface.
(ii) In a stationary liquid, pressure is same at all points on a horizontal plane.
(iii) Pressure is same in all directions about a point in the liquid.
(iv) Pressure at same depth is different in different liquids. It increases with the increase in the density of liquid.
(v) A liquid seeks its own level.