[3 Mark Question Answer]

Question 13 Marks

Describe the function of a simple mercury barometer. State its disadvantages.

Answer

A simple mercury barometer is used to measure atmospheric pressure. Some of its disadvantages are listed below.
The barometer is not compact, and thus, not portable.
The level of mercury in the open trough increases or decreases with change in atmospheric pressure. Hence, a
permanent scale cannot be marked on the glass tube.
The trough is open and there are chances of impurities getting mixed with the mercury in the trough.

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Question 23 Marks

List any three characteristics of liquid pressure.

Answer

The characteristics of liquid pressure are given as follows.
Liquids exert pressure on the walls of the container.
The pressure exerted by a liquid is directly proportional to its density. More the density of a liquid, greater is the pressure exerted by it.
Pressure exerted by a liquid increases with depth.

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Question 33 Marks

Define pressure. Write the factors on which the pressure exerted by a body depends.

Answer

Pressure is defined as the amount of force acting per unit area. It can be mathematically expressed as
Pressure $(P)=\frac{\text { Force }(F)}{\text { Area }(A)}$
It depends on the following two factors $–$ magnitude of force and the area of contact.
Pressure increases if the magnitude of force $($thrust$)$ increases, or the area of contact decreases.
Pressure decreases if the magnitude of force $($thrust$)$ decreases, or the area of contact increases,

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Question 43 Marks

How are force, area and pressure related to each other?

Answer

Force, area and pressure are related by the following expression.
Pressure $(P)=\frac{\text { Force }(F)}{\text { Area }(A)}$
If the force applied is doubled for the same area, then the pressure will also double.

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Question 53 Marks

If the magnitude of force acting on a surface is doubled and area over which it acts is halved, then what will be the effect on pressure ?

Answer

When,
magnitude$-$of force $= F$, and
area over which the force acts $= A$
then, the pressure $(P)$ is given by
$P=\frac{F}{A}$
Now, if the force is doubled $(2F)$ and area over is halved $(A/2)$, then the pressure is given by $–$
Pressue $=\frac{2 F}{A / 2}=\frac{4 F}{A}= AP$
So, when the magnitude of force acting on a surface is doubled and area over
which it acts ¡s halved, then the pressure becomes $4$ times.

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Question 63 Marks

A motorcycle weighs $1200 N$. If the pressure exerted by one of its types is $120$ Pa, then find the area of the type in contact with the road.

Answer

Weight of the motorcycle $= 1200 N$
Force exerted by the motorcycle on the road, $F =$ Weight
of the motorcycle $= 1200 N$
Pressure exerted by one of its tyres, $P = 120$ Pa
Area of the tyre in contact with the road,$ A =$ ?
From the formula, $P =\frac{F}{A}$ weget, $120$ Pa $=\frac{1200 N }{ A }$
or $A=\frac{1200 N }{120 Pa }=10 m ^2$

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Question 73 Marks

The tip of a drawing pin has an area of $0.01\ mm ^2$ Find the pressure exerted by it if a force of $20\ N$ is applied by it.

Answer

Force exerted by the tip of a drawing pin$, F = 20\ N$
Area of the tip of the drawing pin$, A = 0.01\ mm^2\frac{0.01}{1000 \times 1000}\ m ^2$ Pressure exerted by the tip of the pin,$P =\frac{F}{A}=\frac{20\ N }{0.01 /(1000 \times 1000) \ m ^2}= 2,00,00,00,000\ Pa$

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Question 83 Marks

Calculate the force applied on a body producing a moment of force of $16 N$ m such that the perpendicular distance between the point of application of force and the turning point is $80 \ cm$.

Answer

Moment of force $= 16\ N m$
Moment arm $( d )=80 \ cm =\frac{80}{100} m =0.8 m$
Force $(F) =$ ?
From the formula,
Moment of force $= F \times d$
we get,
$16 \ N m = F \times 0.8\ mF=\frac{16 Nm }{0.8 m}=20 N$

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Question 93 Marks

Calculate the force which will produce a moment of $60 N$ m when the perpendicular distance between the point of application of force and the fixed point is $60 \ cm.$

Answer

Here,
Moment of force $= 60\ N m$
Moment $\operatorname{arm}( a )=60 \ cm =\frac{60}{100} m =0.6 m$
By using the formula,
Moment of force $F \times d$
we get,
$60\ Nm = F \times 0.6\ mF =\frac{60 Nm }{0.6 m }=100 N$

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Question 103 Marks

A force of $250\ N$ produces a moment of force of $500\ N m$. Calculate the moment arm.

Answer

Here,
Force $(F) = 250 N$
Moment of force $500\ N m$
Moment arm $(d) = ?$
By using the formula,
Moment of force $= F \times d$
we get,
$500\ N m = 250 N \times d =\frac{500 Nm }{250 N }=2 m$

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Question 113 Marks

A nut is opened by a spanner of length $30 \ cm$. If the applied force is $20 N$, find the moment of force needed to loosen the nut.

Answer

Here,
Force $(F) = 120 N$
Moment arm $( d )=30\ cm =\frac{30}{100}=0.3 m$
Moment of force $= F \times d$
$= (120 N) \times (0.3 m) = 36\ N m$
So, the moment of force needed to loosen the nut is $36\ N m.$

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Question 123 Marks

Explain in brief why birds that walk on the leaves floating on the surface of water have wide feet.

Answer

Birds that walk on leaves floating on the surface of the water have wide feet. This is because wide feet increases the area of contact with the leaves. So, they exert less pressure on the leaves, allowing them to walk on these leaves.

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PHYSICS [3 Mark Question Answer]

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