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CBSE Questions for Class 8 Physics Force And Pressure Quiz 9 - MCQExams.com
CBSE
Class 8 Physics
Force And Pressure
Quiz 9
A force of $$100N$$ is applied on an area of $$4m^2$$. the pressure being applied on the area is
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$$50Pa$$
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$$25 Pa$$
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$$12.5 Pa$$
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$$400Pa$$
Explanation
Given, $$froce=100N$$
and $$area=4m^2$$
$$\therefore$$$$pressure=\dfrac{force}{area}=\dfrac{100N}{4m^{2}} =25 Pa$$
Ink does not spill out of an ink dropper when the atmospheric pressure and ____ pressure are equal.
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external
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internal
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air
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vaccuum
Explanation
As the pressure inside the dropper, i.e, the internal pressure and the atmospheric pressure are equals, Ink does not spill out.
State whether the following statements are true or false?
(i) the pressure inside our body is equal to the atmospheric pressure
(ii) pressure is inversely proportional to the area.
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Both are true
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Both are false
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(i) True
(ii) False
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(i) False
(ii) True
Explanation
(i) True, because we do not get crushed by the atmosphere as the pressure inside our body and atmospheric pressure are equal.
(ii) True, as $$pressure=\dfrac{force}{area}$$. So, for constant force, $$pressure\propto\dfrac{1}{area}$$
What happens when pressure inside a sealed can is less than the atmospheric pressure?
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The can is crushed
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The can bursts
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No change is observed
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The can swells
Explanation
When the pressure inside a sealed can become less than the atmospheric pressure, the atmospheric pressure pushes the surface of the can inside and the can is crushed.
Mark the correct statements
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Force exists only as a result of an interaction of two objects
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Newton's first law of motion, gives us notion of external forces
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Contact forces are forces in which the two interacting objects are physically in contact with each other
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All
Explanation
Force exists only as a result of an interaction
of two objects.A force can not exist without the interaction of two bodies.
The newton first law states that an object at rest will stay at rest, and an object in motion will stay in motion unless acted on by a net external force.
Hence Newton
s first law of motion introduces about external forces.
Based on whether physical contact is present or not , forces are categorised into two types.
(1) contact forces and
(2) non-contact-forces
Contact forces are forces in which the two interacting
objects are physically in contact with each other. Examples of contact forces
are:
Buoyant force
Friction force
A non-contact force is a force which acts on an object without coming physically in contact with it.The most familiar non-contact force is weight acting on a body due to gravity.
Two vessels, one cylindrical and another cubical are filled by water to the same height. Near the base, lateral pressure exerted by the liquid on the wall
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will be more for cylindrical vessel
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will be more for cubic vessel
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will be same for both the vessels
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will be zero
Explanation
Liquid pressure depends on the depth of the liquid. It does not depend on the shape of the container. So, the lateral pressure at a given depth will be the same in both the vessels.
To move a roller, a force of $$300 N$$ is required. You alone can produce a force of $$30 N$$. How many of your friends who are equally strong as you should join you to move the roller?
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9
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10
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5
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None of these
Explanation
Force applied by you $$= 30 N =$$ force applied by any of your friends.
So, let there be $$x$$ people in total to push the roller.
When $$x$$ people, each giving a force of $$30N$$, give force in same direction, total force = $$(30 \times x)$$ newton
To move the roller, $$30x=300$$
So, $$x=\dfrac{300}{30}=10$$
$$\therefore$$ number of friends to join you $$=(10-1)=9$$ (excluding you)
The tremendous pressure exerted by air can be demonstrated by :
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Crushing can experiment
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Hydraulic press
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Altimeter
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All
Explanation
Crushing can experiment:
Take an empty and absolutely clean oil can and put in it about $$0.25$$ litres of water. Heat the can in fire till all the air is expelled out and the steam comes out freely from its mouth. Screw the mouth of the can with a lid and immediately remove it away from the fire. Pour cold water on the can. It is found that the can gets crushed. The reason is that on pouring cold water, the steam changes into water, thereby causing a vacuum inside the can. Thus the tremendous atmospheric pressure exerts pressure from outside and crushes the can.
Pressure which a fluid exerts on the sides of a container , below its free surface is known as
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Fluid pressure
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lateral pressure
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Atmospheric pressure
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None
Explanation
Definition of lateral pressure, whereas the pressure at the base of vessel is simply called pressure.
A force can change:
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shape of a body
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size of a body
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direction of motion
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all the above
In crushing can experiment, the vaccum inside the can causes .......in pressure.
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Increase
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Decrease
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No change
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All
Explanation
Crushing can experiment ,Take an empty and absolutely clean Mobil oil can and put in it about 0.25 liters of water. Heat the can in fire till all the air is expelled out and the steam comes out freely from its mouth.screw the mouth of the can with a lid and immediately remove it away from the fire .pour cold water on the can. It is found that the can gets crushed.the reason is that on pouring cold water, the steam changes into water ,thereby,causing a vaccum inside the can. Thus the tremendous atmospheric pressure,exerts pressure from outside and crushes the can.
In crushing can experiment the can gets crushed because of:
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decrease in internal pressure
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Increase in internal pressure
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increase in atmospheric pressure
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decrease in atmospheric pressure
Explanation
Crushing can experiment: Take an empty and absolutely clean can and put in it about $$0.25 \ l$$ of water. Heat the can in the fire till all the air is expelled out and the steam comes out freely from its mouth. Screw the mouth of the can with a lid and immediately remove it away from the fire. Pour cold water on the can. It is found that the can gets crushed.
The reason is that, on pouring cold water, the steam changes into water thereby, creating a partial vacuum inside the can, and hence the pressure exerted on the inner walls of the can decreases. Thus, the air pressure, exerted from outside is greater and that crushes the can.
When we press the bulb of dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to:
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Pressure of water
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Gravity of the earth
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Shape of rubber bulb
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Atmospheric pressure
Explanation
The rise of water in the dropper is due to atmospheric pressure.
When the bulb of a
dropper is pressed, the pressure inside the dropper decreases and this causes the water to go inside the dropper.
On a body, a force of $$5N$$ is applied towards North and another force of $$2N$$ is applied towards South. Net force on the body is
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$$3N$$
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$$7N$$
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$$5N$$
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$$2N$$
Explanation
When forces are applied inn the opposite directions, the forces get subtracted.
So, net force on the body$$=(5-2)N=3N$$
At what position the fluid experiences more pressure
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At the bottom of container
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At top of container
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In the middle of container
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Outside the container
Explanation
The pressure at a point in the fluid is due to the weight of the fluid above that point, acting on unit area.
Since the weight above the bottommost point is the highest, the pressure is maximum at that point.
When pressure exerted on fluids by container, then the force is always parallel to the wall. Is the statement true or False
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True
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False
Explanation
When pressure exerted on fluids by container, then the force is always perpendicular to the wall. Mathematically it is given by
$$\vec{F}=P\vec{A}$$
Which of the following are the common consequences of atmospheric pressure in our daily life.
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Sucking a drink with a straw
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Filling a syringe with a liquid
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Filling ink in a fountain pen
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All of the above
Explanation
Liquid flows from higher pressure region to the lower pressure region.
Thus sucking a drink with a straw, filling a syringe with a liquid and filling ink in a fountain pen are the consequences of atmospheric pressure.
In terms of physics, which of the following are considered as fluids?
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Solids and Gases
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Gases and liquids
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Only liquids
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Liquids and solids
Explanation
A fluid is defined as a substance that has no fixed shape and yields easily to external pressure. They are free to flow, and hence the name fluid. They are of two kinds- Gases and Liquids.
Solids are not classified as fluids as they have a definite shape and volume.
The fluid itself exerts pressure whose force component is equal to its ___________. Complete the sentence.
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Height
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Weight
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Depth
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None
Explanation
The pressure due to a fluid, for example, the pressure due to water, is due to the weight of the fluid above that point on the point.
$$\text{Force = Pressure} \times \text{Area}$$
Hence the force component for the pressure is the weight of the fluid.
the fluid on the bottom has much greater pressure than the fluid on the top, due to the weight of the fluid above it.
Fluid pressure comprises of which of the following pressures?
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Air pressure
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Water pressure
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Gauge pressure
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Both A and B
Explanation
A fluid is defined as a substance that has no fixed shape and yields easily to external pressure. They are of two kinds- Gases and Liquids.
The fluid pressure is, thus, of two kinds.
One is due to gases(air) and is called air pressure.
The other is due to liquids(water) and is called water pressure.
Identify the following shows the application of pressure in our everyday life?
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The blade of Ice-skating shoe is very sharp
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It is easier to drive a sharp tipped nail into wood
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Sharp knife cuts better
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Foundation of high-rise building are kept wide
Explanation
Since all examples show an increase in pressure except D as an outcome of decrease in area, i.e, $$P = \dfrac{F}{A},$$ they are all correct applications of pressure in our lives. Option D is an example of decrease in pressure as area is increased
A block of mass of a $$2kg$$ with dimensions $$5cm\times 20cm\times 10cm$$ respectively. The ratio of minimum to maximum pressure it exerts on the change in orientation is:
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$$1:1$$
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$$1:2$$
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$$1:4$$
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$$4:1$$
Explanation
Pressure is the force per unit area. So, $$P=F/A$$
Thus, minimum surface area $$(5 cm \times 10 cm) $$ will give the maximum pressure and maximum area $$(20 cm\times 10 cm)$$ will give the minimum pressure.
So, $$P_{max}=\dfrac{2g}{50\times 10^{-4}} N/m^2$$ and
$$P_{min}=\dfrac{2g}{200\times 10^{-4}} N/m^2$$
Thus, $$\dfrac{P_{min}}{P_{max}}=50/200=1/4$$
The option C will be right.
Why does a sharp knife enable us to cut through things more easily?
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The pressure exerted is lesser when the same force is used
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The pressure exerted is greater when the same force is used
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The sharp edge can pass through the material slowly
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The sharp edge is not felt when cutting through the material
Explanation
The pressure is defined as the force per unit area. i.e $$P=F/A$$
So the pressure depends on the force F and area A of contact.
If force is constant, the pressure is only depends on the area.
A sharp knife edge has small area of contact and less area of contact will produce the more pressure.
Thus, the option B can be correct.
Liquid pressure at a point in a liquid does not depend on the
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Density of liquid
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Shape of the vessel in which the liquid is kept
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Depth of the point from the surface
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Acceleration due to gravity
Explanation
The expression of the pressure at a point in a fluid is given by $$P=h\rho g$$
where.
$$h$$; depth of the point from the surface,
$$\rho$$; be the density of the fluid and
$$g$$; acceleration due to gravity
Thus, the pressure in liquid at a point does not depend on the shape of the vessel in which the liquid is kept.
With the increase in the area of contact of an object the pressure
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increases
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decreases
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is not affected
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none of these
Explanation
The pressure is the force per unit area. So, $$P=F/A$$
Thus the pressure P is
inversely
proportional to contact area A.
So when area increases , the pressure will be decreasing.
Compare the pressure exerted by a sharp needle on a surface and the pressure exerted by a blunt needle.
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The pressure exerted by a sharp needle is more
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The pressure exerted by a blunt needle is more
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The pressure exerted by both is equal
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None of these
Explanation
Pressure is defined as $$P=\dfrac{Force \ applied}{Area}$$
So the pressure applied is inversely proportional to the area to which it is applied.
Since in a sharp needle, surface area on which the force is applied, is less than that for a blunt needle.
Hence, pressure exerted by a sharp needle is more than blunt needle.
Which of the following is a correct statement regarding the pressure due to fluids?
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Force acting on the wall due to pressure is always perpendicular to it because of collision between the molecules takes place perpendicular to it.
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Force acting on the wall due to pressure is always perpendicular to it because of collision between the molecules takes place oblique to it.
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Force acting on the wall due to pressure is always oblique to it because of collision between the molecules takes place perpendicular to it.
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Force acting on the wall due to pressure is always perpendicular to it because of collision between the molecules takes place at any direction to it but its normal reaction on the wall is always perpendicular to it.
Explanation
The net force acting on a wall of fluid containing vessel is always normal to it because effective force is due to collisions of molecules to the wall of the vessel.
The resultant of two opposite parallel forces $$12\ N$$ and $$7\ N$$ is _____ N
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$$12$$
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$$5$$
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$$19$$
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$$7$$
Explanation
$$\vec{F}_{resultant}=\vec{F_{1}}+\vec{F_{2}}$$
$$\left | \vec{F_{1}}+\vec{F_{2}} \right |=\sqrt{F_{1}^{2}+F_{2}^{2}+2F_{1}F_{2}cos\theta }$$
$$\theta =\pi $$
$$\left | \vec{F_{1}}+\vec{F_{2}} \right |=\sqrt{144+49-168}$$
$$=5N$$
In the superhit film 'Raja Hindustani', Amir Khan greets his beloved by shaking hand. What kind of force do they exert?
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Nuclear
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Gravitational
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Weak
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Electromagnetic
Explanation
Shaking hands will involve electromagnetic forces because their hands interaction is kind of friction between their hands and
Friction force is a type of Electromagnetic force.
Therefore option $$D$$ is correct
The two femurs each of the cross-sectional area 10 cm$$^2$$ support the upper part of a human body of mass 40 kg. The average pressure sustained by the femurs is then (Takes g= 10 m s$$^{-2}$$)
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2 x 10$$^3$$ N m$$^{-2}$$
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2 x 10$$^4$$ N m$$^{-2}$$
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2 x 10$$^5$$ N m$$^{-2}$$
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2 x 10$$^6$$ N m$$^{-2}$$.
Explanation
Given that,
$$Area=10 \ cm^{2}$$
$$Mass=40 \ kg$$
$$g=10 \ m/s^{2}$$
Total cross-sectional area of the femurs is,
$$A = 2 \times 10 \ cm^2$$
$$ = 2 \times 10\times 10^{-4} m^2 $$
$$ = 20 \times 10^{-4} m^2$$
Force acting on them is
$$F= mg =40\,kg \times 10\,m\,s^{-2} =400\,N$$
$$\therefore$$ Average pressure sustained by them is
$$\displaystyle P=\dfrac{F}{A}= \dfrac {400\,N}{20 \times 10^{-4}\,m^2}$$
$$ = 2 \times 10^5\,N\,m^{-2}$$
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