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CBSE Questions for Class 8 Physics Force And Pressure Quiz 7 - MCQExams.com
CBSE
Class 8 Physics
Force And Pressure
Quiz 7
If a body is moving with a constant speed along a straight line; then in order to change its direction of motion, an external force has to be applied in a direction normal to the direction of motion.
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True
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False
Explanation
Newton's First law of motion - "Every body continues to be in its state
of rest or of uniform motion in a straight
line unless compelled by some external
force to act otherwise".
According to Newton's First law of motion, first half of the statement is true, an external force has to be applied. But "in a direction normal to the direction of motion" is false. The applied force doesn't have to be always normal to the direction of motion to change its direction of motion. For example, a bat hitting a ball applies a force in a direction opposite to its motion and yet it can change its direction of motion.
Pressure varies inversely with area (A) provided the thrust is same.
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True
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False
Explanation
pressure = $$\dfrac{thrust}{area}$$
So if thrust is the same then pressure varies inversely with the area.
So option (A) is correct
Pressure varies with force (F) as (provided area is same)
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$$P\propto F$$
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$$P\propto \dfrac {1}{F}$$
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$$P\propto F^2$$
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$$P\propto \dfrac {1}{F^2}$$
Explanation
We know, Pressure is the
force exerted divided by area.
Pressure $$=\dfrac{force}{Area},$$ When A is consfont,
Pressure is
directly proportional to F. ie
$$P\propto F$$
Hence, Option A
A force produces translational motion:
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When the body is free to move and applied force pass through the rotation axis
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When the body is hindered to move
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When the body is rigidly fixed
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When the body is free to move and applied force does not pass through the rotation axis
Explanation
For the application of translational motion, the body must be free to move and after applying force, all the particles in that body move with the same velocity. So applied force must pass through the rotation axis otherwise force can rotate the body.
Which of the following force doesn't change with distance?
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Electrostatic force
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Gravitational force
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Frictional force
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Magnetic force
Explanation
Gravitational, magnetic and electrostatic forces follow the inverse square law, i.e., the forces vary inversely as square of the distance. But frictional force does not depend on distance.
It is better to use a sharp tipped nail than a blunt nail because :
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The tip with the smallest area will produce a higher pressure when a force is applied.
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A sharp tipped nail needs a larger force to drive it into the wooden surface
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The sharp tip saves cost as less raw materials are required to make such nails
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The sharp tip has a spiralling effect when the nail is hit on the head
Explanation
Sharp tipped nail have a low area of contact which results in more amount of pressure even for a small amount of force, whereas blunt nail has more area of contact so it would provide less pressure for the same applied force. As pressure and area of contact are inversely related.
Hence option (A) is correct
With the increase in the area of contact of an object, the pressure:
(Note : Thrust remains same)
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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
Since,
$$P = \frac{F}{A}$$
Thus pressure is inversely proportional to area,
$$i.e. \,\,P \propto \frac{1}{A}$$
With the increase in the area of contact of an object the pressure decreases because pressure is inversely proportional to area of contact.
Which of the statements about force are true?
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It can be seen
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It causes an object to move
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It is pull or a push
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It can change the motion of an object
Explanation
A force is any influence that, that can change or try to change the state of motion of an object on which it is acting. It can be Pull or Push.
Hence Options B,C, and D are correct
The pressure exerted by a women wearing shoes with pointed heels is ...... than what an elephant with one foot can exert on ground :
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Much lesser
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Much greater
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Both equal
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None of these
Explanation
The pressure exerted by a women wearing shoes with pointed heels is much greater than what an elephant with one foot can exert on ground. This because of lesser area of contact of pointed heels.
Fill in the blank.
Dams are ____ at the base.
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thicker
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thinner
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same
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can't say
Explanation
$$Pressure=\frac{Force}{Area}$$
So pressure is inversely proportional to area of contact.
That is why area at base of dam is more as compared to the top so there is less pressure on base so it will be safe.
Which of the following has infinite range?
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Gravitational force
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Electromagnetic force
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Strong nuclear force
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Both a and b
Explanation
undefined
The same force acting on a smaller area exerts a larger pressure, and a smaller pressure on a larger area.
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True
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False
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Ambiguous
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Data insufficient
Explanation
Answer is A.
The same force acting on a smaller area exerts a larger pressure, and a smaller pressure on a larger area. This is the reason why a nail has a pointed tip, knives have sharp edges and buildings have wide foundations.
For example, When standing, a person will sink more into sand than when he lies flat stretched on sand. The body weight acts on a smaller area, in standing position, than when the person lies stretched over a larger area.
Hence, the statement is true.
A box exerts a force of $$420N$$ on a floor. The bottom of the box has an area of $$0.7m^2$$. What is the pressure exerted by the box on the floor?
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$$600 Pa$$
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$$500 Pa$$
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$$400 Pa$$
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$$700 Pa$$
Explanation
Given, force, $$F=420N$$; area$$=0.7m^2$$;
Pressure, $$P=?$$
Pressure, $$P=\displaystyle\frac{Force}{Area}=\frac{F}{A}=\frac{420}{0.7}=600Pa$$
Ask a friend to drive a small nail into a piece of wood placed on top of a pile of books on your head. Why doesn't this hurt you?
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Because the nail has a sharp point and thus, exert less pressure on wood
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The force is exerted on the wood, thus it has no effect on out head
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The relatively large mass of the books and block on your head resists being moved
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Action-reaction forces cancel out each other
Explanation
The hammer delivers a big force for a very short time. That force on a very small nail may change its velocity quickly (big acceleration with big force because of small mass.)
However the big force does not last long enough to change the speed of the pile of books much (big force, big mass, small acceleration.) Therefore the pile of books does not transmit much force on to your heard because they have large inertia .
A force $$\displaystyle F_{1}$$ accelerates a particle from rest to a velocity v. Another $$\displaystyle F_{2}$$ decelerates the same particle from v to rest, then:
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$$\displaystyle F_{1}$$ is always equal to $$\displaystyle F_{2}$$
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$$\displaystyle F_{2}$$ is greater than $$\displaystyle F_{1}$$
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$$\displaystyle F_{2}$$ may be smaller than, greater than or equal to $$\displaystyle F_{1}$$
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$$\displaystyle F_{2}$$ cannot be equal to $$\displaystyle F_{1}$$
Explanation
We cannot say anything about the magnitude of the forces because it is not mentioned by how much time each of the forces has taken to increase/decrease the velocity.
Porters (coolies) place a round piece of cloth
on their heads
, to ______ the area of contact of the load with their head.
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increase
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decrease
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maintain the same
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Cannot say
Explanation
Increase
Coolie or porter place a round piece of cloth on their heads when they have to carry heavy loads. By doing this they increase the area of contact of the load with their head. As pressure is inversely proportional to the area of contact, the pressure on their head is reduced and the porters find it easier to carry the load.
It is easier to cut with a sharp knife than with a blunt one, This is because
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sharp knife exerts more pressure
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sharp knife exerts less pressure
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both knives exert equal pressure
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sharp knife has a better grip
Explanation
It is easier to cut with a sharp knife than with a blunt one,
Blunt Blades have larger area compared to the sharp-edged blade.
Thus the applied force produces a lower pressure in case of blunt blades, which makes it difficult to cut the object.
Buildings have wide foundations so that they exert less force on the ground or Earth.
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True
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False
Explanation
The force exerted by the building on the ground, $$F=m\times g$$
where, $$m=mass$$ and $$g= acceleration\ due\ to\ gravity$$
Force does not depend on the area.
We know,
$$Pressure=\dfrac{force}{Area}$$
Buildings have wide foundations so that they exert less pressure on the ground or Earth.
A $$1.5m$$ wide by $$2.5m$$ long water bed weighs $$1025N$$. Find the pressure that the water bed exerts on the floor. Assume that the entire lower surface of the bed makes contact with the floor.
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$$273.3Pa$$
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$$373.5 Pa$$
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$$173.3Pa$$
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$$473.3Pa$$
Explanation
Pressure exerted by water bed will be force applied per unit area.
$$P=\dfrac {F}{A}$$
Given:
Force= $$1025 \ N$$
Area= $$1.5 m \times 2.5m = 3.75 m^2 $$
Pressure
$$=\dfrac { 1025 \ N }{ 3.75 \ m^2} =273.3 \ N/m^2 = 273.3 \ Pa$$
Which of the following is NOT true?
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A sharp knife can easily cut due to its small cutting surface.
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It is easier to walk in sand than on road.
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A battle tank can move easily on soft ground because its tracks have bigger surfaces.
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The pressure exerted by a needle is much more than the foot of an elephant.
Explanation
Due to lack of friction on the sand, it is difficult to walk on sand than on road.
A Sharp knife can cut easily because it has a small cutting surface.
Battle tank has a large surface area so it can move easily on soft ground.
The pressure exerted by the needle is much larger because it has a very small surface area while the elephant's foot has a larger surface area.
State whether the given statement is TRUE or FALSE.
The foundations of high-rise buildings are kept wide so that they may exert more pressure on the ground.
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True
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False
Explanation
We know,
$$\text{Pressure}=\dfrac{\text{Thrust}}{\text{Area}}$$
So, if the area of contact increases then the pressure reduces.
Wide foundations exert lesser pressure because the increase in the area of contact reduces the pressure on the ground and the buildings will not sink.
So the statement is False.
Force can only increase or decrease the speed of a moving object or change its direction.
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True
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False
Explanation
An external force applied can:
1) I
ncrease or decrease the speed of a moving object
2) C
hange its direction
3) Can change the shape and size of the body
State whether the given statement is TRUE or FALSE.
A liquid in a container exerts pressure in all directions- on the bottom of the container, on its walls, and even upwards.
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True
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False
Explanation
According to Pascal's law, a liquid exerts pressure in all directions. Thus, a liquid in a container will exert pressure on the bottom of the container and on its walls.
Tick the correct statements.
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air exerts pressure because it occupies space and has weight.
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air is more dense near the earth. It goes on thinning out as we go up.
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oxygen content of the air goes on decreasing as we go up.
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air is uniformly dense right fron the earth to the sky.
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air pressure is very high near the earth and it goes on decreasing as we go up.
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$$99\%$$ of air exists up to a height of nearly $$30\;km$$.
Explanation
The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force exerted on a surface by the air per unit area of surface Atmospheric pressure is commonly measured with a barometer.
Earth's gravity pulls air as close to the surface as possible.Hence more air molecules are present near the surface of earth hence density of air is more at surface and decreases as we go up.Pressure is directly proportional to density hence pressure is also maximum at surface and goes on decreasing as we go up.For the same reason oxygen decreases as we go up because total air is decreasing hence oxygen which is nearly $$20$$ percent of air also decreases.
Because of earth's attraction most of air is present within $$30km$$ from earth surface. In figures, nearly 99% of air exists up to a height of nearly 30km.
A mountain climber experiences a nose bleed due to:
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Decrease in atmospheric pressure
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Increase in atmospheric pressure
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More gravitational pull
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Effect of high altitude
Explanation
A mountain climber experiences a nose bleed due to a decrease in atmospheric pressure.
The atmospheric pressure decreases with height as the air becomes less dense as we go up in altitude. Thus, the internal body pressure becomes higher than atmospheric pressure causing a nose bleed.
If we apply the brakes to a moving car, it comes to rest. This shows that force can make a moving object go slower.
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True
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False
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Ambiguous
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Data insufficient
Explanation
Answer is A.
Friction prevents objects from moving or slows them down. It also causes wear on surfaces as they rub against each other and generates heat. Thus energy is wasted in overcoming friction. Hence, the force of friction can slow down an object.
Hence, the statement is true.
A contact force cannot act through
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Empty space
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Touching
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Touching with a metal rod
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Touching with a wooden rod
Explanation
Answer is A.
Contact forces are such forces that are exerted on objects by direct contact. In other words, The forces which acts on bodies when they are in actual contact are called the contact forces.
Hence, they cannot act through empty space.
(a) A force of $$200\;N$$ acts on an area of $$0.02\;m^2$$. Find the pressure in Pascal.
(b) What force will exert a pressure of $$50,000\ Pa$$ on an area of $$0.5\;m^2$$?
(c) Find out the area of a body which experiences a pressure of $$500\ Pa$$ by a force of $$100\;N$$.
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(a)$$\;10000\;Pa$$, (b)$$\;25000\;N$$, (c)$$\;0.2\;m^2$$
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(a)$$\;1000\;Pa$$, (b)$$\;2500\;N$$, (c)$$\;2\;m^2$$
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(a)$$\;100000\;Pa$$, (b)$$\;250000\;N$$, (c)$$\;0.2\;m^2$$
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(a)$$\;100\;Pa$$, (b)$$\;250\;N$$, (c)$$\;0.2\;m^2$$
Explanation
We know
$$Pressure(P)=\dfrac{Force(F)}{Area(A)}$$
(a) $$F=200N$$ $$A=0.02m^2$$
$$P=\dfrac{200}{0.02}=10000Pa$$
(b) $$P=50000Pa$$ $$A=0.5m^2$$
$$F=PA=50000\times 0.5=25000N$$
(c) $$P=500Pa$$ $$F=100N$$
$$A=\dfrac{F}{P}=\dfrac{100}{500}=0.2m^2$$
Which of the following class of force is different from others?
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Magnetic force
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Electrical force
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Gravitational force
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Stretching of a spring
Explanation
Answer is D.
Contact forces:
They are such forces which are exerted on objects by direct contact. The forces which acts on bodies when they are in actual contact are called the contact forces.
The following forces come under Contact Forces.
Frictional forces
Normal reaction force
Tension force as applied through strings
Force exerted by a spring
Force exerted during collision
Force exerted by a spring
Force exerted during collision
Force applied as a pull or push
Non Contact Force:
The forces which act on bodies without being physically touched, are called the non-contact forces.
The following forces come under non-contact forces :
Gravitational Force
Electrostatic Force
Magnetic Force
Hence, the odd man out is stretching force of a spring.
$$a$$. To stretch the bow, the archer applies a force that causes a change in its ______
$$b$$. The force applied by the archer to stretch the bow is an example of ______ force.
$$c$$. The type of force responsible for a change in the state of motion of the arrow is an example of a _____ force.
$$d$$. While the arrow moves towards its target, the forces acting on it are due to ______ and that due to _______ of air.
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Shape
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Muscular
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Contact
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Gravity, Friction
Explanation
a. To stretch the bow, the archer applies a force that causes a change in its shape.
b. The force applied by the archer to stretch the bow is an example of muscular force.
c. The type of force responsible for a change in the state of motion of the arrow is an example of a contact force.
d. While the arrow moves towards its target, the forces acting on it are due to gravity and that due to Friction of air.
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