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PHYSICS [2 Mark Question Answer]

Force and Pressure · ICSE STD 8 (ICSE - English Medium). 25 questions.

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[2 Mark Question Answer]

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25 questions · timed · auto-graded

Question 12 Marks
In Fig. a force $F$ is applied in a direction passing through the pivoted point $O$ of the body. Will the body rotate? Give reason to support your answer.
Answer
No, the body will not rotate as
Turning effect $=$ Force $\times \perp$ of the force from the pivoted point.
$= F\times 0 = 0$
$\perp$ distance $= O$ or force is parallel to the point of application of force.
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Question 22 Marks
State two factors on which affect moment of force.
Answer
Factors on which moment of force depends
$1.$ The magnitude of the force applied.
$2.$ The distance of the line of action of the force from the axis of rotation.
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Question 32 Marks
State the effect of force $F$ in of the following diagram.
Answer
Wheel turns about the axis of rotation i.e. about pivot on applying force.
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Question 42 Marks
State the effect of force $F$ in of the following diagram.
Answer
The ball moves in the direction of magnitude of force $f$ and continues to move in that direction
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Question 52 Marks
Give reason for the following:
Water does not run out of a dropper unless its rubber bulb is pressed.
Answer
Atmospheric pressure acting from outside the dropper balances the pressure exerted by water and water does not come out of a dropper. On pressing the dropper inside pressure of water becomes more than outside atmospheric pressure and water run out.
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Question 62 Marks
Describe the crushing tin can experiment. What do you conclude from this experiment?
Answer
Take a tin$-$can having an airtight cap $($screw cap$)$. Remove cap and boil some water in it, so that steam comes out and in this way air from inside goes out. While boiling replace the cap and allow it to cool. Vapours inside condense and form water creating a vacuum above them. We see the can crumbles due to air pressure from outside. This proves that air exerts pressure.
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Question 72 Marks
State two effects of a force when applied on a body.
Answer
TWO EFFECTS OF APPLIED FORCE :
$(i)$ It can change the state of body.
i.e. It can stop a moving object and also a force can move a stationary object.
$(ii)$ A force can change the shape $($and size$)$ of an object.
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Question 82 Marks
Describe a simple experiment to show that at a given depth, a liquid exerts same pressure in all directions.
Answer
Experiment : We have often seen children playing with polythene bag filled with water and having small holes at various places on this bag and sprinkling water on others. This is because water enclosed in bag exerts pressure in all directions.
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Question 92 Marks
It is easier to cut with a sharp knife than with a blunt one. Explain.
Answer
It is easier to cut with a sharp knife than with a blunt one because, for the same applied force, the pressure exerted $($force$/$area$)$ is more in the case of the sharp knife than in the case of a blunt one.
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Question 102 Marks
Why is the bottom part of the foundation of a building made wider?
Answer
$P ∝ 1/A$ i.e. pressure is inversely proportional to surface Area
Greater the area, lesser is the pressure.
In order that building should exert less pressure, the foundation of the building should have more area or be wider. So that building should not sink into the earth.
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Question 112 Marks
Why do camels or elephants have broad feet?
Answer
Camels have to walk on sand. To walk fast, the feet should not sink in sand, the feet area broad i.e. increase in area. Hence, lesser pressure will help to sink less and so camels have broad feet.
Elephants have broad feet in proportion to their size. Because of their weight and size, elephants need to distribute the weight better.
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Question 122 Marks
Name two factors on which the pressure on a surface depends.
Answer
Factors affecting the pressure : $(i)$ The surface area $(A)$ on which thrust acts. More area, lesser is pressure. $($Area$)$
$(ii)$ The magnitude of thrust $(F)$ acting on the surface. $($Force$) \because P=\frac{\text { Thrust }}{\text { Area }}=\frac{F}{A}$
More thrust more is pressure.
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Question 132 Marks
How is the thrust related to pressure?
Answer
Pressure is directly proportional to thrust $P µ$ Thrust
i.e. greater the thrust, greater is the pressure and smaller the thrust, smaller is the pressure.
Force simply refers to push or pull on an object whereas thrust refers to the reaction force acting on an accelerated object due to the force which is applied to that object. Therefore, the relationship between thrust and pressure is that pressure is directly proportional to thrust.
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Question 142 Marks
Define the term ‘pressure’ and state its unit.
Answer
$\text{PRESSURE}$ : “The thrust on unit area of the surface is called $\text{PRESSURE.”}$
Pressure $=$ Thrust $/$ Area
$\text{S.I.}$ unit $= Nm^{-2}$
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Question 152 Marks
On what factors does the effect of thrust on a surface depend?
Answer
The effect of thrust depends on the area of the surface on which it acts. Smaller the area of the surface on which a thrust acts, larger is its effect. But the effect of a thrust is less on a larger area.
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Question 162 Marks
It is easier to open the door by pushing it at its free end.
Answer
Moment of force is a product of force and perpendicular distance.
Torque is defined as Force $\times$ distance. Larger the distance from the fixed axis or hinge, larger the torque and hence it is easier to open the door
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Question 172 Marks
The hand flour grinder is provided with a handle near the rim.
Answer
The stone of hand flour grinder is provided with a handle near its rim $($i.e., at maximum distance from centre$)$ so that it can easily be rotated about the iron pivot at its centre by applying a small force at the handle.
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Question 182 Marks
The steering wheel of a vehicle is of large diameter.
Answer
The larger the radius the greater the torque. Another way of stating that is that a smaller force on a large wheel could generate the same torque as greater force on a smaller wheel. Hence, the steering wheel of heavy vehicles has a large diameter.
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Question 192 Marks
State one way to obtain greater moment of a given force about a given axis of rotation.
Answer
Moment of a force is the product of the force and the perpendicular distance of force from axis of rotation. So, one way to increase the moment would be to increase the distance from the axis of rotation where the force would act.
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Question 202 Marks
How much thrust will be required to exert a pressure of $20,000\  Pa$ on an area of $1/ cm^2.$
Answer
Pressure $=20,000\  Pa$
Area $= 1 \ cm^2$
$= 10^{-4} m^2$
Force = Pressure $\times$ area
$=20,000 \times 10^{-4} = 2$ Newton
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Question 212 Marks
Find the thrust required to exert a pressure of $50000$ pascals on an area of $0.05\  m^2$ ?
Answer
Pressure $=$ Force $/$ Area
Force $=$ Pressure $\times$ Area
$= 50,000 \times 0.05 = 2,500$ Newton
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Question 222 Marks
A normal force of $200\ N$ acts on an area of $0.02\ m^2.$ Find the pressure in pascal.
Answer
Pressure $=\frac{\text { Force }}{\text { Area }}$
$ \frac{200\  N }{0.02\  m ^2}$
$ =\frac{200 \times 100}{2}=10000 N / m ^2 $ or $10000$ Pascal 
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Question 232 Marks
A spanner of length $10 \ cm$ is used to open a nut by applying a minimum force of $5.0\  N$. Calculate the moment of force required.
Answer
$\perp$ Distance $= 10\ cm = 10/100 = 1/10\  m$
Force applied on spanner $5\  N$
$\therefore$ Moment of force $= F \times \perp$ distance
$= 5 \times 1/10 = 0.5\  nm.$
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Question 242 Marks
The moment of a force of $25\  N$ about a point is $2.5\  N m$. Find the perpendicular distance of force from that point
Answer
Moment of force $= 2.5 N m$
Force applied $= 25$
$\perp$ distance from the point of rotation ?
Moment of force $=$ Force $\times \perp$ distance
$2.5 = 25 \times x$
$\therefore x= \perp$ distance$=\frac{2.5}{25}=\frac{1}{10} m$
$=1/10 \times 100 = 10\ cm$
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Question 252 Marks
Find the moment of force of $20\  N$ about an axis of rotation at distance $0.5\  m$ from the force.
Answer
$F = 20 N \perp$ distance $= 0.5\  m$
Moment of force $=$ Force $\times$ perpendicular distance of force from the point of rotation
$= F \times \perp$ distance
$= 20 N \times 0.5 m$
$= 10\  Nm$
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