9th Physics Full Book Constructed response questions
9th Physics Full Book Constructed response questions. We will give the solution of only revised smart syllabus 2025 class 9th class physics.
There are total 37 CROs which are included in 9th class revised smart syllabus 2025 Punjab board.
Chapter 1
1.3 The minimum main scale reading of a micrometre screw gauge is 1 mm and there are 100 divisions on the circular scale. When thimble is rotated once, 1 mm is its measurement on the main scale. What is the least count of the instrument? The reading for thickness of a steel rod shown in the figure. What is the thickness of the rod?
Least count of micrometre screw gauge is 0.01 mm or 0.001 cm.
The edge of the thimble in the figure is just pass the 3mm mark on the main scale. The reference line coincides with 70 divisions on the circular scale.
Thickness = 3+70(0.01)
=3+0.70= 3.70 mm
Hence thickness of steel rod is 3.70 mm.
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1.5 The end of a metre scale is worn out. Where will you place a pencil to find the length?
We will avoid the worn out mark and place the pencil at any clear mark. We will read the scale at the outer end and subtract the starting reading to get the true length.
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1.6 Why is it better to place the object close to the metre scale?
It is better to place the object close to the meter scale to read the marking correctly. This will reduce the chance of error.
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1.7 Why a standard unit is needed to measure a quantity correctly?
A standard unit will ensure accurate, uniform and universally understood measurement.
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1.9 It is difficult to locate the meniscus in a wider vessel. Why?
It is difficult to locate the meniscus ( curved surface of liquid inside a container ) in a wider vessel because the liquid surface becomes flat. It makes the meniscus less curved and hard to see.
Chapter 2
2.2 When a bullet is fired, its velocity with which it leaves the barrel is called the muzzle velocity of the gun. The muzzle velocity of one gun with a longer barrel is lesser than that of another gun with a shorter barrel. In which gun is the acceleration of the bullet larger? Explain your answer.
$$v_f^2-v_i^2=2as\\Bullet\;starts\;from\;rest\\So,\;initial\;velocity\;will\;be\;0\\v_f^2-0=2as\\v_f^2=2as\\\frac{v_f^2}{2s}=a\\a=\frac{v_f^2}{2s}$$
The above relation shows that acceleration is inversely proportional to the distance. So, the shorter barrel gun will have the larger acceleration.
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2.6 Is it possible that the velocity of an object is zero at an instant of time, but its acceleration is not zero? If yes, give an example of such a case.
Yes it is possible. When we throw a ball vertically upward its velocity will be zero at the highest point but its acceleration will not be zero at that instant. At that point its acceleration will be acceleration due to gravity i.e 9.8 meter per second square.
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Chapter 3
3.1 Two ice skaters weighing 60 kg and 80 kg push off against each other on a frictionless ice track. The 60 kg skater gains a velocity of metre per second. Considering all the relevant calculations involved, explain how Newton’s third law applies to this situation.
According to Newton’s third law of motion both the skaters will apply equal and opposite forces on each other.
Law of conservation of momentum
Initial momentum = Final momentum
0 = (60)(4) + (80)(v)
-80v = 240
v= -3 meter per second
The negative sign shows that the 80 kg skater will move in opposite direction with a speed of 3 meter per second.
Hence Newton’s third law tells us that the lighter skater accelerates more and will gain a greater velocity but the heavier skater accelerates less and will gain smaller velocity.
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3.2 Inflatable airbags are installed in the vehicles as safety equipment. In terms of momentum, what is the advantage of airbags over seatbelts?
When suddenly vehicle stops, the passenger’s momentum will change to zero. Airbags increase the time over which the momentum changes.
This increase in time will reduce the force because rate of change of momentum is equal to the applied force. Hence airbags reduce the force on the passenger and provide better protection.
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3.4 When a cricket ball hits high, a fielder tries to catch it. While holding the ball he/she draws hands backward. Why?
When the ball is caught, momentum changes to zero. By drawing the hands backward, the fielder increases the time taken to stop the ball. This increase in time will reduce the force because rate of change of momentum is equal to the applied force. This prevents injury.
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3.5 When someone jumps from a small boat onto the river bank, why does the jumper often fall into the water? Explain.
Due to Newton’s third law of motion the boat exerts a backward force on the person. This makes the boat move and reduces stability, so the person may fall into the water.
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3.6 Imagine that it friction vanishes suddenly from everything, then what could be the scenario of daily life activities?
If friction vanishes suddenly from everything, daily life will be completely disturbed. Because holding, driving and walking will be impossible.
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Chapter 4
4.1 A car travels at the same speed around two curves with different radii. For which radius the car experience more centripetal force? Prove your answer.
If same car travels with same speed around two curves with different radii, for smaller radius car will experience greater centripetal force. Because mass and speed will be constant and radius of the circle will be inversely proportional to the centripetal force.
$$F\;=\;\frac{m\;v^2}r\\If\;m\;and\;v\;are\;cons\tan t\\then\\F\;\propto\frac1r$$
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4.2 A ripe mango does not normally fall from the tree. But when the branch of the tree is shaken, the mango falls down easily. Can you tell the reason?
It happens due to inertia. When branch moves, mango want to stay in its original state due to inertia. Due to this, stem of the mangoes experiences a force and the mango falls down. This is an example of Newton’s first law of motion.
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4.3 Discuss the concepts of stability and centre of gravity in relation to objects topping over. Provide an example where an object’s centre of gravity affects its stability, and explain how altering its base can influence stability.
Stability means after tilting how likely the object is to return to its original position. Centre of gravity of an object is a point where the entire weight of the object appears to be acting.
A cupboard in a room is has high centre of gravity. So a small push can tilt it. When it tilts, centre of gravity can move outside the base, causing the cupboard to topple over. If we increase the base, the vertical line through the centre of gravity still falls inside the base, so cupboard will not topple.
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4.4 Why an accelerated body cannot be considered in equilibrium?
An accelerated body cannot be considered in equilibrium because acceleration means forces are unbalanced.
A body is said to be in equilibrium if all the forces acting on it are balanced and its state of motion does not change
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4.5 Two boxes of the same weight but different heights are lying on the floor of a truck. If the truck makes a sudden stop, which box is more likely to tumble over? Why?
Two boxes of the same weight but different heights are lying on the floor of a truck. If the truck makes a sudden stop the taller box will more likely tumble over due to its higher centre of gravity.
Chapter 5
5.1 Can the kinetic energy of a body ever be negative?
No, Kinetic energy will never be negative. Because mass and square of speed will always be positive.
$$K.E=\frac12mv^2$$
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5.3 A car is moving along a curved road at constant speed. Does its kinetic energy change?
$$K.E=\frac12mv^2$$
kinetic energy depends on mass and speed. Due to constant speed its kinetic energy will not change.
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5.4 Comment on the statement. ” An object has one joule of potential energy.”
An object has one joule of potential energy is incomplete sentence because no reference level is given. Sentence is complete If we say an object has one joule of gravitational potential energy with respect to earth when it is raised to a certain height above the earth’s surface. One joule of potential energy means one joule of work is done to raise an object against the gravity when reference level is earth.
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5.6 While playing cricket on a street, the ball smashes a window pane. Describe the energy changes in this event.
The moving ball has kinetic energy. When it hits its energy changes into sound energy and heat energy and some of the energy is used to break the glass. After break pieces of glass move with kinetic energy.
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5.7 A man rowing boat upstream is at rest with respect to the shore. Is he doing work?
Yes, he is doing work. Boat is at rest but the man is using energy to balance the opposing force of water. So, he is doing the work.
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Chapter 6
6.2 Springs are made of steel instead oof iron. Why?
Main reason is the higher elastic limit of steel. Steel allows the spring to stretch or compress under the force and return to its original shape with permanent deformation.
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6.5 What will happen to the pressure in all parts of a confined liquid if pressure is increased in one part? Give an example from your daily life where such principle is applied.
If pressure is increased in one part of a confined liquid, it will be distributed equally in all the parts of the liquid due to Pascal’s law. A hydraulic car lift is the best example of Pascal’s law. When we apply small force on a piston, the pressure is transmitted through the hydraulic fluid, allowing a larger piston to lift a heavy car
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Chapter 7
7.2 Why is mercury usually preferred to alcohol as a thermometric liquid?
We use Mercury as thermometric liquid due to its wide temperature range. It expands uniformly with temperature and it does not stick to glass. Its boiling point is also high. It does not freeze easily.
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7.3 Why is water not suitable for use in thermometers? Without calculations, guess what is equivalent temperature of 373 k on Celsius and Fahrenheit scales?
Water gives a narrow range of temperature. Its melting point is 0 degree Celsius and boiling point is 100 degree Celsius. It expands evenly with temperature.
$$Celsius\\T_c=373-273\\=100^\circ C\\Fahrenheit\\T_f=\frac95\times100+32\\=212^\circ F$$
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7.6 Why are there no negative numbers on the Kelvin scale?
There will be no negative number on Kelvin scale because temperature cannot be below absolute zero (0 k). Kelvin scale starts from absolute zero.
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7.9 Which is greater: an increase in temperature 1 degree Celsius or one 1 degree Fahrenheit?
1 degree Celsius is greater than 1 degree Fahrenheit because 1 degree Celsius corresponds to 1.8 degree Fahrenheit.
7.13 Discuss whether the Sun is matter.
Yes, sun is matter. Sun consists of 4th state of matter i.e Plasma. It has mainly Hydrogen and Helium atoms.
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Chapter 8
8.1 Two bar magnets are stored in a wooden box. Label the poles of the magnets and identify P and Q objects.
Right Magnet
South (blue) pole is at top and North (red) pole is at the bottom
Left Magnet
South (blue) pole is at the bottom and North (red) pole is at the top
P and Q are non-magnetic or test rods used to detect the magnetic field
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8.2 A steel bar has to be magnetised by placing it inside a solenoid such that end A of a bar becomes N-pole and end B becomes S-pole. Draw circuit diagram of solenoid showing steel bar inside it.
When current flows through the solenoid, it produces magnetic field around its own axis. If we curl the fingers of our right hand in the direction of current flowing through the coils, our thumb points towards the North-pole of solenoid. When we place the steel bar inside the solenoid along its axis and current flows through the solenoid, end of steel bar facing the North-pole of solenoid becomes North-pole and end of steel bar facing the South-pole of solenoid becomes South-pole.
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8.4 Electric current or motion of electrons produce magnetic field. Is the reverse process true, that is the magnetic field gives rise to electric current? If yes, give an examples and describe it briefly.
Yes, the reverse process is true. a magnetic field can give rise to an electric current.
An electric generator works on this principle.
In an electric generator, when a coil is rotated in a magnetic field, the changing magnetic field induces an electric current in the coil, which produces electricity.
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Chapter 9
9.1 Is the theory of science an ultimate truth? Describe briefly.
No, scientific theory is not an ultimate truth. Scientific theory is a well tested explanation of observed natural phenomena but it can be replaced or modified if new evidences are discovered.
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9.2 Do you think that the existing laws of nature may need a change in future?
Yes, existing laws of nature may need change in the future because better explanations and new discoveries can lead to modification of scientific laws.
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9.3 Describe three jobs that need the use of scientific knowledge.
Doctors, scientists, pharmacists, meteorologist and mathematicians, and engineers are the jobs that need the use of scientific knowledge.
Doctors use scientific use of biology and medicine. Pharmacists use scientific use of chemistry to prepare medicine. Engineers use scientific use of physics.
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9.4 Describe when a theory is rejected or needs its modification.
Scientific theory is rejected or modified when new experimental results contradicts its predictions. A Scientific theory is also rejected or modified when a better theory is developed that explains the same facts more accurately.
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9.7 If a hypothesis is not testable, is the hypothesis wrong? Explain.
No, a hypothesis that is not testable is not necessarily wrong. But that hypothesis will not be scientific. A hypothesis must be testable, otherwise it cannot be confirmed or rejected using scientific methods.
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