Physics Class x - Scientific Reasons - (Old Book)

Scientific Reasons

Q.No.1: Why do passengers move forward when the bus suddenly stops?
Answer: Passenger in a bus also in motion with same velocity, due to inertia and will continue their motion when brakes are applied and move forward.

Q.No.2: Why is it dangerous to jump from fast moving vehicle?
Answer: Person in a fast moving vehicle is also in motion with the same velocity. When he jumps out of the vehicle, he continuous this state due to inertia and when he collides with anything outside, he may be hurts severely.

Q.No.3: Why does a ball thrown vertically upward fall down?
Answer: When the ball is thrown vertically upward, the force of gravity continuously reduces its velocity till it stops and pulls the ball downward.

Q.No.4: Why is it difficult to stop a loaded truck then an unloaded truck?
Answer: Mass is the measure of inertia. So, due to greater mass of loaded truck it will take more force to stop than an unloaded truck.

Q.No.5: Why do a ball and a feather take the same time to reach the ground from some height when dropped freely, when there is vacuum or no fluid friction?
Answer: The acceleration due to gravity is irrespective of mass so it is acting equally on both the objects hence they will take some time in reaching the same height.

Q.No.6: Why does a loaded truck collide more forcefully than a motorcycle moving with same velocity?
Answer: Due to greater mass of loaded truck, it has greater momentum than a motorcycle. That's why, it will collide more forcefully.

Q.No.7: A person walks 20 steps forward and 20 steps backward and covers a distance of 40 steps while his displacement will be zero. why?
Answer: Distance is a scalar quantity so it is added arithmetically while displacement is a vector quantity and it is added by head to tail rule. So, they cancel each other, due to opposite direction.

Q.No.8: Why does a lighter boy balance are heavier boy on seesaw?
Answer:  When the heavier boy moves closer to the axis of rotation reduces the torque is reduced to, balance the torque produce by lighter boy.

Q.No.9: Why does a kite fly in air?
Answer: The thrust of air is balanced by the force applied on the string attached to the kite to keep its state in equilibrium according to first condition of equilibrium hence it flies.

Q.No.10: Why does a paratrooper fall with uniform velocity?
Answer: Upthrust on parachute balances the weight of the paratroopers and hence it becomes in dynamic equilibrium according to first condition of equilibrium. Hence, he falls with uniform velocity.

Q.No.11: Why does the pirate ship in Aladin Park not fall during swinging?
Answer: The point  of support of that ship is above its centre of gravity. Therefore, it becomes in stable equilibrium.

Q.No.12: Why is a Ferris Wheel in Aladdin in equilibrium while swinging?
Answer: The Stand of the wheel holds it like centre of gravity and the cabins on either side keep the wheel in second condition of equilibrium. Hence, it rotates freely.

Q.No.13: How does a motorcyclist ride  uniformly in a death well?
Answer: The wall of death well provides centripetal force to the  motorcycle while its weight provides the centrifugal force. So, they cancel each other and hence the motorcyclist rides uniformly.

Q.No.14: Why is the weight of a body is maximum at the poles than at equator?
Answer: The acceleration due to gravity is maximum at the poles than at equator which increases the weight of the body at the poles.

Q.No.15: Why does a body become weightless in space?
Answer: The gravitational forces acting on the object due to other planets and stars cancel each other which keeps the body weightless in space.

Q.No.16: Why is the weight of a body greater on the earth than on the moon ?
Answer: The product of the mass of a body and the mass of the earth is greater than the product of the mass of  body and the mass of moon. Hence, the weight of the body increases due to Gravitational force at the earth.

Q.No.17: Why does an object, thrown vertically upward, always fall with the same velocity ( if there is no air resistance) ?
Answer: We know that the total energy of a system will remain constant if there is no resistance. So, the energy of the object remain constant and it will return back with the same velocity.

Q.No.18: Why are gaps left between two adjacent rails? OR Why are gaps left in between rails while laying a railway track?
Answer: When a heavy train runs over the rails, due to friction and heat of the surroundings, the rails expand due to linear thermal expansion. If there are no gap between them, they may bend, causing accidents. So, gaps between them.

Q.No.19: Why is the efficiency of screw jack always less than 100%?
Answer: The efficiency of screw Jack is less than 100% to overcome the frictional forces between different parts of the screw jack, as lot of heat energy is consumed.

Q.No.20: For an ideal machine, output = input but practically, output is not equal to input . Why?
Answer: It is impossible that the output is equal to input because the machine overcomes the internal and external resistance by the same input. So, the value of output is reduced and hence, it is said that output = input for an ideal machine.

Q.No.21: Why is it better to use wheel and axle in place of pulley?
Answer: In pulley, we apply the same effort as compared to load while small effort is required by the wheel to lift the load from axle.

Q.No.22: Why is Iron more elastic than rubber?
Answer: More force is required to deform iron which has more elastic limit while a very small force is applied to deform rubber. So,  Iron is more elastic than rubber.

Q.No.23: Why is rolling friction much lesser than sliding friction?
Answer: The physical contact of rolling objects is very small and also it rotates easily as compared to the physical contact of sliding object.

Q.No.24: When a bullet is fired from a gun, why does it have a great momentum?
Answer: The spring attached to the recoil of gun stores elastic potential energy when gun is loaded. Hence, when it is fired, due to great potential energy of the spring attached to the recoil of heavy gun, it transfers its energy into the bullet. That is why it has great momentum.

Q.No.25: Why is a person thrown backward when,  he fire the bullet from a gun?
Answer: To every action, there is an equal and opposite reaction. The gas produced behind the bullet when the recoil strikes, throws the gun backward. So in reaction, the bullet moves forward.

Q.No.26:  Why does the weight of stone seem heavier in air  than in water?
Answer: The density of water as compared to air is greater. So, the particles of water apply more thrust more than the particles of air. Hence, the weight of stone scenes less in water.

Q.No.27: Why do we use Steel bar inner roofs during construction instead of aluminium which is very light?
Answer: Steel is more elastic than aluminium and can be maximum load, that's why steel is used in roofs during construction.

Q.No.28: Why is it easy to come out from the depth of sea as compared to the depth of lake?
Answer: The density of sea water is more then the density of  lake water and also it is stagnant. That is why, lake water pushes the diver download while sea water pushes the diver upward.

Q.No.29: Why it is better to use hydraulic brake in heavy vehicles as compared in mechanical brake?
Answer: Enclosed liquid transmits pressure equally all dimensions, according to Pascal's principle. So, it can apply more force as compared to the force applied by the person on brake.

Q.No.30: Why does a nail sinks in water but a ship having huge load floats?
Answer:  The weight of a ship is equal to its thrust . That is why, it floats while the weight of the nail is greater than its thrust, it sinks in water.

Q.No.31: Why does a hot air balloon start rising initially, then floats in air when reaching a certain height?
Answer: The thirst of air is more than the weight of the balloon near the surface of earth. That is why, it rises up  while, when reaching a certain height due to low air pressure, the weight of the balloon becomes equal to its thrust, that is why it is starts floating.

Q.No.32: How is a submarine able to sink in water rise in water and float?
Answer: The submarine has a tank of air or gas  when it is unfilled with water it weight becomes equal to thrust it is starts floating, while when water is filled inside the tank, it starts sinking because the weight become greater than the thrust and if the tank is unfilled with water, its weight becomes lesser than thrust. So it rises up.

Q.No.33: In summer, why are the wires tied separately by a stick instead of tightened among the poles?
Answer: If the wires among the poles are tightened during summer and then during winter, the wires tighten themselves, due to contraction and may break, that is why the wires are tied separately by the sticks.

Q.No.34:  Why does ice float on water?
Answer: The density of ice at 0 °C is less than the density of water that's why it is starts floating on water.

Q.No.35: Why do we use utensils of aluminium for cooking Institute of clay pots?
Answer: The thermal conductivity of aluminium is much greater than that of clay. So, it transfer heat more rapidly which helps us in fast cooking.

Q.No.36: Why are the holders of cooking pots or utensils made of wood and plastics?
Answer: Wood and plastics are bad conductors or insulators of heat so they do not get hot.

Q.No.37: Why is more heat required to raise the temperature of 1 kg of water than 1 kg of iron through 1°C ?
Answer: The specific heat of water is greater (i.e. 9 times greater) than iron so that water needs more heat to raise its temperature through 1° C.

Q.No.38: Why is burning by steam more severe than water at its boiling point?
Answer: The heat possessed by steam at 100 ° C is more due to latent heat of vaporization as compared to water at the same temperature.

Q.No.39: Why do the pipe lines in cold areas burst during winter?
Answer: In winter, the temperature gets below the freezing point. So, the pipes contract while the water inside expands due to anomalous expansion and hence it bursts.

Q.No.40: Why do water bottles burst in deep freezers?
Answer: Due to anomalous expansion of water, water expense while the bottles contract, that is why they burst.

Q.No.41: How does marine life survive in frozen areas?
Answer: In frozen areas, the temperature is for below the freezing point. So, the upper layer of the lake, sea or river becomes ice and floats on sea water due to low density. While below this layer, water remains in its state and high density water remains at the bottom. Hence, marine life survives in frozen areas.

Q.No.42: Why are pressure cookers used in areas of high altitudes?
Answer: The atmospheric pressure at high altitude is low which decreases boiling point of water. So, the cooking takes more time. Therefore, pressure cookers are used to increase the boiling point of water.

Q.No.43: How are Ice balls made by crushed ice?
Answer: The ice melts under pressure but when crushed pieces join together, they  refreeze again due to regelation and hence, ice balls are made.

Q.No.44: Ice Hockey is played more quickly compared to normal hockey. Why?
Answer: The players wear shoes which have vertical blades at the bottom with which pressure is exerted on ice which will reduce friction. That is why the players slide over the molten which ice again refreezes due to regelation.

Q.No.45: When a flask, having narrow neck, full of water is heated, why does the water surface in the next falls  in the beginning?
Answer: On heating, everything expands. So, the liquid inside expends as well as the flask. This gives some space which is occupied by what water. Hence, the level falls.


Q.No.46: Why is the climate of the region near the sea or lakes moderate?
Answer: The specific heat of water is maximum so it takes heat in the day of Summer but, at night, the cold air above the sea takes place of the air on land (i.e. sea breeze) keeps the temperature moderate.

Q.No.47: Why can we not hear the sound of explosion produce on the sun?
Answer: Sound needs a material medium and vacuum is not a material medium. So, we cannot hear the sound of explosions produced on the sun.

Q.No.48: Why are soldiers instructed to break their steps while crossing a bridge?
Answer: When soldiers are marching over a bridge, it starts vibrating with greater amplitude due to resonance and hence it may collapse. That is why, they are instructed to break their steps while crossing a bridge.

Q.No.49: Why is the flash of lightning seen earlier than the sound of thunder?
Answer: the speed of light is greater than the speed of sound. That is why, flash of lightening is seen earlier than the sound of thunder.

Q.No.50: Why is parabolic concave mirror used in headlights and searchlights?
Answer: When source of light is near the focus of concave mirror it throws the light at a long distance. That is why concave mirror is used in headlights and searchlights.

Q.No.51: Why is convex mirror used for rear view mirror?
Answer: Since convex mirror diverges the parallel beam of light, used properly in contrast as it can view a large portion as compared to plane mirror. Hence, convex mirror is used for rear view mirror.

Q.No.52: Why does light persists after sunset and come before sunrise?
Answer: In the morning or in the evening, when sun is not exactly in front of us, it's light is scattered from dust particles all around in our atmosphere. That is why, light remains before sunrise and persist after sunset.

Q.No.53: Why does the depth of a swimming pool seem less deep as compared to its actual depth?
Answer: When light travels from air into water, it will bends towards the normal while when traveling from denser to rarer medium, it bends away from the normal. Due to this property, depth of a pool seems lesser as compared to its actual length.

Q.No.54: Why does a spoon seem bigger when dipped in a glass full of water?
Answer: The glass full of water behaves like a lens. So, the spoon seems to be an object lying under its focal length. That is why, we see its virtual and magnified image.

Q.No.55: Why does light that enters from one end of an optical fibre, come out with the same intensity from the other end?
Answer: When light enters in the core of optical fibre, due to internal reflection, there is no loss of energy within the optical fibre. So, it comes out with the same intensity.

Q.No.56: Why does a car parked under the sun for a long time, become extremely hot inside as compared to its surrounding?
Answer: The glass allows the sunlight to come inside the car but the heat waves or the heat is not allowed to come out from these glasses. Hence it show the green house effect. So it becomes hotter inside then the outer surroundings.

Q.No.57: Why is plastic or rubber used to cover the electrical wires?
Answer: Plastics and rubber are bad conductors of heat and electricity. That is why, they are used to cover the electric wires to prevent electric shocks and short circuits.

Q.No.58: Why does the thunder produced by the clouds fall to the ground?
Answer: The earth is at zero potential for the electrical energy and we know that energy flows from high level to low level.  That's why it falls to the ground.

Q.No.59: When ebonite scale is rubbed with hair, why does it attract paper pieces?
Answer:  When ebonite scale is rubbed with hair, it give excess of electrons. When kept close to the paper pieces, they get positively charged under its influence. So, it attracts paper pieces  due to electrostatic force.

Q.No.60: Why is the equivalent resistance of the circuit, when resistance are connected in parallel always less than the individual resistance?
Answer : When resistors are connected in parallel, the total current flowing through the circuit is divided between all the resistance. That is why, equivalence resistance is always less than the individual resistances.

Q.No.61: Why is it not possible to measure the alternating current with the help of the moving coil galvanometer?
Answer: The magnetic field, produced in the coil, depends upon the direction of current and when A.C. passes through the coil, the direction continuously changes in a second. Therefore, pointer vibrates around the zero mark rapidly and we cannot measure the alternating current from the moving coil of Galvanometer.

Q.No.62: Why are highly pure Silicon and Germanium crystal almost insulators, especially at low temperatures?
Answer: Pure Silicon and Germanium crystals form strong covalent bonds with their neighbouring atoms. Since the electrons are tightly held in covalent bonds, there are no, really free electrons to form electric current on the application of a potential difference. Hence, they are mostly complete insulators.

Q.No.63: Why is the transistor used as a switching device?
Answer: Any thing which can switch on a small base current toward transistor will release a large collector current to operate relay loudspeaker etc.

Q.No.64: Why do beads emit light in darkness?
Answer: Beads have a coating of radium which is a radioisotope. So, it emits radiation. That is why, they emit light in darkness.

Q.No.65: Why are seven colours seen in the rainbow?
Answer: During or after rainfall, sun rays fall obliquely on the raindrops which behave like a prism and white light split into seven colours which are seen in the form of rainbow in the sky.

Q.No.66: Why does the surface of water at rest behave as plane mirror?
Answer: The surface of water at rest is the planest surface in the world. So, it behaves as plane mirror.

Q.No.67: Why is the magnetic field stronger inside the solenoid then outside?
Answer: The magnetic lines of force cannot pass easily in air.

Q.No.68: Why does a magnet become demagnetized when it is placed inside the solenoid, having alternating current?
Answer: The alternative current reverses  its direction several times in a second which also changes the polarity. So, the magnet is demagnetized.

Q.No.69: Why do pulses and meat take longer time to cook at high altitude?
Answer: At high altitude, due to low pressure, the boiling point of water is lower. So pulses and meat take longer time to cook.

Q.No.70: Why is it difficult to walk on ice?
Answer: The ice under the feet of a person melts due to regelation,  which behave as a lubricant and hence it reduces the friction.

Q.No.71: Why are exhaust fans always fitted near the roof on the wall?
Answer: The hot and polluted air in the room moves up, due to low density. So, exhaust fans are always fitted above to pull this air out.

Q.No.72: Why do ripened fruits fall down from the tree when its branches are shaken?
Answer: The contact with the branches of ripe fruits is comparatively less. So, when branches are shaken, they fall to the ground due to gravity.

Q.No.73: Why is white light disperse when it passes through a prism?
Answer: When white light passes through a prism, the colour which has  the greater  frequency deviates more  while colour having low frequency deviates less. So light splits up into its seven colours , according to their frequencies or wavelengths.

Q.No.74: Why is a concave lens of suitable focal length used to remove short sightedness of eyes?
Answer: The eye lens becomes more converging. So, it forms the image in front of retina. That's why a diverging or concave lens is used to produce a clear image on retina.

Q.No.75: Why is radioactive substance placed in Lead box, with the lid made of lead?
Answer: Lead never allows radiation to pass through it because of its greater density. So substances are placed in a lead Box with a lid made up of lead.

Q.No.76: Why does a parrot toy on a support not fall down after it is given a sufficient Jerk?
Answer: When a parrot toy is disturbed after given a sufficient jerk, its centre of gravity raises. Hence it comes in stable equilibrium. So, it does not fall.

Q.No.77: Why are houses made up of wood, in frozen areas?
Answer: Wood is bad conductor of heat and electricity. So, in frozen areas, houses are made up of wood as it prevents the flow of heat out of the houses.

Q.No.78: Why is it difficult to drive a motorcycle on an oily road?
Answer: Due to oil on the road, the friction is decreased. So, it is difficult to drive motorcycle on it.

Q.No.79: Why does a person have to bend down while carrying a load in his back?
Answer: To keep the centre of gravity in line with his feet, a person bends down while carrying a load on his back.

Q.No.80: Why does the direction of magnetic lines of force change if an iron piece is placed in its field?
Answer: Iron has its own magnetic field and when an iron piece is placed in his field, it disturbs the magnetic lines of force and hence changes the direction.

Q.No.81: Why is the index refraction of ruby is less than the index of refraction of diamond?
Answer: The sine of the critical angle of ruby is greater than the sine of the critical angle of diamond due to the fact that,  ղ= 1/ Sin C.

Q.No.82: Why should liquid used in a thermometer be a good conductor of heat?
Answer: The liquid used in a thermometer should be a good conductor of heat which expand rapidly on gaming heat and shows accuracy in the measurement of temperature.

Q.No.83: Moon revolves around the earth under the influence of its gravitational field. Why then does it not fall on to the ground due to the same gravitation pull?
Answer: Due to centrifugal throw on moon by other planets and stars, it comes in equilibrium. That is why, it does not drop on to the earth.

Q.No.84: Why is it difficult to climb up flight of stairs then to walk an equal distance on the ground?
Answer: It is difficult to climb a flight of stairs as compared to walking on the ground because moving against gravity is more difficult than moving along the ground.

Q.No.85: In an Ammeter, why is low resistance parallel with the coil?
Answer: To prevent the coil from breaking, a low resistance is connected in parallel which splits the current between them and allows lesser amount of current to pass through the coil.

Q.No.86: In a Voltmeter, why is a high resistance connected in series?
Answer: To prevent the coil from breaking, it divides potential difference across both of them.   Greater potential difference arise across the high resistor.

Q.No.87: Why is convex lens used to remove long sightedness?
Answer: The eye lens becomes less converging. So, it forms image behind the retina. Hence, we use converging or convex lens of suitable focal length to form image on the retina.

Q.No.88: Why is Diode used as a rectifier?
Answer: When p-type substance is connected with the positive terminal and vice versa, diode allows the current to flow through it. But when p-type substance is connected with the negative terminal, diode shows reverse bias and no current will be drawn through it.

Q.No.89: When current is flowing in upward direction in a wire, why do magnetic lines revolve in anticlockwise direction?
Answer: According to the right hand rule, when current is flowing in a wire in upward direction,  the magnetic lines of force represented by fingers moving in the direction of palm i.e. in anticlockwise direction.

Q.No.90: Why are the stars seem twinkle in the sky?
Answer: Due to refraction from different air patches, having different optical densities, the stars seem to the twinkling.

BY Miss Samra Khan