Physics Ix Notes.docx

MEHRIA ACADMEY DHOKE JILLANI BHARA KAU (ISLD) IMPORTANT CONCEPTUAL QUESTIONS FOR CLASS IX PHYSICS Q1. Why do we need to measure extremely small interval of time? We need extremely small interval of time “Δt” as the smaller is the time interval better resolution of time interval is measured. Q2. How is precision related to the significant figures in a measured quantity? The greater the number of significant figures means greater precision. Each significant figure increases the precision by a factor of ten. Q3. What is the difference between speed and velocity? SPEED VELOCITY The distance covered by an object in a The rate of displacement of a body unit time is called its speed. is called its velocity. Speed =distance Velocity=displacement Time taken Time taken Distance = speed ×time d=v/t S=vt V=dt Speed is a scalar quantity. Velocity is a vector quantity. SI unit of speed is meter per second SI unit of velocity is same as speed 1 (ms ) but direction is also stated. Q4. What is the difference between distance and displacement? DISTANCE Length of the path between two points is called distance between two points. Distance is scalar quantity. Distance is denoted by S S= vt SI unit is ms-1

DISPLACEMENT Shortest distance between two points with direction is called displacement. Distance is a vector quantity. Distance is denoted by d. d= vt SI unit is also ms-1 1

Q5. Can a body moving at a constant speed have acceleration? Yes, when a body is moving with constant speed and its direction changes and acceleration is produced. For example if the body is moving in a circle with constant speed it will have acceleration. Q6.How do riders in a Ferris wheel possess translational motion but not circular motion? Riders in a Ferris wheel possess translational motion because their motion is in circle without rotation. Q7. Why vector quantities are not added or subtracted like scalar quantities? As scalar quantities have no direction so they obey arithmetic and algebra. Since vector quantities have direction also so they obey special rules of vector addition. Q8. What is the difference between Mass and Weight? MASS The quantity of matter contained in a body is called its mass. The mass of the body remain constant everywhere. Mass is a scalar quantity. SI unit of mass is kilogram. m=w/g

WEIGHT Weight is the force with which earth attract a body towards its center. Weight of the body is not a constant quantity but its value is different at different places. Weight is a vector quantity and always directed towards the center of the earth. SI unit of weight is Newton. w=mg

Q9. What is rolling and sliding friction? A force between the sliding objects which possess relative motion is called sliding friction.

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Force of friction between a rolling body and surface over which it rolls is called rolling friction. Q10. Why it is dangerous to ride on the roof of the bud? Because the force of air friction acting on passenger try to turn over which is dangerous for passenger while at low portion remain at rest w.r.t the roof of the roof of the bus due to inertia. Q11. Why does a passenger move outward when a bus takes a turn? Due to the absence of necessary centripetal force a passenger move outward when bus takes a turn? Q12.When a gun is fired it recoils. Why? As gun and bullet both are at rest before firing the gun so total momentum of the system is zero .when the gun is fired bullet shoot out of the gun with some momentum. To conserve the momentum of the system the gun recoils. Q13. Why rolling friction is less than the sliding friction? When a body rolls over the surface of another body it has only one point of contact so it has small sliding friction. When a body move on the other surface there is relative motion between two surfaces thus friction has some maximum value. That is the rolling friction is less than the sliding friction by 100 times. Q14. Why seatbelt saves from serious injury? The more time is required for stretching seat belt. This increases the stopping time for momentum to change and reduce the effect of collision and save from serious injury. Q: 15 what is the difference between torque and couple? TORQUE COUPLE Torque is a special kind of force Couple is formed by unlike that has the capacity to rotate an parallel forces of same object about an axis. 3 magnitude but not along the same line.

Q16. Give an example of a moving body which is in equilibrium? A car moving with uniform velocity on leveled road and plane flying in the air with uniform velocity are examples of bodies in equilibrium. Q17. Why the height of the vehicles is kept as low as possible? As whole weight of the body acts on center of gravity so, so the center of gravity is kept close to the earth so that there is less chance of overturning of vehicle. Q18. Why a vehicle is made heavy at its bottom? A vehicle is made heavy at it bottom to keep its center of gravity as low as possible. A lower center of gravity keeps it stable. (Moreover the base of vehicle is made wide so that the vertical line passing through center of gravity should not get out of its base during a turn. Q19. What is meant by force of gravitation? There exists a force due to which every body of the universe attracts every other body. This force is called force of gravitation? Q20. Gravitational forces are consistent with third law of motion. Explain? It is to be noted that m1 attracts m2 with F1 and m2 attracts m1 with F2 which is same in magnitude but opposite in direction. If F1 is action, then F2 is considered as reaction. Action and reaction due to gravitational forces are equal but opposite in direction. This is consistence with Newton’s third law of motion Q21. What is meant by gravitational field strength?

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In gravitational field of the earth, the gravitational force per unit mass is called gravitational field strength of the earth. It is 10N kg-1 near the surface of the earth.

Q22. On what factors does orbital speed of a satellite depends? As orbital speed v0 = √ gh (R +h) As clear orbital speed depends upon g(gravitational acceleration),R(radius of earth ) and h(height from the earth surface).

Q23. Why communicational satellites are stationed at geostationary orbits? Communicational satellites are given geostationary orbit so that the antennas that communicate with them do not have to move to track them, they can be pointed permanently.

Q24. Why fossils fuels are called non-renewable form of energy? Fossil fuels are known as renewable sources because it took millions of years for them to attain the present shape. Q25. Name 3 devices that convert electrical energy into mechanical energy? •

Electric fan

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Drill machine

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Juicer machine

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Q26. Write three postulates of kinetic molecular model of matter? Matter is made up of particles called molecules These molecules remain in continuous motion Molecules attract each other. Q27. it is easy to remove air from a balloon but it is very difficult to remove air from a glass bottle? This is because the air inside the balloon is at fairly high pressure than the atmospheric pressure on air outside the balloon. On the other hand the air inside the glass bottle is already equal to the atmospheric pressure, so it is difficult to remove air from the glass bottle. Q28. why does a piece of stone sink in water but a ship with a huge weight floats? It is due to the Archimedes principal. Density of ship is less it displaces more water, experience more up thrust and floats where as density of stone is more it displaces less water experience less up thrust and sinks. Q29. Differentiate heat capacity and specific heat capacity? Specific heat Amount of heat required to raise the temperature of 1 kg mass through 1 Kelvin. c= SI unit is Jkg-1 k-1

Specific heat capacity Amount of heat required to raise the temperature of a body through 1K. Heat capacity = mc Unit of heat capacity is Jk-1

Q30. Why gapes are left in railway tracks? Gapes are left in railway tracks to compensate thermal expansion during hot season. Railway tracks buckled on a hot summer day due to expansion if gapes are not left between the sections. 6

Q31. Why gapes are left in bridges with roller? Bridges made with girders expand during the hot summer day and contract during night. They will bend if there edges are fixed. To allow thermal expansion one end is fixed and other end rests on rollers in the gap left for the expansion. Q32. Why overhead transmission lines also given a certain amount of sag? Overhead transmission lines are also given a certain amount of sag so that they can contract during winter without snapping. Q33. Why coefficient of volume expansion of liquid is greater than the solid? The molecules of solids are held tightly on the other hand molecules of liquid are free to move so on heating the amplitude of liquid molecules increases so they need more space to occupy. That is why coefficient of volume expansion of liquid is greater than the solid. Q34. Why heat flows from hot body to cold body? Heat flows from hot body to cold body due to the difference of temperature of hot and cold part of body. Q35. Why does a cup of tea become cold after some time? When a body is at a temperature higher than its surrounding, then it radiates heat. A cup of hot tea, radiate more heat, than it absorb. As a result temperature goes on decreasing till it become equal to the temperature of surrounding. Q36. Why does a glass of chilled water become hot after some time? 7

When a body is at temperature lower than its surrounding than it absorb energy till its temperature becomes equal to the temperature of its surrounding. So a glass of chilled water becomes hot after some time by absorbing energy. Q37. How heat is prevented to enter or leave the thermos flask? In thermos flask most of heat is prevented to enter or leave the system by suitable measures to stop conduction convection and radiation. Anything kept in it maintains its temperature for a long time. Q38. Why deserts get hot during day and soon get cold during night why? The desert land has low specific heat capacity that is why it will soon get heated or cooled. Hence desert nights are colder and days are very hot. Q39. What is greenhouse effect and global warming? GREEN HOUSE EFFECT: Ultraviolet and infrared radiations of shorter wave length from the sun enter from glass or polythene sheets and warm up the objects inside it. Objects reflect waves of longer wavelength and glass or plastic sheet does not allow passing them. This maintains the temperature of green house. This is called greenhouse effect. GLOBAL WARMING: CO2 and water present in the atmosphere act in the same way to the radiation as glass or polythene. It causes greenhouse effect and maintains temperature of earth. During the recent years the amount of CO2 has been increased, this causes an increase in the average temperature of the earth. This is called Global Warming. -------------UNITY ----------------------FAITH--------------------DISCIPLINE--------8