34 Objective Questions on Physics Part-1 (Civil Service Examination)

A. Mechanics

5. A simple machine helps a person in doing: [1996]

(a) less work

(b) the same amount of work with lesser force

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(d) the same amount of work much faster

6. The tendency of a liquid drop to contract and occupy minimum area is due to: [1997]

(a) surface tension (b) viscosity

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7. Which one of the following is a vector quantity? [1997]

(a) Momentum (b) Pressure

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8. A girl is swinging on a swing in sitting position. When the girl stands up, the period of swing will: [1997]

(a) be shorter

(b) be longer

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(d) not change

10. The working principle of a washing machine is: [1997]

(a) centrifugation (b) dialysis

12. A ball is dropped from the top of a high building with a constant acceleration of 9.8 m/s². What will be its velocity after 2 seconds? [1998]

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(a) 9.8 m/s (b) 19.6 m/s

13. A hunter aims his gun at a point between the eyebrows of a monkey sitting on a branch of a tree. Just as he fires, the monkey jumps down. The bullet will: [1999]

(a) hit the monkey at the point aimed

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(b) hit the monkey below the point aimed

(d) miss the monkey altogether

14. For which one of the following is capillarity not the reason? [1999]

(a) Blotting of ink

(b) Rising of underground water

(d) Rising of water from the roots of a plant to its foliage

15. Consider the following features of newer models of motor cars: [2000]

1. Radial tyres

2. Streamlined body

3. Multipoint fuel injection

4. Catalytic converter with exhaust

Which of these features make the new models of motor cars more ‘fuel efficient?

(a) 1 and 2 (b) 2 and 3

16. Assertion (A): A man standing on a completely frictionless; surface can propel himself by whistling. [2000]

Reason (R): If no external force acts on a system, its momentum cannot change.

Codes:

(a) Both A and R are true and R is the correct explanation] of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

17. The mass of a body on earth is 100 kg (acceleration due to gravity, g_e = 10 m/s². If acceleration due to gravity on the moon is g_e / 6, then the mass of the body on the moon is: [2001]

(a) 100/6 kg (b) 60 kg

18. Consider the following statements: [2001]

A simple pendulum is set into oscillation.

Then:

1. the acceleration is zero when the bob passes through the mean position.

2. in each cycle the bob attains a given velocity twice.

3. both acceleration and velocity of the bob are zero when it reaches its extreme position during the oscillation.

4. the amplitude of oscillation of the simple pendulum decreases with time.

Which of these statements are correct?

(a) 1 and 2 (b) 3 and 4

20. Assertion (A): With the increase of temperature, the viscosity of glycerine increases. [2002]

Reason (R): Rise of temperature increases kinetic energy of molecules. Codes:

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

21. Assertion (A): An iron ball floats on mercury but gets immersed in water. [2002]

Reason (R): The specific gravity of iron is more than that of mercury. Codes:

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

22. A solid cube just gets completely immersed in water when a 0. 2.kg mass is placed on it. If the mass is removed, the cube is 2 cm above the water level. What is the length of each side of the cube? [2002]

(a) 12 cm (b) 10 cm

23. Assertion (A): The weight of a body decreases with the increase of altitude on earth. [2003] Reason (R): The earth is not a perfect sphere.

Codes:

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

24. Consider the following statements: [2003]

A 4-wheel vehicle moving in a sharp circular path at high speed will:

1. overturn about its outer wheels

2. overturn about its inner wheels

3. skid outwards

4. skid inwards

Which of these statements are correct?

(a) 1 and 3 (b) 2 and 4

25. An oil tanker is partially filled with oil and moves forward on a level road with uniform acceleration. The free surface of oil then: [2003]

(a) remains horizontal

(b) is inclined to the horizontal with smaller depth at the rear end

(d) assumes parabolic curve

26. If the radius of the earth were to shrink by one per cent, its mass remaining the same, the value of g on the earth’s surface would: [2003]

(a) increase by 0.5% (b) increase by 2%

27. A car is running on a road at a uniform speed of 60 km/h. The net resultant force on the car is: [2004]

(a) driving force in the direction of car’s motion

(b) resistance force opposite to the direction of car’s motion

(d) equal to zero

28. A spherical body moves with a uniform angular velocity (w) around a circular path of radius r. Which one of the following statements is correct? [2004]

(a) The body has no acceleration

(b) The body has a radial acceleration directed toward centre of path

(d) The body has an acceleration co² tangential to its path

29. A weightless rubber balloon is filled with 200 cc of water. Its weight in water is equal to: [2004]

(a) 9.8/5 N (b) 9.8/1 ON

30. Which one of the following is the correct sequence of the given substances in the decreasing order of their densities? [2005]

(a) Steel > Mercury > Gold

(b) Gold > Mercury > Steel

(d) Gold > Steel > Mercury

31. What is the approximate mean velocity with which the earth moves round the sun in its orbit? [2006]

(a) 20 km/s (b) 30km/s

32. Four wires of same material and of dimensions as under are stretched by a load of same magnitude separately. Which one of them will be elongated maximum? [2007]

(a) Wire of I m length and 2 mm diameter

(b) Wire of 2 m length and 2 mm diameter

(d) Wire of 1 m length and 1 mm diameter

33. Three identical vessels A, B and C are filled with water, mercury and kerosene respectively up to an equal height. The three vessels are provided with identical taps at the bottom of the vessels. If the three taps are opened simultaneously, then which vessel is emptied first?[2007]

(a) Vessel B

(b) All the vessels A, B and C will be emptied simultaneously

(d) Vessel C

B. Heat and Thermodynamics

34. The clouds float in the atmosphere because of their low: [1995]

(a) Temperature (b) velocity

36. Low temperatures (cryogenics) find application in: [I999]

(a) space travel, surgery and magnetic levitation

(b) surgery, magnetic levitation and telemetry

(d) space travel, magnetic levitation and telemetry

37. Assertion (A): A piece of copper and a piece of glass are heated to the same temperature. When touched, thereafter, the copper piece appears hotter than the glass piece.

Reason (R): The density of copper is more than that of glass. [2001]

Codes:

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

38. Assertion (A): The boiling point of water decreases as the altitude increases.

Reason (R): The atmospheric pressure increases with altitude. [2001]

Codes:

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

39. When water is heated from 0°C to I0°C. Its volume: [2001]

(a) increases

(b) decreases

(d) first decreases and then increases

40. A hollow sphere of radius R, a hollow cube of side R and a thin circular plate of radius R, made up of the same material, are all heated to 20°C above room temperature. When left to cool in the room, which of them will reach the room temperature first? [2002]

(a) Circular plate

(b) Cube

(d) All of them will reach the room temperature at the same time

41. Consider the following statements: [2003]

1. Steam at 100°C and boiling water at 100°C contain same amount of heat.

2. Latent heat of fusion of ice is equal to the latent heat of vaporization of water.

3. In an air-conditioner, heat is extracted from the room air at the evaporator coils and is rejected out at the condenser coils.

Which of these statements is/are correct?

(a) 1 and 2 (b) 2 and 3

42. Cloudy nights are warmer compared to clear cloudless nights, because clouds: [2001]

(a) prevent clod waves from the sky from descending on earth

(b) reflect back the hear given off by earth

(d) absorb heat from the atmosphere and send it towards earth

Answers:

A. Mechanics

5. (b) A simple machine is a mechanical device that changes the direction or magnitude of a force. In general, they can be defined as the simplest mechanisms that use mechanical advantage to multiply force. Thus, simple machine helps us in doing same amount of work with lesser force. Few examples of simple machines are pulley, lever .wheel, screw, etc.

6. (a) It is observed that liquid drop tend to contract and appear spherical when it is set free from external forces like gravity, etc. Surface tension is the property among liquids due to which they tend to occupy minimum surface area. That’s why water droplet appears spherical because for a given volume sphere has minimum surface area. Due to this property of surface tension liquid surface stretches and behave like a stretched membrane.

7. (a) Scalars are quantities that have magnitude only; they are independent of direction. Vectors have both magnitude and direction. Momentum is the product of the mass and velocity of an object (p = mv) momentum is a vector quantity, since it has a direction as well as a magnitude .Whereas, the rest of quantities in option pressure, work and energy have magnitude but not direction.

8. (a) Effective length is the length of point of suspension to the centre of gravity(mass) of a body. Let it be l when girl is sitting and I’ when girl stands up. Here l > l’. Thus time period of swing will decrease because time period of swing is given by formula

T = 2∏√ l/g

So, it is clear from above formula that time period of system is directly proportional to effective length.

10. (a) Washing machine works on the principle of centrifugation. Centrifugation is a process that involves the use of the centrifugal force for the separation of mixtures with a centrifuge, used in industry and in laboratory settings. More-dense components of the mixture migrate away from the axis of the centrifuge, while less-dense components of the mixture migrate towards the axis.

12. (c) The velocity after time t is given by

v = u + gt

Here, u = 0, t = 2s, g=9.8 m/s²

v = 0 + (9.8x 2) = 19.6ms

13. (a)

14. (b) Capillary action, or capillarity, is a phenomenon where liquid spontaneously rises in a narrow space such as a thin tube, or in porous materials such as paper or in some non-porous materials such as liquefied carbon fibre. This effect can cause liquids to flow against the force of gravity or the magnetic field induction. In blotting of ink, spread of water drop on a cotton cloth, rising of water from the roots of a plant to its foliage.

15. (b) A “streamlined” design is one in which objects that move through a gas or liquid are shaped to match these lines, and therefore, reduce the energy required to produce that motion. Multi point fuel injection system injects fuel into individual cylinders, based on commands from the ‘on board engine management system computer-popularly known as the Engine Control Unit/ECU. These techniques result not only in better ‘power balance’ amongst the cylinders but also in higher output from each one of them, along with faster throttle response. The rest two options radial tyres and catalytic converter with exhaust do not contribute to make car more fuel efficient.

16. (b) First statement can be explained on the basis of laws of motion. First statement involves the use of Newton’s third law of motion, “For every action there is equal and opposite reaction”. In first case while whistling a man using his breathe as a propellantto propel himself. The second statement can be explained on the basis of law of conservation of linear momentum according to which the momentum of body remains conserved until any external force act upon it.

17. (c) Mass is a universal constant. The mass of a body remains unchanged in any part of universe. Mass of a body does not change with respect to gravity. It is weight that changes with gravity.

20. (d) Statement 1 is false. The temperature dependence of liquid viscosity is the phenomenon by which liquid viscosity tends to decrease (or, alternatively, its fluidity tends to increase) as its temperature increases. Thus, with increase of temperature viscosity of glycerine decrease. Statement 2 is correct because with the rise of temperature in case of liquid, the kinetic energy increases as kinetic energy of liquid molecules is directly proportional to absolute temperature.

21. (c) Statement 1 is true but statement 2 is false. Specific gravity is the ratio of the density (mass of a unit volume) of a substance to the density (mass of the same unit volume) of a reference substance. Mercury has a specific gravity of 13.56 whereas specific gravity iron is 7.21 .Thus, iron ball floats on mercury.

22. (b)

23. (b)Weight is basically the measurement of the gravitational force that acts on an object. Weight of any body is given by formula

W= mg

m is the mass of body. g refers to the acceleration that the Earth imparts to objects on or near its surface. It is called acceleration due to gravity. Now, mass is universal constant it will remain same at: earth surface or at any point above the surface of earth. Acceleration due to Gravity decreases with altitude, since greater altitude means greater distance from the Earth’s centre.

The following formula approximates the Earth’s gravity variation with altitude:

Where

i. is the gravity measure at height above sea level.

ii. is the Earth’s mean radius.

iii. is the standard gravity.

Earth is not a perfect sphere because its two poles on the earth cause it to bulge out in the middle. It spins on its axis and the centrifugal force makes the part at the equator bulge out slightly.

24. (a) In case of object moving along a circular path two types of forces centripetal and centrifugal acts upon it. As they have opposite impact thus, object remains in balance. When centrifugal force gets disturbed then the object will leave the balance and skid toward outward direction.

25. (c) Here, free surface of oil will incline with respect to horizontal with larger depth at the rear end.

26. (b) The value of g on earth’s surface is given by

g = GM/R²

∆g/g = -2 (∆R/R) (if M is constant)

Thus, if radius of the earth shrinks by 1%, the value of g will increase by 2%.

27. (d) By Newton’s second law F = ma. If a = 0, F_net = 0

Thus, the resultant force is zero.

28. (b) A body moving with a uniform angular velocity co on a circular path of radius r has radial acceleration equal ω²r directed towards the centre of the path.

29. (d) In this case, balloon is filled up with water & then put into water. Initially, after some oscillations it become stable because the downward directing force exerted by weight of water in balloon is balanced by the upward directing buoyancy force of water. Hence, the resultant weight in water is zero as there is no resultant force acting upon it.

30. (b) Density of Gold is 19.30 g/cm³. The density of ultra pure liquid mercury is 13.534 g/cm³ and the density of Steel is 7.80 g/cm³.

31. (b)

32. (c) Elongation (Al) in a wire is given by

Y = F/A x l/∆l

=> ∆l = Fl/AY

For wires of same material and same load,

∆l α l/A => ∆l α l/r²

Therefore, the extensions will be maximum in case of the wire with length 3 m and diameter 1.5 mm.

33. (d)

B. Heat and Thermodynamics

34. (a) They are made of water and ice particles. A cloud’s individual particles are so small, in fact, that warm air rising from the earth’s surface is able to keep them floating in the air. It’s similar to how dust motes swirl in a shaft of sunlight. When the warm air keeps clouds afloat it cools as goes above, thus water vapour condenses to the water droplets. At a certain point the droplets become heavy enough to overwhelm the force of the rising air, and all that water falls to the ground. That’s what we call rain.

36. (a) In physics, cryogenics is the study of the production of very low temperature (below ? 150 °C, ?238 °F or 123 K) and the behavior of materials at those temperatures. Application of cryogenics is in MRI. MRI is a method of imaging objects that uses a strong magnetic field to detect the relaxation of protons that have been perturbed by a radio-frequency pulse. This magnetic field is generated by electromagnets, and high field strengths can be achieved by using superconducting magnets. Traditionally, liquid helium is used to cool the coils because it has a boiling point of around 4 K at ambient pressure. Cryogenic liquids are also used in the space program. For example, cryogens are used to propel rockets into space. A tank of liquid hydrogen provides the fuel to be burned and a second tank of liquid oxygen is provided for combustion.

Cryogenic processes are also used to supply “banks” storing eye corneas, blood, and sperm for future surgical procedures. In 1961 American surgeon Irving S. Cooper introduced a freezing technique called cryosurgery. Cryosurgery is relatively bloodless because the low temperatures used constrict the blood vessels, stemming the flow.

37. (b) Both statement 1 and statement 2 are correct. On heating up to same temperature copper piece is found hotter then in comparison to glass piece because copper being metal is a good conductor of heat and electricity while glass on the other hand is a poor conductor of heat. The density of copper is 8.92 gin/cm³ and of glass is 2.6 gm/cm³.

38. (c) The boiling point of an element or a substance is the temperature at which the vapour pressure of the liquid equals the environmental pressure surrounding the liquid. As altitude increases, atmospheric pressure decreases, so boiling point decreases, so the liquid would boil at a lower temperature.

39. (d) Initially at start of heating from 0°C to 4°C there will be a contraction as a result of which volume decreases. On further heating beyond 4°C to 10°C the molecules gain kinetic energy and start moving more randomly. Thus, intermolecuiar distance increases as a result of which its volume increases.

40. (c) In this case, the sphere will reach first at room temperature. For a given volume sphere has minimum surface area:

A = 4∏r²

Therefore, in case of sphere less area is exposed to heat and it will attain lesser temperature and comes down to room temperature early.

41. (d) Only statement 3 is correct. An air conditioner (often referred to as AC) is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. Steam (at 100 degree Celsius ) has more energy than water ( at 100 degree Celsius ) because it takes energy to break the bonds that keep it a liquid. Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a change of state that occurs without a change in temperature. Latent heat of fusion of water is 334 kJ/kg whereas Latent heat of vaporization of water is 2260 kJ/kg.

42. (b) Cloudy nights are warmer than clear nights because, the cloud cover provides a shield which act as an insulator and store up the heat radiated by the earth and do not permit heat to escape.