Physics (Part-I) (UPSC)

Total Questions: 50

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

(a) 20 km/s
(b) 30 km/s
(c) 40 km/s
(d) 50 km/s

Correct Answer: (b) 30 km/s

Solution:To calculate the mean velocity of the Earth in its orbit around the Sun, we can use the basic formula for orbital velocity:

V = (2pi*r)/T

where :

v is the orbital velocity.

r is the average distance from the Earth to the Sun . (approximately 1 astronomical unit or 1AU \approx 1.496 * 10 ^ 8 * km )

T is the orbital period of the Earth (1 year, which is approximately 365.25 * 24 * 3600 seconds). First, we find the circumference of the Earth's orbit (assuming a circular orbit for simplicity):

Circumference - 2pi*r \approx 2pi * 1.496 * 10 ^ 8 * km \approx 9.4 * 10 ^ 8 * km

Then, we convert the orbital period into seconds:

T \approx 365.25 * 24 * 3600 seconds = 3.156 × 107 seconds

Now, we calculate the velocity:

v = (2pi*r)/T \approx (2.4 * 10 ^ 8 * km)/(3.156 * 10 ^ 2 * s) \approx 29.8km / s

Rounding this value, we get approximately 30 km/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 Im length and 2 mm diameter
(b) Wire of 2 m length and 2 mm diameter
(c) Wire of 3 m length and 1.5 mm diameter
(d) Wire of 1 m length and 1 mm diameter

Correct Answer: (c) Wire of 3 m length and 1.5 mm diameter

Solution:Elongation (Al) in a wire is given by

For wires of same material and same load,

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

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
(c) Vessel A
(d) Vessel C

Correct Answer: (d) Vessel C

Solution:Viscocity of Kerosene is least among all, so it will emptied first.

Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid

34. Consider the following statements: [2012-1]

If there were no phenomenon of capillarity

1. it would be difficult to use a kerosene lamp

2. one would not be able to use a straw to consume a soft drink

3. the blotting paper would fail to function

4. the big trees that we see around would not have grown on the Earth

Which of the statements given above are correct?

(a) 1, 2 and 3 only
(b) 1, 3 and 4 only
(c) 2 and 4 only
(d) 1, 2, 3 and 4

Correct Answer: (d) 1, 2, 3 and 4

Solution:Kerosene Lamp: The wick of a kerosene lamp uses capillary action to draw the kerosene up from the reservoir to the flame. Without capillarity, the wick would not be able to transport the fuel.

1. Straw: Drinking through a straw primarily involves the reduction of pressure inside the straw, allowing atmospheric pressure to push the liquid up. However, capillary action also contributes to the ability of the liquid to rise within the straw, especially in narrower straws.

2. Blotting Paper: Blotting paper absorbs ink through capillary action, where the liquid travels along the tiny gaps within the fibers of the paper. Without capillarity, this absorption process would not occur.

3. Big Trees: Trees rely on capillary action to transport water and nutrients from the roots to the leaves through narrow tubes called xylem. Without this phenomenon, it would be impossible for trees to move water upwards against gravity.

Therefore, all the statements (1, 2, 3 and 4) are correct

35. Ball bearings are used in bicycles, cars, etc., because: [2013-1]

(a) the actual area of contact between the wheel and axle is increased
(b) the effective area of contact between the wheel and axle is increased
(c) the effective area of contact between the wheel and axle is reduced
(d) None of the above statements is correct

Correct Answer: (c) the effective area of contact between the wheel and axle is reduced

Solution:Ball bearings are used to reduce friction and friction is directly proportional to effective surface area. So if effective surface area is reduced then friction will also reduce.

36. Consider the following actions: [2023-1]

1. Detection of car crash/collision which results in the deployment of airbags almost instantaneously

2. Detection of accidental free fall of a laptop towards the ground which results in the immediate turning off of the hard drive.

3. Detection of the tilt of the smart phone which results in the rotation of display between portrait and landscape mode

In how many of the above actions is the function of accelerometer required?

(a) Only one
(b) Only two
(c) All three
(d) None

Correct Answer: (c) All three

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

(a) temperature
(b) velocity
(c) pressure
(d) density

Correct Answer: (d) density

Solution:Clouds float in the atmosphere primarily due to their low density compared to the surrounding air. Clouds are composed of tiny water droplets or ice crystals that are small enough to be suspended in the air. Despite water being denser than air, the cloud particles are spread out over a large volume, giving clouds a lower overall density. This reduced density allows them to remain buoyant and float in the atmosphere.

38. Strips of two metals A and B are firmly jointed together as shown in the figure. [1999]

On heating, A expands more than B does. If this jointed strip is heated, then it will appear as :

Correct Answer: (b)

Solution:Thermal expansion is the tendency of matter to change in volume in response to a change in temperature. In this case A expands more than B. Therefore, expansion of volume will be more for A in comparison to B on application of same amount of heat. Therefore, on heating it will appear like figure (b).

39. Low temperatures (cryogenics) find application in: [1999]

(a) space travel, surgery and magnetic levitation
(b) surgery, magnetic levitation and telemetry
(c) space travel, surgery and telemetry
(d) space travel, magnetic levitation and telemetry

Correct Answer: (a) space travel, surgery and magnetic levitation

Solution: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 are 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 loss of blood.

40. The following items consist of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer these items using the code given below. [2001]

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.

(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
(c) A is true but R is false
(d) A is false but R is true

Correct Answer: (c) A is true but R is false

Solution:A is true but R is false. Assertion (A): It is true that when a piece of copper and a piece of glass are heated to the same temperature, the copper piece appears hotter than the glass piece when touched.

This is due to the different thermal conductivities of copper and glass. Copper has a much higher thermal conductivity, so it transfers heat to your hand more quickly, making it feel hotter.

Reason (R): The reason provided is that the density of copper is more than that of glass. While it is true that copper is denser than glass, density is not the reason why copper feels hotter.

The sensation of heat is related to thermal conductivity, not density. Thus, the reason given does not correctly explain the assertion.

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