Physics (Part-I) (UPSC)

Total Questions: 50

11. A smooth plane inclined at an angle Q with the horizontal as shown in the given figure. A body starts from rest and slides down on the inclined surface. The time taken by the body to reach the bottom is: [1997]

Correct Answer: (c)

Solution:An object on an inclined plane with inclination → moves with an acceleration g sin theta down the plane. The time taken to reach the bottom is given by

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]

(a) 9.8 m/s
(b) 19.6 m/s
(c) 29.4 m/s
(d) 39.2 m/s

Correct Answer: (b) 19.6 m/s

Solution:The velocity after time is given by

v = u + gt

Here, u = 0 t = 2s g = 9.8m / (s²)

v = 0 + (9.8 * 2) = 19.6m / 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
(b) hit the monkey below the point aimed
(c) hit the monkey above the point aimed
(d) miss the monkey altogether

Correct Answer: (a) hit the monkey at the point aimed

Solution:When the hunter fires the bullet, both the bullet and the monkey experience the same gravitational acceleration downwards. As the bullet travels forward, it also falls towards the ground, just like the monkey after jumping off the branch. Since the initial aim was at the monkey's eyebrows, both the monkey and the bullet fall the same vertical distance due to gravity during the bullet's flight time. Thus, the bullet hits the monkey as aimed, demonstrating the independence of horizontal and vertical motion under gravity.

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

(a) Blotting of ink
(b) Rising of underground water
(c) Spread a water drop on a cotton cloth
(d) Rising of water from the roots of a plant to its foliage

Correct Answer: (b) Rising of underground water

Solution:Rising of underground water is primarily driven by pressure differences and permeability of soil and rock layers, rather than capillarity. Therefore, the rising of underground water is not primarily caused by capillarity but rather by hydrostatic pressure and other geological factors.

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
(c) 2, 3 and 4
(d) 1, 3 and 4

Correct Answer: (b) 2 and 3

Solution: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 other two options radial tyres and catalytic converter with exhaust do not contribute to make the car more fuel efficient.

The amount of heat produced by the complete combustion of unit mass of a fuel is known as fuel efficiency or calorific value.

16. The following item consists of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer these item using the code given below. [2000]

Assertion (A): A man standing on a completely frictionless surface can propel himself by whistling.

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

(a) Both 4 and R are true and R is the correct explanation of A
(b) Both 4 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 & is true

Correct Answer: (b) Both 4 and R are true but R is not a correct explanation of A

Solution: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 an equal and opposite reaction". In first case while whistling a man uses his breathe as a propellant to 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 acts upon it.

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

(a) 100/6 kg
(b) 60 kg
(c) 100 kg
(d) 600 kg

Correct Answer: (c) 100 kg

Solution: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 the weight that changes with gravity.

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
(c) 1, 2 and 4
(d) 2, 3 and 4

Correct Answer: (c) 1, 2 and 4

Solution:The bob of a simple pendulum is attached to a string which pulls the bob along its length.

Here B and C are extreme positions whereas A is mean position. The speed of the bob increases as it approaches the mean position A and continues to move till it reaches C. At C the speed becomes zero. Due to the unbalanced force the bob moves towards the mean position. The speed of the bob is maximum at the mean position and is zero at the extreme positions. Thus, it is clear that in each cycle bob velocity increases from zero to maximum. This means that it attains a given velocity twice. Suppose the bob of the pendulum reaches up to B while oscillating, then AB is the amplitude. For the next oscillation the bob fails to reach B but it will reverse the direction from point B' instead of B. The amplitude of oscillation in the second case is AB' which is less than AB. That means, a retarding force is acting on the bob thereby reducing the amplitude of oscillation. This retarding force is nothing but air-resistance or air-friction. At extreme position, acceleration is maximum. So statement 3 is not correct.

19. Which of the following distance-time graph (x-1) represents one-dimensional uniform motion? [2001]

Correct Answer: (a)

Solution:Figure (d) correctly represents the time-distance graph for one dimensional uniform motion.

20. The following two 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. [2002]

Assertion (A): With the increase of temperature, the viscosity of glycerine increases.

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

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

Solution: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.

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