All questions of Physics for Mechanical Engineering Exam

**Explanation:**

The correct answer is option **B) Distance**.

A light-year is a unit of distance used in astronomy to measure large distances in space. It is defined as the distance that light travels in one year in a vacuum. Since light travels at a constant speed of approximately 299,792 kilometers per second (or about 186,282 miles per second), the distance covered by light in one year is immense.

To better understand the concept of a light-year, let's break down the distance calculation and the significance of using this unit in astronomy:

**Definition of a Light-Year:**
- A light-year is the distance that light travels in one year.
- Light travels at a speed of about 299,792 kilometers per second (or about 186,282 miles per second).
- Therefore, in one year, light can travel about 9.46 trillion kilometers (or about 5.88 trillion miles).

**Importance of Light-Years in Astronomy:**
- The vast distances between celestial objects in space make it impractical to use conventional units like kilometers or miles.
- Astronomers use light-years to measure the distances between stars, galaxies, and other objects in the universe.
- Light-years allow us to comprehend the enormous scale of the universe and the time it takes for light to travel across such vast distances.

**Examples of Light-Years:**
- The nearest star to Earth, Proxima Centauri, is approximately 4.24 light-years away.
- The Andromeda Galaxy, our closest neighboring galaxy, is about 2.537 million light-years away.
- The observable universe is estimated to be about 93 billion light-years in diameter.

In conclusion, a light-year is a unit of distance used in astronomy to measure the vast distances between celestial objects. It represents the distance that light travels in one year and is an essential tool for understanding the scale and size of the universe.

Explanation:
A mirage is an optical illusion that occurs due to the bending of light rays. It is a phenomenon where distant objects appear to be shimmering, distorted, or displaced from their actual position. The correct answer is option 'D' because a mirage fits the description of an optical illusion.

Definition of a Mirage:
A mirage is a phenomenon that occurs when light rays bend due to the variation in air temperature. It usually happens in hot, desert-like environments where the ground is significantly heated. The bending of light causes an apparent displacement of objects, creating the illusion of water or reflections.

Causes of Mirage:
1. Temperature Gradient: Mirages occur due to the temperature gradient in the air. The air close to the ground is hotter than the air higher up. This temperature difference causes the light rays to bend as they pass through the layers of air with varying densities.

2. Total Internal Reflection: When light travels from one medium to another, it bends or refracts. In the case of a mirage, the temperature gradient causes the light rays to bend more than usual, leading to total internal reflection. This reflection creates the illusion of water or a shiny surface.

Types of Mirage:
1. Inferior Mirage: An inferior mirage is the most common type of mirage. It occurs when the air close to the ground is hotter than the air above. This causes the light rays to bend upwards, creating an image of objects below the actual position.

2. Superior Mirage: A superior mirage occurs when the air close to the ground is colder than the air above. In this case, the light rays bend downwards, creating an image of objects above their actual position. Superior mirages are often seen in cold Arctic regions.

Characteristics of a Mirage:
1. Shimmering Effect: Mirages create a shimmering effect, making the reflected image appear unstable or wavering.

2. Displacement: Mirages can displace the position of objects, making them appear higher or lower than their actual location.

3. Illusion of Water: One of the common characteristics of a mirage is the illusion of water. Due to the bending of light, distant objects may appear as if they are reflecting off a water surface.

4. Distance: Mirages often occur at a distance, particularly in desert areas where the ground is heated.

In conclusion, a mirage is an optical illusion that occurs due to the bending of light rays caused by a temperature gradient in the air. It creates the illusion of shimmering, displaced, or distorted objects, often resembling water or reflections.

Galvanizing: Protecting Iron from Rusting

Galvanizing is the method of protecting iron from rusting by coating it with a thin layer of zinc. It is a widely used technique to prevent corrosion and extend the lifespan of iron and steel objects. Let's delve into the details of galvanizing and why it is an effective method of protection.

1. What is Galvanizing?
Galvanizing is a process in which a layer of zinc is applied to the surface of iron or steel to create a protective barrier. The zinc coating acts as a sacrificial anode, meaning it corrodes instead of the iron or steel beneath it. This sacrificial action ensures that the iron or steel remains protected from rusting.

2. How is Galvanizing done?
The galvanizing process involves several steps:

2.1 Surface Preparation:
The surface of the iron or steel object is cleaned to remove any dirt, grease, or oxide layers. This step is crucial as it ensures proper adhesion of the zinc coating.

2.2 Immersion in a Zinc Bath:
The cleaned iron or steel object is immersed in a bath of molten zinc at a temperature of around 450°C. The object is carefully dipped into the bath, allowing the zinc to adhere to its surface.

2.3 Metallurgical Reaction:
During immersion, a metallurgical reaction occurs between the iron or steel and the molten zinc. This reaction forms a series of zinc-iron alloy layers on the surface of the object.

2.4 Cooling and Finishing:
After the object is removed from the zinc bath, it is allowed to cool, allowing the zinc coating to solidify and adhere firmly to the iron or steel surface. The galvanized object is then typically inspected for any defects or imperfections.

3. Advantages of Galvanizing:
Galvanizing offers several advantages as a method of protecting iron from rusting:

3.1 Corrosion Resistance:
The zinc coating provides excellent corrosion resistance to the iron or steel object, preventing rust formation even in harsh environments.

3.2 Longevity:
Galvanized objects have a longer lifespan compared to bare iron or steel. The zinc coating acts as a durable protective layer, extending the life of the object.

3.3 Cost-Effective:
Galvanizing is a cost-effective method of protection as it requires minimal maintenance. The initial investment in galvanizing pays off in terms of reduced repair and replacement costs.

3.4 Aesthetic Appeal:
Galvanized objects have a visually appealing silver-gray finish that is often desirable in architectural and decorative applications.

4. Applications of Galvanizing:
Galvanizing is widely used in various industries and applications, including:

- Construction: Galvanized steel is used in roofing, fencing, structural components, and other construction applications.
- Automotive: Galvanized parts are used in automobile manufacturing to enhance corrosion resistance.
- Agriculture: Galvanized equipment and structures are commonly used in farming and agricultural settings.
- Electrical: Galvanized electrical conduits and cable trays provide protection against corrosion.

In conclusion, galvanizing is a highly effective method of protecting iron from rusting by coating it with a thin layer of zinc. This process creates a barrier that prevents the iron

Eclipses occur due to Rectilinear Propagation.

Explanation:
Eclipses are fascinating astronomical events that occur when one celestial body passes through the shadow of another celestial body. They can be categorized into two types: solar eclipses and lunar eclipses.

1. Solar Eclipses:
Solar eclipses occur when the Moon passes between the Sun and the Earth, casting its shadow on a portion of the Earth's surface. During a solar eclipse, the moon blocks the direct sunlight from reaching certain areas on the Earth.

- Shadow Formation:
The shadow of the Moon consists of two parts: the umbra and the penumbra. The umbra is the darkest part of the shadow, where the Moon completely blocks the sunlight. The penumbra is a lighter shadow where only a portion of the sunlight is blocked.

- Path of Totality:
The path of totality refers to the region on the Earth's surface where the Moon completely covers the Sun, resulting in a total solar eclipse. Only the observers within this narrow path can witness the complete blocking of the Sun. Outside this path, the observers witness a partial solar eclipse where only a portion of the Sun is obscured.

2. Lunar Eclipses:
Lunar eclipses occur when the Earth comes between the Sun and the Moon, casting its shadow on the Moon. During a lunar eclipse, the Earth blocks the sunlight from directly reaching the Moon.

- Shadow Formation:
Similar to solar eclipses, the Earth's shadow also consists of two parts: the umbra and the penumbra. The umbra is the region where the Earth completely blocks the sunlight, and the penumbra is the region where only a portion of the sunlight is blocked.

- Types of Lunar Eclipses:
There are three types of lunar eclipses: total lunar eclipses, partial lunar eclipses, and penumbral lunar eclipses. A total lunar eclipse occurs when the Moon passes through the Earth's umbra, resulting in the complete darkening of the Moon. A partial lunar eclipse occurs when the Moon passes partially through the Earth's umbra. A penumbral lunar eclipse occurs when the Moon passes only through the Earth's penumbra, resulting in a subtle darkening of the Moon.

In conclusion, eclipses occur due to the optical phenomenon of rectilinear propagation. During a solar eclipse, the Moon's shadow is cast on the Earth, and during a lunar eclipse, the Earth's shadow is cast on the Moon. These phenomena are a result of the straight-line propagation of light, where the light rays travel in straight lines until they encounter an obstacle (in this case, the Moon or the Earth), causing an eclipse to occur.

Explanation:

Density Measurement:
- Pycnometer is an instrument used to measure the density of liquids or solids.
- It is a small glass bottle with a stopper that has a capillary tube attached to it.
- The pycnometer is weighed empty and then filled with the substance whose density is to be measured.
- The weight of the filled pycnometer is then recorded.
- By knowing the weight of the substance and the volume of the pycnometer, the density can be calculated using the formula: Density = Mass/Volume.

Other Options:
- Intensity of solar radiation is measured using instruments like pyranometer or radiometer.
- Intensity of earthquakes is measured using seismometers and seismographs.
- High temperatures are measured using thermometers or pyrometers.
Therefore, the correct answer is option 'A', as pycnometer is specifically designed for measuring density.

Explanation:
When two vectors are added, they can either give zero or a resultant vector. However, when three or more vectors are added, they can never give zero as the resultant vector. Let's understand why:

Two vectors:
When two vectors are added, they can either give zero or a resultant vector. If the two vectors are of the same magnitude and opposite in direction, they will give a resultant vector of zero. For example, if two forces of 5 N each are applied in opposite directions, they will cancel out each other and the net force will be zero.

Three vectors:
When three vectors are added, they can never give zero as the resultant vector. The reason is that three vectors can form a triangle, and the sum of any two sides of a triangle is always greater than the third side. Therefore, there will always be a resultant vector when three vectors are added.

Four or more vectors:
When four or more vectors are added, they can never give zero as the resultant vector. The reason is that four or more vectors can form a polygon, and the sum of any two sides of a polygon is always greater than the third side. Therefore, there will always be a resultant vector when four or more vectors are added.

Therefore, the minimum number of unequal vectors which can give zero resultant are two.

Understanding Auditory Sensation Persistence
The phenomenon of sound sensation persisting in our brain is known as auditory sensory memory or echoic memory. This is a crucial aspect of how we experience and interpret sounds in our environment.
Duration of Sound Persistence
- The correct answer is 0.1 seconds.
- This brief duration allows the brain to process and integrate sounds effectively.
Mechanism of Echoic Memory
- Echoic memory is a type of sensory memory. It holds auditory information for a short period, typically around 0.1 seconds.
- This fleeting persistence enables us to perceive sound sequences and comprehend speech without interruption.
Importance in Daily Life
- The ability to retain sound information for a short time is vital for language comprehension.
- It helps in distinguishing between different sounds and recognizing patterns in music or conversation.
Comparison with Other Sensory Memories
- Visual sensory memory (iconic memory) lasts longer, around 0.25 seconds.
- However, auditory sensations are processed rapidly, which is essential for effective communication and reaction to auditory stimuli.
Conclusion
Understanding the persistence of sound sensations in our brain is important for grasping how we process auditory information. The brief duration of 0.1 seconds allows us to engage with our environment, facilitating communication and interaction. This insight is particularly relevant in fields such as psychology, cognitive sciences, and education.

Understanding Audible Sounds
Sounds are classified based on their frequencies, which are measured in Hertz (Hz). The human ear can perceive a specific range of frequencies, which is crucial for communication and interaction with the environment.
Frequency Range of Audible Sounds
- Audible Sounds: The range of sound frequencies that humans can hear is approximately 20 Hz to 20,000 Hz (20 kHz).
- This range is commonly referred to as audible sound.
Other Sound Classifications
- Ultrasonics: Frequencies above 20 kHz (20,000 Hz) are known as ultrasonics. These sounds are inaudible to humans but can be detected by certain animals and used in various technologies (e.g., ultrasound imaging).
- Infrasonics: Frequencies below 20 Hz are classified as infrasonics. These sounds are also inaudible to humans and can occur naturally (e.g., earthquakes) or be generated artificially.
- Megasonics: This term is less commonly used and generally refers to very high-frequency sounds, typically above ultrasonics, often used in specialized applications.
Conclusion
In summary, sounds within the frequency range of 20 Hz to 20,000 Hz are termed audible sounds, making option 'A' the correct answer. This classification helps in understanding how humans interact with sound and its applications in technology, medicine, and environmental studies.

Understanding Hydraulic Brakes
Hydraulic brakes in automobiles operate based on the principles of fluid mechanics, particularly Pascal's law. This principle states that a change in pressure applied to an enclosed fluid is transmitted undiminished throughout the fluid.

Key Aspects of Pascal's Law
- **Pressure Transmission**: When the brake pedal is pressed, force is applied to the brake fluid in the master cylinder. According to Pascal's law, this pressure is transmitted equally in all directions through the fluid.
- **Force Amplification**: The system uses two cylinders of different diameters (master and slave cylinders). A small force applied in the master cylinder results in a larger force at the slave cylinder. This amplification enables vehicles to have effective braking with minimal effort.

Components of Hydraulic Brakes
- **Master Cylinder**: Initiates the hydraulic pressure when the brake pedal is pressed.
- **Brake Lines**: Transport the pressurized brake fluid to the brake calipers.
- **Brake Calipers**: Actuate the brake pads against the rotors, creating friction that slows down or stops the vehicle.

Advantages of Hydraulic Brakes
- **Efficiency**: Provides greater stopping power with less effort.
- **Consistency**: Offers uniform braking performance across various conditions.
- **Durability**: Less wear on mechanical components compared to traditional braking systems.
In conclusion, the operation of hydraulic brakes in automobiles is a direct application of Pascal's law, which is fundamental in ensuring efficient and reliable braking performance.

Electrons

Communication by Artificial Satellites

Introduction:
Artificial satellites play a crucial role in modern communication systems. They are used for various purposes, including communication, weather monitoring, navigation, and scientific research. In the context of communication, artificial satellites rely on specific types of waves to transmit signals between the satellite and the ground station. Among the given options, micro waves are used for communication by artificial satellites.

Explanation:
1. Micro waves:
Micro waves are a type of electromagnetic waves with wavelengths ranging from one millimeter to one meter. They are commonly used for communication purposes due to their ability to easily penetrate the Earth's atmosphere and their high bandwidth. Artificial satellites use micro waves for long-distance communication with ground stations.

Advantages of Micro waves:
- High Bandwidth: Micro waves have a high bandwidth, which means they can carry a large amount of information simultaneously.
- Low Atmospheric Absorption: Micro waves are not significantly absorbed by the Earth's atmosphere, allowing them to travel long distances without significant signal loss.
- Directivity: Micro waves can be focused in a specific direction, allowing for efficient communication between the satellite and ground stations.
- Low Interference: Micro waves are less susceptible to interference from other sources, making them ideal for reliable communication.

2. Radio waves:
Radio waves are also a type of electromagnetic waves, but they have longer wavelengths compared to micro waves. While radio waves are used for various communication purposes, they are not commonly used for communication by artificial satellites. Radio waves are typically used for terrestrial communication, such as AM and FM radio broadcasts.

3. A. M. (Amplitude Modulation):
AM is a modulation technique used in radio broadcasts, where the amplitude of the carrier wave is varied to transmit information. While AM is a form of radio wave communication, it is not specifically used for communication by artificial satellites.

4. Frequency of 1016 series:
The frequency of 1016 series does not correspond to any specific type of wave used for communication. It is not a recognized frequency band for communication purposes.

Conclusion:
In summary, artificial satellites use micro waves for communication with ground stations. Micro waves offer advantages such as high bandwidth, low atmospheric absorption, directivity, and low interference, making them ideal for long-distance communication. Radio waves, including AM, are not commonly used for satellite communication. The frequency of 1016 series does not correspond to any specific wave used for communication.