Science & Technology Basics

An amalgam is an alloy of mercury with one or more other metals. Mercury is the essential component that defines an amalgam. Common amalgams include those with silver, gold, and tin used in dentistry.

The Soviet spacecraft Venera 3 crash-landed on Venus on March 1, 1966, making Venus the first planet (other than Earth) to have a spacecraft crash on its surface. Earlier missions to Mars achieved soft landings.

Dry ice is solid carbon dioxide (CO2). When dry ice sublimates, it transitions directly from solid to gas without becoming liquid, producing carbon dioxide gas. This is why dry ice 'disappears' when left at room temperature - it converts to invisible CO2 gas.

Progeria (Hutchinson-Gilford Progeria Syndrome) is a genetic disorder that causes accelerated aging in children. People with progeria age approximately 8-10 times faster than normal, experiencing rapid physical aging symptoms while maintaining normal cognitive development.

In Java threading, the start() method is used to begin execution of a thread. When start() is called, the JVM creates a new thread and invokes the run() method within that new thread. The init() method is not a standard thread method, run() contains the thread's body but doesn't start execution, and resume() is deprecated. The start() method is specifically designed for thread initialization and execution.

In Java's threading model, the run() method contains the body or actual code that the thread executes. When a thread is started using start(), it internally calls run() to execute the thread's task. The start() method only initiates thread execution, stop() is deprecated, and main() is the program entry point unrelated to threading. The run() method must be overridden or implemented to define thread behavior.

The wait(), notify(), and notifyAll() methods are defined in the Object class in Java, not in Thread or Runnable. This design allows all objects to be used as locks or monitors for thread synchronization. Every object in Java inherits these methods from Object. The Thread class provides methods for thread management, while Runnable is an interface for defining thread tasks. Only Object defines these fundamental synchronization methods.

The Runnable interface requires implementing the run() method with public void run() signature. The method must be public (to be called by the thread) and void (no return value). Option A (void run()) lacks the public modifier, making it inaccessible. Option C (public void start()) is incorrect because Runnable doesn't define start(). Option D (void run(int priority)) has wrong signature and parameters. The exact signature public void run() is mandated by the interface contract.

In nuclear reactors, moderators are used to slow down fast neutrons produced during fission. Moderators absorb excess neutrons' kinetic energy (not the neutrons themselves) through collisions, reducing them to thermal energies suitable for sustaining the chain reaction. Option C is the intended answer though the wording 'absorb excess neutron' is imprecise.

Carbon-14 is a radioactive isotope of carbon with a half-life of approximately 5,730 years. This property makes it useful for radiocarbon dating of organic materials up to about 50,000-60,000 years old. The half-life is the time for half of the radioactive atoms to decay.

Hydrogen bombs (thermonuclear weapons) are significantly more powerful than atomic bombs (fission weapons). Hydrogen bombs use both fission and fusion processes, whereas atomic bombs only use fission. The energy released by fusion is much greater, making hydrogen bombs far more destructive.

Elements with atomic number greater than 82 (lead) exhibit natural radioactivity. This is because all nuclei beyond lead are unstable and undergo radioactive decay. The elements from atomic number 83 onwards are naturally radioactive.

Elephants have the longest gestation period among land mammals, approximately 645 days (about 22 months). African elephants have a gestation period of about 22 months, while Asian elephants gestate for approximately 18-22 months.

When you exercise intensely and your muscles need more oxygen than available, they switch to anaerobic respiration, producing lactic acid as a byproduct. This lactic acid buildup contributes to muscle fatigue and soreness after intense exercise.

Virga is the correct meteorological term for precipitation that falls from clouds but evaporates before reaching the ground due to dry air layers beneath. Fog is a cloud at ground level, dew forms on surfaces through condensation, and smog is pollution-based haze. Virga appears as streaks falling from clouds that disappear mid-air.

Uranium and Thorium are the two most common nuclear fuels used in nuclear reactors worldwide. Uranium-235 is the primary fuel in most reactors, while thorium is used in some advanced reactor designs and is more abundant than uranium. The other options incorrectly include elements like polonium and neptunium which are not practical nuclear fuels.