Explanation

Solar energy originates directly from the Sun’s nuclear fusion reactions.

Wind and coal are indirect or non-solar sources, but the Sun is the primary source.

Explanation

Heinrich Rudolf Hertz discovered electromagnetic waves in 1887, experimentally confirming James Clerk Maxwell's predictions.

Hertz demonstrated the existence of radio waves, a type of electromagnetic wave, using a simple apparatus.

Explanation

As we go below Earth’s surface, the outer layers exert no gravitational force (by the shell theorem).

Only the mass beneath contributes, so g decreases gradually and becomes zero at the Earth’s center.

Explanation

A gas-filled balloon stores potential energy due to the compressed gas inside creating pressure.

This stored energy can be released when the balloon bursts or expands.

Explanation

A projectile follows a parabolic path under the influence of gravity and initial velocity.

This happens because of the combination of uniform horizontal motion and accelerated vertical motion.

Explanation

In mechanics, the work done by a machine is termed as its output.

The input is the work supplied to the machine, while output is the useful work delivered.

Explanation

Work is done only when a force produces displacement.

Since the wall does not move, displacement = 0 → Work = 0.

Explanation

For equal distances, average speed = harmonic mean = 2𝑎𝑏/𝑎+𝑏 = 2⋅40⋅60/40+60 = 48.

Not the arithmetic mean (which would be 50) because travel times differ.

Explanation

The Greeks are considered pioneers of physics due to early contributions by philosophers like Aristotle, Archimedes, and Democritus.

They laid the foundations of natural philosophy, which later evolved into modern physics.

Explanation

Mechanical waves require a medium to travel and can be:  

- Transverse (e.g., water waves)  

- Longitudinal (e.g., sound waves)