Why does a mechanical wave require a medium for transmission?
- the medium provides the required energy needed to propagate the wave.
- the wave is only transmitted when particles bump into each other.
- the medium keeps the wave from propagating too far.
- the medium provides an opposing force to slow down the wave.

- The wave is only transmitted when particles bump into each other.

Explanation:

A mechanical wave is a type of wave that propagates through a medium, carrying energy through the medium due to the oscillations of the particles inside the medium.

Due to its nature, a mechanical wave is transmitted through the collisions of the particles of the medium with each other: the closer the particles are, the faster the wave is transmitted (this is why mechanical waves travel faster in solids than in liquids or gases). Mechanical waves cannot be transmitted in vacuum, because there are no particles that can bump each other transmitting the oscillation.

Based on this definition, the correct answer is

- The wave is only transmitted when particles bump into each other.

the part of the wave he is travelling on is the crest

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.