Suppose a parachutist is falling toward the ground, and the downward force of gravity is exactly equal to the upward force of air resistance. Which statement is true?

a. The velocity of the parachutist must be zero.
b. The velocity of the parachutist is not changing with time.
c. The velocity of the parachutist is increasing with time.
d. The velocity of the parachutist is increasing with time.

Option b. is correct.

Explanation:

In the given question a parachutist is falling toward the ground .

Also, the downward force of gravity is exactly equal to the upward force of air resistance.

So, net force applied to the parachutist is equal to zero ( because both force acts in opposite direction ).

Now by first law of motion  :

An object will be in rest or in constant speed unless and until no external force is applied on it .

So, in the question the velocity of the parachutist is not changing with time.

Therefore, option b. is correct.

Hence, this is the required solution.

no. dot product   =0 when vecs at right angles

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia