What is the difference between a wave and a particle?
a. A particle is a discrete unit of energy; a wave is a
continuous flow of energy
b. A wave carries energy; a particle carries light
c. A wave moves at the speed of light; a particle can
move at a range of speeds
d. A wave carries electricity; a particle carries a
magnetic charge

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.

prndulum needs thread, weight clock to time swings

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.