After bombarding a gold foil sheet with alpha particles, scientists concluded that atoms consist mainly of

Mainly of empty space (or low-mass electron cloud).

Explanation: The outcome of the experiment (done by Rutherford) was that most alpha particles fly unaffected through the atom. Only a small fraction of them gets deflected due to collision with the relatively small but massive nucleus.

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.