What is the difference between 1d motion and 2d motion?
i need explanation, .

Explanation:

1D motion is motion in only one direction.  To put it simply, it's motion along a line.  For example, a train on a straight track has 1D motion.

2D motion is motion in two directions.  A good example of this is a projectile launched at an angle.  It has both vertical motion and horizontal motion.

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.

3. dark matter

empty space could also be a simple vacuum