Scientists understood that the world was round, but in 1687, and English scientist named Isaac Newton explained why people do not fall off the earth. Newton realized that everything in the universe was attracted to everything else, and that the greater and closer the object, the greater its pull. We call this force gravity, which comes from a Latin word meaning heaviness. Everything and everybody has gravity-even you-but the earth has a mass of 5,972,000,000,000 pounds. The earth is very large, so people and objects are attracted to its gravitational pull. The earth orbits the sun because of the sun's gravitational pull on the earth.


Isaac newton knew this when he felt and apple drop/saw an aple drop on his head:












These are Newton's 3 laws of motion:
1:Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

2:For every action there is an equal and opposite reaction.

3:The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.

Law 3 is the most important9as well as complicated law)
This is the most powerful of Newton's three Laws, because it allows quantitative calculations of dynamics: how do velocities change when forces are applied. Notice the fundamental difference between Newton's 2nd Law and the dynamics of Aristotle: according to Newton, a force causes only a change in velocity (an acceleration); it does not maintain the velocity as Aristotle held.

This is sometimes summarized by saying that under Newton, F = ma, but under Aristotle F = mv, where v is the velocity. Thus, according to Aristotle there is only a velocity if there is a force, but according to Newton an object with a certain velocity maintains that velocity unless a force acts on it to cause an acceleration (that is, a change in the velocity). As we have noted earlier in conjunction with the discussion of Galileo, Aristotle's view seems to be more in accord with common sense, but that is because of a failure to appreciate the role played by frictional forces. Once account is taken of all forces acting in a given situation it is the dynamics of Galileo and Newton, not of Aristotle, that are found to be in accord with the observations.

this explanation cwas taken from :http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html

Now here's a video of newton's 3 laws of motion
object :




(PS: you'll enjoy this video if you like listening to the mission impossible song, i can guarantee that :)
(PSS:look at the quicksand post and there'll be a new video, its called ''forest of death'', relax there's no horror scence, just people getting pulled by vines in quicksand:)