1. what
1.1. There are 4 fundamental forces that have been identified. In our present Universe they have rather different properties. there are...
1.1.1. The strong interaction is very strong, but very short-ranged. It acts only over ranges of order 10-13 centimeters and is responsible for holding the nuclei of atoms together. It is basically attractive, but can be effectively repulsive in some circumstances.
1.1.2. The electromagnetic force causes electric and magnetic effects such as the repulsion between like electrical charges or the interaction of bar magnets. It is long-ranged, but much weaker than the strong force. It can be attractive or repulsive, and acts only between pieces of matter carrying electrical charge.
1.1.3. The weak force is responsible for radioactive decay and neutrino interactions. It has a very short range and, as its name indicates, it is very weak.
1.1.4. The gravitational force is weak, but very long ranged. Furthermore, it is always attractive, and acts between any two pieces of matter in the Universe since mass is its source.
2. how
2.1. The key point here is that if there is no net force acting on an object (if all the external forces cancel each other out) then the object will maintain a constant velocity. If that velocity is zero, then the object remains at rest. If an external force is applied, the velocity will change because of the force.
3. who
3.1. Sir Issac Newton, was the first ever person tho find the algorithm of forces and gravity. then made his three laws to help understand the laws of gravity...
3.1.1. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
3.1.2. II. 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.
3.1.3. III. For every action there is an equal and opposite reaction