Dynamics is a branch of physics concerned with the study of forces and torques and their effect on motion,
as opposed to Kinematics, which studies the motion of objects without reference to its causes.
From Isaac Newton, force can be defined as an exertion or pressure which can cause an object to move. 
The concept of force is used to describe an influence which causes a free object to accelerate.
It can be a push or a pull, which causes an object to change direction, have new velocity, or to deform temporarily or permanently. 

From Wikipedia - http://en.wikipedia.org/wiki/Dynamics_(mechanics)

1 Newton (N) = 1 kg m / s2

1 picoNewton (pN) = 10−12 N

Forces involved at the biological level

Entropic forces ~ kBT/nm = 4 pN

- Breaking of a covalent bond (C-C): 1eV/0.1nm ~ 1600 pN
- Breaking of a noncovalent bond (biotin/streptavidin): 1eV/1nm ~ 160 pN
- Breaking single dsDNA molecule by pulling on opposite ends ~ 65 pN
- Unziping single dsDNA molecule at room temperature ~ 10 pN
- Breaking of a weak bond (Hydrogen bond) ~ 4 pN
- Developped by a molecular motor, walking on microtubule ~ 4 pN
- Langevin force on E-coli for 1 second ~ 0.01 pN