A force represents the action of one body on another. A force is characterised by its point of action, magnitude, and direction. If the resulting force acting on a particle is zero, the particle will remain at rest (if originally at rest) or move with a constant speed in a straight line (if originally in motion) Gravitation: two particles of mass m and m are mutually attracted with equal and opposite force f and -f of magnitude: f=g(mm/r2), g=6. 67 x 10-11. The attraction of the earth to a particle located on it"s surface is a particular case of newton"s law of gravitation. The force f exerted by the earth on the particle is defined as the weight of the particle. w=mg, g=9. 81m/s2. Weight v. mass: mass (kg, weight (n) Measurement of the amount of matter something contains. Varies in the universe: f=ma, w=mg.