Interactive free flash animation to learn that in an Galilean reference frame, if the velocity of the center of inertia of a solid varies, the sum of the forces acting on it is not zero. The vector "sum of forces" has the same direction and same direction as the vector "speed variation" between two close moments.
Definition - Statement
Newton's second law of motion : The resulting force F exerted on a material point, of given mass m, is equal to the product of the mass of the point and its acceleration a.
Σ Fext = ma