A computer-aided design system and computer-implemented method define initial conditions for dynamic simulation of an assembly of objects in a three-dimensional scene of a computer-aided design. The system and method provide (S1) the assembly of objects with information relative to kinematic joints linking objects of the assembly. The system/method provide (S2) a manipulating tool (M) embedded in the scene; attach (S3) said manipulating tool (M) to one object of the assembly; select (S4) a degree of freedom of the manipulating tool (M) attached to the object of the assembly; and input (S5) at least one initial condition (IC) according to said selected degree of freedom, an initial condition comprising an initial position and/or an initial speed and/or an initial acceleration. The tool has a referential (reference) with three-axes allowing for each axis a degree of freedom in translation and a degree of freedom in rotation. Next the system and method calculate (S6) and display in real time the dynamic simulation taking into account the initial conditions (IC) previously defined.