G_acc overrides SetPos_dtdt. What's the point?

[Tymo]

There is a problem with SetPos_dtdt.

coords_dtdt.pos seems to be overwritten with G_acc, so what is the point of SetPos_dtdt?

ChSystemNSC sys;
auto body = make_shared<ChBody>();
sys.AddBody(body);
body->SetPos_dtdt(Vector(0, 1, 0));
cout << body->GetPos_dtdt().y() << endl;//1
sys.DoFrameDynamics(1);
cout << body->GetPos_dtdt().y() << endl;//-9.81 value overwritten
sys.Set_G_acc(Vector(0, 0, 0));
body->SetPos_dtdt(Vector(0, 1, 0));
cout << body->GetPos_dtdt().y() << endl;//1
sys.DoFrameDynamics(2);
cout << body->GetPos_dtdt().y() << endl;//0 value overwritten again

[Radu Serban]

Tymo,

 

You are doing a dynamics analysis, more precisely “forward dynamics”. In this case, the motion of the system is dictated by its initial state (positions and velocities) and the set of applied forces.  Forward dynamics analysis calculates the accelerations (think Newton’s second law a = F/m) and then integrates to find the new velocity and position.

 

In the examples you show, gravity is the only applied force so, as expected, the body’s acceleration will end up being the gravitational acceleration.

 

The function SetPos_dtdt can be used for other types of analysis, for example inverse dynamics.

 

--Radu

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[Tymo]

Thank you for clarification