for (int j = 0; j < forces.size(); j++) {
if (forces.get(j) instanceof ConstantForceRelativistic){
forces.set(j, new ConstantForce());
}
if (forces.get(j) instanceof SimpleGridForceRelativistic){
forces.set(j, new SimpleGridForce());
}
}
}
switch(i) {
case 1: