如何在OpenGL中启用引力？

#include "btBulletDynamicsCommon.h"

btDefaultCollisionConfiguration* collisionConfiguration = new btDefaultCollisionConfiguration();
btCollisionDispatcher* dispatcher = new btCollisionDispatcher(collisionConfiguration);
btBroadphaseInterface* overlappingPairCache = new btDbvtBroadphase();
btSequentialImpulseConstraintSolver* solver = new btSequentialImpulseConstraintSolver();
btDiscreteDynamicsWorld* dynamicsWorld = new btDiscreteDynamicsWorld(dispatcher, overlappingPairCache, solver, collisionConfiguration);

// 设置重力值
dynamicsWorld->setGravity(btVector3(0, -9.81, 0));

btCollisionShape* groundShape = new btBoxShape(btVector3(btScalar(100.), btScalar(50.), btScalar(100.)));
btDefaultMotionState* groundMotionState = new btDefaultMotionState(btTransform(btQuaternion(0, 0, 0, 1), btVector3(0, -50, 0)));
btScalar mass = 0;
btVector3 inertia(0, 0, 0);
groundShape->calculateLocalInertia(mass, inertia);
btRigidBody::btRigidBodyConstructionInfo groundRigidBodyCI(mass, groundMotionState, groundShape, inertia);
btRigidBody* groundRigidBody = new btRigidBody(groundRigidBodyCI);
dynamicsWorld->addRigidBody(groundRigidBody);

dynamicsWorld->stepSimulation(1.0f / 60.0f, 10);

btTransform trans;
groundRigidBody->getMotionState()->getWorldTransform(trans);

// 获取位置和旋转
btVector3 pos = trans.getOrigin();
btQuaternion rot = trans.getRotation();

// 应用位置和旋转到OpenGL对象