Collisions between game objects and the floor with regards to gravity?

private long previousTime = System.currentTimeMillis();
private long currentTime = previousTime;

public void updateGame()
{
    currentTime = System.currentTimeMillis();
    float elapsedSeconds = (currentTime - previousTime) / 1000f;

    foreach(GameObject gameObject in gameObjects)
    {
        // Apply gravity to velocity vector
        gameObject.velocity.y += (gravityConstant * elapsedSeconds);

        // Move objects x/y position based off it's velocity vector
        gameObject.position.x += (gameObject.velocity.x * elapsedSeconds);
        gameObject.position.y += (gameObject.velocity.y * elapsedSeconds);

    }

    checkCollisions();

    previousTime = currentTime;
}