At sea, ships are moved by the motion of the waves, by the swell, although different types of ship move in different ways. There are two main points of force that affect a ship at sea – the centre of buoyancy, through which there is an upthrust that keeps the ship afloat, and the centre of gravity, the opposing downthrust. As the sea pushes against the side of the ship and causes her to heel over, the opposing forces of buoyancy and gravity lever her upright again. As the ship is righted, though, the momentum then causes her to heel over the other way. Thus the ongoing rolling motion is created.