Forces Governing Ocean Motion

In the first lecture, we saw why the ocean moves. In this second lecture, Nicholas Hall looks at how – the forces that act on every parcel of water in the sea.

Starting from Newton’s laws, the lecture builds up the forces one by one:

• Gravity – pulling water downward
• The pressure gradient force – the push from high pressure toward low pressure that drives water sideways
• Shear stress – the frictional force between layers of water moving at different speeds, and the route by which wind’s pull enters the ocean
• The Coriolis force – the apparent deflection that arises because we live on a rotating planet, always turning moving water to the right in the northern hemisphere and to the left in the southern

The lecture then gathers these forces together into the primitive equations – the full set of relations that describe ocean motion – and shows how they can be simplified into the shallow-water equations, a powerful tool you’ll meet again when we study waves and tides.

Don’t worry about the mathematics: the aim is to build a physical feel for what each force does. If you come away knowing which force pushes which way, and why rotation matters, you have what you need for the rest of the week.