Newton's Second Law
Forces and Motion

Floating and sinking

Physics Narrative for 11-14 Supporting Physics Teaching

Buoyancy forces cause things to rise in fluids

Here are four situations where a non-zero resultant force predicts a change in motion:

  • Hold some balsa wood underwater. The buoyancy force will be greater than the gravity force. Remove the force applied by your hands. The block rises.
  • Hold a helium filled balloon and its tether. The buoyancy force is again greater than the pull of gravity, so if you release the tether the balloon will rise.
  • Hold a stone below the water. The buoyancy force is less than the pull of gravity. Release the stone and it will sink.
  • Hold a tennis ball in air. The buoyancy force is less than the pull of gravity. Release the ball and it will sink.

In all cases the resultant force determines the change in motion: from rest to moving off in the direction determined by the sum of the forces.

A volume of warmer air or warmer water can take the place of the balsa wood or the helium filled balloon. The buoyancy forces on it are greater than the gravity forces, so it starts to rise. Similarly a volume of colder fluid will sink, as the gravity force will be greater than the buoyancy force.

These are convection currents.

Are convection currents important? Yes! Convection streams dominate our global environment, appearing in ocean currents and the winds and driving both plate tectonics and the dynamo that produces the Earth's magnetic field.

Convection currents: volumes of warmed fluid moving around

In the process of convection, volumes of already warmed liquids and gases move around. Convection works by convection currents. These are streams of liquids and gases which carry warm volumes from a hot place to cooler parts of the system. So, as with conduction, convection still relies on particles to shift the energy (SPT: Energy topic). The crucial difference from conduction is that in convection there is mass movement of the particles.

How do convection streams get started, and what drives them? It's all a matter of floating and sinking.

Energy shifted from the thermal store associated with the hot object (such as the radiator in our example) to the thermal store of a volume of particles warms that volume (the air above the radiator). This process is conduction: the pathway is heating by particles. The volume of air expands, but still contains the same number of particles (in other words its density is reduced). This air then rises, floating up through the cooler, denser air around it. As the air moves it takes a quantity of energy away with it. It's not a case of heat rising, but of the warmed air floating up through the surrounding cooler air.

As the warm air rises, cool air moves in to take its place. The process is continuous, with a rising stream of air moving away from the hot radiator. This upward movement is a clear pointer to distinguish convection from conduction. Although the mechanism of convection has been described here in relation to the warm air rising from the radiator example, precisely the same processes occur when convection currents are set up in liquids.

Newton's Second Law
is expressed by the relation F=ma
can be used to derive Kepler's First Law

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