Conceptual Physics

Finally, the net force is the sum of the buoyancy upward and the weight of the bait downward.

The negative value of the net force indicates that the bait combination is pulling down on the line.

The quantities involved in this problem are rather small: The weight of the bait, its buoyancy and the net downward force all have magnitudes in

the thousandths of newtons. Bait like this will sink, but not very quickly, due to the resistance of water to its motion. For this reason, fishermen

often use lead weights called “sinkers” to cause the bait to sink faster to a depth where fish are feeding.

Step Reason

10.F = Fb +(–mhg) +(–mwg) net force

11. evaluate

13.9 - Sample problem: buoyancy of an iceberg

You may have heard the expression “it’s just the tip of the iceberg”; icebergs are infamous for having nine-tenths of their volume submerged

below the surface of the sea. The composite photograph above shows just how dangerous icebergs can be for navigation. The submerged

portion not only extends downward a great distance, it may also extend sideways to an extent that is not evident from above. A ship might

easily strike the submerged portion without passing especially close to the visible ice.

Use the values stated below for the densities of ice and of seawater at í1.8°C, which is the freezing point of seawater in the arctic. The

iceberg is in static equilibrium, moving neither up nor down.

Variables

The upward buoyant force on the iceberg is Fb.

Strategy

1. Use equilibrium to state that the buoyant force acting on the iceberg equals its weight. Archimedes’ principle allows you to express the
   buoyant force in terms of the weight of the displaced water. Replace the equilibrium equation with one stating that the mass of the
   iceberg equals the mass of the displaced water.


2. Use the definition of density to replace the masses in the previous equation by products of density and volume. Solve for the ratio
   VH2O/Vice, and evaluate it.
   Physics principles and equations

Newton’s second law.

ȈF = ma

Archimedes’ principle states that the buoyant force on an object in a fluid equals the weight of the fluid it displaces.

The definition of density is

What fraction of this iceberg is

submerged below the water?

iceberg displaced water

density ȡice = 917 kg/m^3 ȡH2O= 1030 kg/m^3

volume Vice VH2O

mass mice mH2O

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