4.2 Forces: Density & Pressure

To prevent the overloading and sinking of ships markings, known as load lines, are required on ships hulls since the Middle Ages.

As the ship is loaded it sinks lower in the water. Once the load line meets the waterline, the ship is deemed to be fully loaded.

In 1876 Samuel Plimsoll refined the load line to reflect the needs of increasingly international trading routes. Today the 'Plimsoll line' has markings to show safe loading levels in fresh, saltwater, and warm and cold temperatures.

The Plimsoll line markings on ships today are shown in Fig. 1.1. The expected labels, their meanings and indications of temperature are written below.

Explain why the ships sit lower in the water when they are fully loaded.

With reference to upthrust

In a simple classroom demonstration of the principle behind the Plimsoll Line, a teacher places an egg in a beaker of distilled water at close to 0°C.

The egg sinks to the bottom of the beaker as shown in Fig. 1.2.

Fig. 1.2.

Suggest and comment on the likely next steps in this demonstration.

Hot air balloons consist of an envelope with an open mouth through which air can be heated using a burner. The air inside becomes much hotter than its surroundings. Lifted by this method, the balloon can carry a large amount of weight across very long distances.

A very large capacity balloon, as shown in Fig. 1.1. can carry 25-30 people.

Fig. 1.1.

The envelope has a diameter at its widest point of of 32 m when it is filled with air. The total mass of the balloon, air, equipment and passengers is 20 900 kg. When the air temperature is 15 °C the density of the air outside the balloon is 1.225 kg m⁻³.

When the air inside the envelope is hot enough, the balloon is released.

The balloon is taken to the Middle East, where the temperature is 45 °C.

Discuss how this would affect the ability of the balloon to fly and any changes the pilot could make to fly in these temperatures.

The air density at 45 °C is 4 % less than the air density at 15 °C.

For the hotter conditions

Party shops sell helium-filled balloons for various events. One such balloon is shown in Fig. 1.1.

Fig. 1.1

The ribbon is tied to a chair to stop the balloon floating away.

By modelling the balloon as a point mass, sketch a free body force diagram for the balloon and ribbon.

At its widest point, the balloon has a diameter 40 cm. The density of the surrounding air is 1.30 kg m^–3.

Calculate the upthrust on the balloon.

Balloons filled with air do not float.

 Explain why a balloon filled with helium floats.

The Galileo thermometer was invented in the 17th century and it is still popular today. The thermometer consists of a column of liquid containing roughly spherical, weighted, glass bulbs, as shown in Fig. 1.1.

Fig. 1.1

The spheres move up or down in the column as the temperature changes.

At a certain temperature, one of the spheres begins to fall through the liquid at an increasing speed. 

Sketch arrows on Fig. 1.2 to show the relative sizes of the forces acting on the sphere as it falls.

  

Fig. 1.2

Show that the upthrust on the sphere is about 0.07 N.

The spheres in the column are identical but have different masses due to the counterweight attached to them. 

State and explain where you would expect to find a sphere of weight 0.069 N.

A submarine below the surface of the sea is stationary in a region where the density of the seawater is 1.03 × 10³ kg m⁻³.

Calculate the upthrust exerted on the submarine by the seawater.

Explain why the mass of the submarine must be 6.0 × 10⁶ kg.

The submarine moves into a region of the sea where the water is less salty, and the density of the water reduces to 1.01 × 10³ kg m⁻³ .

Explain what would happen to the submarine as it moves into this region of lower density seawater.

The submarine alters its weight by pumping water in or out of its internal tanks.

Determine the mass of water that the submarine should pump, in or out of its tanks, to maintain its depth below the surface of the sea.

A hollow brass cylinder with closed ends is floating on the surface of the water.

The cylinder has a length of 4.0 cm and an external diameter of 2.1 cm as shown in Fig. 1.1.

Fig. 1.1

63% of the volume of the cylinder is submerged. The cylinder contains negligible weight of air.

The density of water is 1.0 × 10³ kg m⁻³.

Show that the mass of the cylinder is about 9 × 10⁻³ kg.

Deduce whether an identical hollow cylinder made of gold would also float.

Assume that the volume of gold is the same as the volume of brass.

• Density of gold = 19.3 × 10³ kg m⁻³
• Density of brass = 8.7 × 10³ kg m⁻³