Why does a ship float

The lump of dough immediately sinks to the bottom of the bowl when you put it in. The boat-shaped lump, on the other hand, floats, even though it is the same weight as the first lump.

First place the ball-shaped lump in the water and watch it sink. Now form the second lump into a boat and place it on the water.

• 2 lumps of dough of the same size,
• 1 bowl of water,
• possibly paper handkerchiefs to dry the dough,

The aim of the experiment is to show that plasticine, which sinks as a lump, can float when given a special shape. This corresponds to the phenomenon that a ship weighing several tons floats, although the iron would sink as a lump.

The technical background to the experiment

Whether objects can float or not depends on the relationship between buoyancy and weight. If an object is immersed in water, different cases can occur:

Buoyancy < weight = the body sinks.

Buoyancy = weight = the body floats.

Buoyancy > weight force = The body rises and floats.

In the case of solid bodies (such as a wooden ball, solid rubber ball, cork, etc.), the density of the objects (i.e. how many grams a cubic centimeter of a body weighs) compared to the density of the liquid alone determines whether the object floats or sinks in the liquid. If the density of the material is less than the density of the liquid, the body floats. For many floating bodies (such as ships, balls, empty bottles, etc.), the shape also comes into play. The decisive factor here is that the bodies displace a lot of water when submerged without water penetrating them. This is also the case in the experiment above: the plasticine boat displaces significantly more water than a plasticine ball made of the same material. Depending on how much water is displaced by a body, the water "resists" to different degrees. The "Archimedes' principle" states that the buoyant force experienced by a body in a liquid is exactly the same as the weight of the liquid displaced by the body. The buoyancy force counteracts the weight force. The submerged lump of plasticine also experiences a buoyant force. However, it is not large enough to compensate for the force of weight. The force of water on a submerged body can be felt very clearly when an object is lifted under water: It appears to be lighter in the water. This effect can be experienced particularly clearly with a water bottle filled with water. It seems to weigh nothing under water because the buoyancy force almost completely balances out the weight force. With a floating plasticine boat, the immersion depth is adjusted so that buoyancy and weight are in balance. If the boat is loaded, the weight force increases. At the same time, the boat sinks further, exactly to the point where the weight of the additionally displaced liquid balances out the weight of the additional load.

Some elementary school materials argue that the plasticine boat offers the liquid a larger surface area to attack than the lump and is therefore held by the water. This explanation is wrong. According to the explanation, a flat plate should float best. However, this is certainly not the case. With the kneaded rubber boat, you quickly realize that the height of the side walls must also be chosen appropriately. It's not just the surface area for the water to attack, but also the immersion depth: the deeper an object is immersed, the more the water presses against it from below. You can easily feel this if you place a bucket (or cup) bottom first on a water surface and press down. If you multiply the surface area and immersion depth, this corresponds exactly to the volume of the displaced liquid! If a body is immersed very slowly, the force exerted by the water on the body increases with every centimeter immersed. If the force of the water corresponds exactly to the weight of the body, the body will not penetrate any further into the water. As soon as water comes onto the body from above, this water pushes the body downwards.

Carolin Schneider & Bastian Fleck