1. To investigate the effects of changes of air pressure inside a submarine in controlling the ascend and descend of a submarine.
2. To help students to understand Archimedes' principle more easily and deeply.
3. To create a teaching aid (model submarine) from recycled material.

(a) Students have a better understanding on the application of Archimedes' principle and air pressure.
(b) Students will be able to cultivate a keen interest in the field of marine engineering.
(c) The model serves as a basic reference to encourage and provide students with an idea to create more efficient designs as means of transportation in water.

 

The model is a simple imitation of a submarine, which is used to show the fundamental principle of the way a real submarine functions. The model consists of a "submarine" created using a plastic soft drink bottle, two pumps and a plastic aquarium.

(i) The "submarine"
The "submarine" is made of a 1.5 litre plastic soft drink bottle. The bottle is laid horizontally so that the bottle cap resembles the head of a submarine, whereas the bottom of the bottle is the tail. Two valves, (A) and (B) are fitted to the bottom of the "submarine". Another valve (C) is placed on the mouth of a PVC tube (D), which is protruding from the bottom to the top, in the inner part of the "submarine". Tube (E) and (F) which are joined to pump (G) and (H) respectively, are linked to the "submarine" at the top. Two brass rods (I) are attached to the lower left and lower right of the "submarine" as shown in 4-D1 (figure showing the basic structure and the water and air flow in the "submarine")

(ii) The valvesValve
(A) and (B) consist of four holes with a suitable round plastic piece covering the holes. These two valves are opened to the inner part of the "submarine" only. The opening and closing of the valve is controlled by water as well as air pressure inside the "submarine".
Valve (C) created using a rubber pipe washer which is tied to rubber bands which serve as the controller of the opening and the closing of the valve.
(See 4-D2 showing the structure of different valves found in the model).

(iii) The PumpsPump (G) is used to pump air into the "submarine". Pump (H) is used to suck the air out from the inner part of the "submarine".

(iv) The brass rods attached to the sides of the "submarine" play a role in balancing the "submarine". They also contribute to a faster submerging of the "submarine", which in turn save the time in doing the experiment.

(v) The plastic aquariumThe plastic aquarium is filled with water and serves as the medium where the experiment is carried out. It enables the submarine to be tested in a surrounding which resembles the ocean.

(vi) The working of the "submarine"We can use Archimedes' principle to explain how submarine ascend and descend. Archimedes' principle tells us the water displaced produces an upthrust. Within the "submarine" there is a large space, which can be flooded with water from the plastic aquarium. Initially, the "submarine" is filled with water to a fixed level which is enough to exert pressure on the valve (A) and (B) to prevent them from opening and hence letting air going out from the valves. When the air in the submarine is suck out using pump (H), the inside of the submarine becomes semi-vacuum and water will be forced to enter through valve (A) and (B). When the whole submarine is filled with water, it becomes heavier and its weight will exceed the upthrust, producing a net force acting downwards, causing the submarine to descend.

The submarine can ascend again by replacing the water in the submarine with air. It is done by pumping air into the submarine using pump (G), forcing the water back out into the aquarium through the PVC tube (D) and valve (C). Hence, the submarine becomes lighter and the upthrust exceeds the weight of the submarine. So the net force will act upwards, causing the submarine to ascend.

(Refer to 4-D3 showing the conditions when the submarine is floating and when it is in the water.)

 

The tested submarine is observed for its efficiency based on:

(a) Air pressureVery low air pressure produces a partial vacuum within the submarine causing water to flood in the submarine. The submarine starts to submerge. High pressure stops water from flooding into the submarine, at the same time forcing water in the submarine to flow out. The submarine starts to emerge.

(b) UpthrustAs water enters the submarine, its overall weight starts to increase. If the weight of the water-filled submarine is greater than the upthrust then the submarine submerges. If the water is pumped out and the weight decreases until it is less than the upthrust of the submarine, then the net force produced will allow the submarine to emerge. Submarine will remain suspended in the middle of the tank of water provided just enough water is filled into it so as to produce a weight which is equal to the upthrust.

Doing experiments using the model submarine created can authenticate all the theoretical description above.

(Refer also 4-P1, 4-P2, 4-P3, 4-P4 and 4-P5)

The construction of the submarine is quite difficult. To achieve the same objectives, the construction of the submarine can be simplified as follows :

One hole is made at the bottom of the submarine and one hole is made at the top of the submarine. The top hole is connected to a PVC tube and to a large syringe (10-20 cm3). The front part of the submarine is covered with bottle cover.

Implementation :
Water is filled into the bottle (submarine) through the mouth of the bottle until the bottle is submerge under the surface of the water. The bottle mouth is closed. Push the piston of the syringe forward, the air pressure in the submarine will push the water out and the submarine will rise. Pull back the piston the air pressure in the submarine will decrease and water flow into the submarine and the submarine sinks.