How Surface Area Affects the Buoyancy of Wood

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Introduction: The upward force on an object that is in water that allows it to float is called buoyancy. The buoyant force is always equal to the weight of the water displaced by the object in the water. This force was discovered by Archimedes, who was in his bathtub when he wondered why objects seemed lighter when he held them under the water. He found out that if an object is lighter than the weight of the water it displaces, it will float, but if it is heavier, it will sink.

The force of buoyancy is important in many different areas, and especially in the making of ships. The surface area that is touching the water of the ship is very large, due to the shape of the hull, and that, beside the density of the ship, is what keeps the ship floating. An important example of how surface area affects buoyancy is when people float in the water. Everyone knows that it is much easier to stay afloat when we are lying on our backs than when we are in a standing position in the water. But how does the surface area of an object affect its buoyancy?

Aim: To investigate how changes in the surface area of pieces of wood of the same mass affects their buoyancy.

Hypothesis: That the larger the surface area of the piece of wood, the more buoyant it will be.

The Independent Variable is the surface area of each piece of wood. The Dependent Variable is the buoyancy of the each piece of wood. The Controlled Variables are:
oThe weight of each piece of wood and its hook
oThe amount of water each piece of wood is measured in
oThe spring balance used to take measurements must be the same (to prevent any inaccuracies) oThe size of the container used to hold the water

4 pieces of wood attached to hooks, each weighing 100g, with different surface areas (e.g. 81cm², 96cm², 110cm², 153cm²) A container large enough to fit each of the pieces of wood without them touching the bottom or sides (e.g. 4L ice-cream container) Retort stand, bosshead, clamp

A spring balance appropriate to the weight of the wood
Enough water to fill the container to ¾. (e.g. 3L)

1.Set up the apparatus as shown in diagram 1 (on pg. 2).
2.Hang each of the pieces of wood from the spring balance in turn, measuring their newtons in air. 3.Put the ice-cream container filled with water under the spring balance as shown in diagram 2 (on pg. 2). 4.Hang each of the pieces of wood from the spring balance in turn, measuring their newtons in water. (Make sure that the spring balance isn’t holding the piece of wood out of the water, or you will not get accurate readings. The best way to do this is to place the piece of wood into the water, and then attach the spring balance so the hook of the balance is only lightly making contact with the hook attached to the piece of wood). 5.Find the difference between the two figures for each piece of wood '' this is their buoyancy. 6.Repeat the experiment twice.

Risk Assessment:

Safety IssuePrecaution

Someone could slip on spilt water.

Keep the water-filled container away from the sides of the bench, clear up any spills immediately.

The edges of the pieces of wood could be sharp '' someone could get a splinter.

Make sure that the pieces of wood are well sanded.


Piece of Wood
(And its surface area in cm²)Repeat Number
Newtons In AirNewtons In Water
(N)Avg. Buoyancy

The results of my experiment show that the larger the surface area, the more buoyant the piece of wood is. This trend is clearly visible in the graph above. This is because the larger the area touching the water, the more upward force the wood receives from the water....
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