Author: Rin Takahashi
Editors: Ian Cho, Sophia Chen
Artist: Olivia Yuan
You may have taken a boat to travel somewhere before—perhaps to a beach, an island, or even another country. But have you ever wondered how these boats stay afloat, even with hundreds of passengers and cars, while significantly lighter rocks sink immediately after landing on the water?
Wherever you are, forces act upon every object. When a person stands up, the gravitational and normal forces prevent us from flying away from outer space. When a box is pushed on a surface by a human, the force of gravity, normal force, friction, and applied force are all in play. Forces explain how an object moves or stays as it is.
When an object like a boat enters water, two forces act upon it: gravitational force, a force downwards, and buoyancy, a force upwards. The gravitational force can be found by Fg=mg, in which the object's weight impacts; Fg represents the force of gravity, m represents the mass, and g represents gravity(9.81m/s^2 in Earth). Buoyancy can be found using Fb=-pgV, which the fluid impacts. Fb represents the force of buoyancy, p indicates the density of the liquid, and V shows the volume change also known as displacement. Buoyancy is the reason why objects stay afloat on water. This phenomenon can also be seen when people wearing life jackets dive into water and remain floating.
You might ask, if every object in water has a gravitational and buoyant force acting upon it, why wouldn’t rocks float? The answer is outlined in Archimedes’ principles, stating that any submerged object will experience a buoyant force equal to the weight of the displaced fluid, which is also shown in the equation Fb=-pgV. In other words, the more fluid an object displaces, the more buoyant it will be. This is related to density because the buoyant force depends on the density of the fluid and the volume displaced, according to Archimedes' principles. Archimedes also found that when an object is submerged in water, the volume of the water displaced must equal the volume of the object submerged. For instance, when you dive into a pool. Taking an average human volume of 75L, if that person is fully submerged, the water level rises by 75L.
Put simply, boats float on the ocean since the steel is dense, but the air in the cabin is not, thus, the ship is less dense overall than the water. However, if the boat were crushed into a small ball with the same mass and eliminating all the air, it would sink. Density is inversely proportional to volume so if you crush the ship into a small ball, the volume will decrease, and the mass will remain the same, causing density to increase.
In conclusion, boats float in the ocean because their density is lower than the density of water, while rocks can’t float because their density is greater than the density of water. To see this for yourself, try putting different objects and fruits in a container filled with water and keep track of their volumes and densities! You may be surprised at what kind of objects you can make it float.
Citations:
“14.4 Archimedes’ Principle and Buoyancy | University Physics Volume 1.”
Courses.lumenlearning.com, 2023, courses.lumenlearning.com/suny-
Gregersen, Erik. “Archimedes’ Principle | Description & Facts.” Encyclopædia Britannica, 5
Dec. 2018, www.britannica.com/science/Archimedes-principle.
McDonald, Amy. “Why Do Ships Float? - Let’s Talk Science.” Letstalkscience.ca, 24 Sept. 2019,
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