How do life jackets keep us afloat on water?« Back to Questions List

Imagine yourself travelling in a boat and all of a sudden you fall off the boat. Luckily you are wearing a life jacket. What happens to you is well explained by Archimedes principle and concept of buoyancy. Let us learn them one by one.

When an object gets submerged in water, it displaces water in accordance to the weight of the object. For example, when a ball filled with air is thrown into water, it displaces very small amount of water. Archimedes discovered that the water will exert an upward force against the immersed object equal to the weight of the water displaced.

Objects that displace an amount of liquid equal to their weight will float because they receive that upward push from the water. So when the amount of water displaced is less, force exerted by water on the immersed object is less. Hence the object sinks. Whereas when the displaced water is more, force exerted by water on the object is more and hence it floats. This upward force exerted by water is called buoyancy.

Buoyancy is the reason we feel light when we are in a swimming pool. The outer shell of life jacket   is made of nylon or vinyl with a material sewn inside. The material traps air when the jacket is immersed. The trapped air weighs much less than the weight of the water it displaces, so the water pushof es up harder than the life jacket pushes down, allowing the life jacket to remain buoyant and afloat. This buoyancy is strong enough to hold up additional weight without sinking.

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