When an object enters the water, it causes the water to be displaced. The object will continue to sink until is has displaced an amount of water equal to its weight. If the displaced amount of water is equal to the weight of the object, it will create a force pushing the object up so it becomes buoyant.
If a 100-pound person were to go into the water, the person would need to displace 100 pounds of water to float. Lighter objects such as foam require much less water to float.
Foam core life jackets have air chambers inside that cause them to displace only a small amount of water and thus float easily.
Other life jackets have air bladders that can be inflated. When inflated, as with the foam core life jackets, it requires little displacement to float.
Because people are already naturally buoyant, the life jacket doesn't have to support the entire weight of the person. Instead, it needs to displace just enough water to keep a person's head above water.
Article Written By Shiromi Nassreen
Shiromi Nassreen has been writing professionally since 2005. She specializes in travel and outdoor topics, and her articles have appeared in various print and online publications, including "DISfunkshion Magazine" and Matador Travel. Nassreen holds a Bachelor of Arts in theatre studies from Rose Bruford College of Speech & Drama.