Why can you float in water if you lie on your back but not if you try to remain vertical in water?

Question

Why can you float in water if you lie on your back but not if you try to remain vertical in water?

Answer 1

What keeps you afloat in the water is an upward force exerted onto your body caused by hydrostatic pressure, which counters the force of gravity of your body that causes you to sink.

According to Pascal's Principle,

Pressure = Force / Area, which also means that:

Force = Pressure * Area

(where the force is normal to the area)

What this relationship says is that, (assuming water pressure remains constant), as the area of the object exposed to the liquid becomes smaller, the force exerted on the object also gets smaller. As the area of the object exposed to the liquid becomes larger, the force exerted onto the object also gets larger.

This means that when you are standing upright in water, the only force acting upwards on your body is due to the water on the small area of the soles of your feet. Due to the small area, only a small amount of force "pushes" your body upwards. When you're lying on your back; however, there is a much greater area of your entire back side, (including arms, legs, etc.), exposed allowing for a greater amount of upward force generated on your body to counter the force of gravity, keeping you afloat.