Buoyant force is the force from the water pushing on you to make you float, and it is related to how much space you take up and how dense you are.

As long as you take up the same amount of space and have the same density, the water will push on you with the same amount of force, regardless of which way you are oriented. If you are upright or laying down in the water, the water will float you the same amount! However, the water floating you the same amount doesn't always mean that is equally easy to breathe or maintain your position in the water. When we are on our backs in the water, our mouth is at the water's surface and we can breathe. Also, we can easily move our arms and legs to keep us floating in that way. When we are upright in the water, our mouth might not be at the water's surface. Also, we have to kick hard to get our head all the way out of the water so we can breathe. It seems like the water is floating us less when we are upright than when we are on our backs, but it mostly has to do with how well we can breathe!

I encourage you to test this if you are curious! In a pool, try floating on your back. Then, try taking a deep breath and tucking yourself into a ball. Do you float to the top? If you make your body straight, but hold a deep breath, do you float up to the top? In each case, you should slowly float to the surface of the water (but remember, your mouth might not be at the surface, so you might have to hold your breath for a few seconds).

Floating while standing is possible, but the orientation can give the appearance of sinking. An object will float in a fluid when the buoyant force, pushing up on the object equals the force from gravity pulling it down.

This essentially comes down to the relative density of the object and the fluid, which is the density of the object compared to the density of fluid. If the density of the object is less than that of the fluid, then the object can float. The magnitude of the buoyant force depends on the volume of fluid (here, water) displaced by the object. When lying down, a person has a large area that is pressing down on the water, so enough water is displaced by sinking only a small amount. When upright, that area is much smaller, and sinking to a greater depth is required to displace enough water. In addition, the density of the "lower" parts of the body (e.g., legs) is higher than that of the chest/torso. Remember that our lungs are in our chests, and lungs are basically big bags of air, which is much less dense than water and floats more easily (basically like inflatable pool toys).

Hello Evelyn! This question really gets at the concept of buoyancy and pressure.

Buoyancy is the upwards force that water applies to an object that is occupying volume in the water.

Pressure is equal to force over area and in a liquid, pressure is pretty much the same everywhere.

When we are floating up, a greater pressure is applied by the water (due to the force of buoyancy) than gravity and the result is a net pressure pointing up. When we sink, more pressure from gravity is being applied and the result is a net pressure pointing down.

When you are lying down in water, the area over which you apply the force of gravity on the water is much larger, meaning the pressure you apply is much smaller. If the pressure in the water remains the same, the pressure you apply when standing is greater than the water pressure, but when you are lying down, it is less. As a result, you will float when lying down but not when standing up.

Archimedes' law: a floating body displaces its weight, while a sinking body displaces its volume.

You actually do float in water if you are 'standing', provided that you have lungs full of air (which makes you float in the first place). However, unless the body of water that you are floating in is deep enough that your feet can't reach the bottom. This is because you do sink partially below the surface, due to Archimedes' law.