Buoyancy

Maria and Sofia are swimming in the sea. Sofia, hold me while I try to float on the surface! I can’t! You’re bigger than me. Just try.

Wow, you feel so light! That’s because the water is helping you, Sofia! It has all to do with forces. When Maria is standing on the beach, gravitational force pulls her towards the Earth, and stops her from floating upwards. The heavier Maria is, the stronger this force will be.

Gravitational force is greater than the force Sofia can produce, so she can’t lift Maria when they are both standing on the beach. When Maria’s in the sea, gravitational force is still acting on her. So why can Sofia lift her now? There must be another force helping Sofia! When an object is immersed in water, water is pushing on the object on all sides.

The higher the pressure, the stronger the pushing force will be. Now, if the force is pushing the object in all directions - - why doesn’t the object start moving in one direction? Because the force pushing on the other side in the opposite direction, is equally strong — the forces acting on the sides of the object cancel each other out. However, the bottom of the object is at a greater depth than the top of the object. Greater depth means higher pressure and therefore a stronger force.

The top of the object, nearer the surface, has a weaker force acting on it. The two forces act in opposite directions, and because of the difference in pressure, the direction of overall force on the object is upwards. We call this upward force buoyant force, and the effect it has, buoyancy. Buoyancy acts in the opposite direction to gravitational force, and so counteracts it. Buoyant force is always present, not only in water but in all liquids and gases!

When Maria is in water, her weight doesn’t change, but the buoyant force counteracts it and pushes her upwards. So with the help of buoyancy, Sofia can hold Maria! Buoyant force can be calculated by subtracting the force acting downwards on the top of the immersed object, from the pressure force acting upwards on the bottom. Let’s look at this juice carton as an example. The force acting on top of the carton is 2.45 newtons, while the force acting on the bottom of the carton is 4.9 newtons — the force at the bottom is twice as large!

Now you can calculate the buoyant force by subtracting the force acting on the top from the force acting on the bottom: 4.9 minus 2.45 newtons. The buoyant force is 2.45 newtons! So, buoyant force is an upward force that fluids exert on immersed objects. It results from the pressure difference between the top and the bottom of an immersed object. I’m getting tired, the buoyant force isn’t helping enough - — your turn to hold me Maria!