When a balloon is pushed into water and released, the force that brings it back to the surface is the buoyant force. Buoyant force is the upward force exerted by a fluid, in this case, water, on an object that is submerged in it. This force is a result of the pressure difference between the top and bottom of the submerged object. According to Archimedes principle, the buoyant force is equal to the weight of the fluid that the object displaces.
Understanding Buoyancy
When the balloon is released, the buoyant force acting on it exceeds the weight of the balloon, assuming the balloon is less dense than water. This causes the balloon to rise to the surface. Once it reaches the surface, the buoyant force continues to act on it until it is fully out of the water. The force at play here is buoyancy, which is the reason why the balloon experiences an upward push.
The Role of Pressure in Buoyancy
The water at any given depth exerts a pressure on everything around it. This pressure is equal to the weight of the water above it plus the weight of the air above the water. Gravity is pulling all of the water and air downward, even if the shape of the water above it isn’t uniform. This explains why the pressure is felt according to the depth.
When the balloon is submerged, the bottom of the balloon is in deeper water than the top. Consequently, the water under the balloon pushes upward with a greater force than the water above the balloon pushes downward. At the same time, the air inside the balloon is pushing on the inside surface of the balloon. The pressure is slightly more pushing downward on the bottom of the balloon because of the weight of the air inside, but the air is so light compared to water that the difference in air pressure pushing the bottom of the balloon down and the air pressure pushing the top of the balloon up is very small. This results in the total upward forces being greater, causing the balloon to rise. The balloon is buoyant.
On the other hand, if the balloon were filled with a denser substance like mercury, the pressure on the bottom of the inside of the balloon would be much higher than the pressure on the top of the inside of the balloon, far exceeding the external water pressure difference. In this case, the downward forces would be greater, and the balloon would not be buoyant."
Illustrating Buoyancy in Action
The concept of buoyancy becomes more vivid when we consider the amount of force required to hold a balloon under water. When you push a balloon full of air under water, it displaces a volume of water equal to the volume of the balloon. The volume of water displaced is much heavier than the air-filled balloon. As a result, the displaced water is pulled back to displace the balloon by gravity, and it takes a strong force from your hands to hold the balloon under water and counteract the buoyancy. The amount of force required to hold the balloon under water is exactly equal to the force of gravity on a mass equal to the difference between the mass of the displaced water and the mass of the air-filled balloon.
Conclusion
The buoyant force is a fascinating force that we can observe daily, as demonstrated by a simple experiment with a balloon. By understanding the principles behind buoyancy, we gain insight into how objects float or sink in different fluids. Additionally, observing the relationship between pressure, density, and buoyancy can lead to a better understanding of various scientific concepts and practical applications in our everyday lives.