![]() If this were the case, even if the object's density were such that the object had the potential to float, i.e. How can water exert a force, creating a greater pressure on the bottom of an object than the top, if the object (take a cube for example) has been placed on the bottom of the container holding the water, so that there is no water below the object (the object is directly against the container), and thus the only force from the water (besides the pressure resulting from the water's force acting on the sides of the object) would be that acting downwards. If that amount of mass on the surface of the object is greater than the mass of the area of the water (or any liquid) underneath it, then the gravitational force pulling downwards on the mass of the object will cause the object to "push aside" the liquid in its way. Now, if an object has a greater density, that means that, per amount of surface area, that object also has a greater mass for that given area. Why did the surface area change? Well, that's because we increased the balloon's volume! For example, an empty balloon has a much smaller surface area than a balloon filled with air. The surface area is related to the volume generally, the greater the total surface area of an object, the greater the object's total volume. Let's look a little closer at that surface area. The force of water above the object is given by rho*g*h, and the buoyant force underneath the object is equal to the (pressure at the bottom of the object)*(surface area of the bottom of the object). This is something difficult to visualize. ![]() The second explanation is considering the tendency of the entire system to lower its gravitational potential energy which can be done if the balloon were replaced with air (due to its greater mass) and therefore all elements of air above the balloon, try to reduce the net energy by coming down in place of the balloon and in the process providing an upward push on it. This is what concept of floating and upthrust is. Had there been normal air there it would have been static as the upward force would have been equal to its weight but since the weight of the balloon is less than the upward force acting on it, it will move up. Therefore, it provides an upward force due to difference in hydrostatic pressure at the bottom of balloon and the top, which is equal in magnitude to the weight of normal air the size of the balloon. One is that atmosphere (which is a fluid in static equilibrium) cannot distinguish between the balloon and an equivalent amount of normal air in its place. There are two explanations as to what is pushing up the balloon.
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