Archimedes' Principle If you have ever held a helium filled balloon or tried to submerge a ball in water, you will have experienced Archimedes' principle. It states that a fluid exerts an upward buoyant force on an object equal to the weight of the displaced fluid. In air this force is usually small because the density of air is small but in water the force can be quite large. Suppose you submerge an object of volume then the buoyant force acting on the object is given by = fluid (1) where fluid is the density of the fluid. If the density of the object is greater than the density of the fluid, the object will sink. However, if the density of the fluid is greater than the object, the object will float. This is the case of a helium balloon in air or a basketball in water. If an object is floating without accelerating, then the sum of forces on the object must be exactly zero. For an object floating in a liquid, the only other force is the weight of the object and therefore, object = = fluid sub (2) where object is the mass of the object and