A fluid exerts an upward buoyancy force on an object immersed in it. In the derivation of Eq. (5.9) in the textbook the buoyancy force exerted on an object by the fluid was ignored. But in some situations, where the density of the object is not much greater than the density of the fluid, you cannot ignore the buoyancy force. For a plastic sphere falling in water, you calculate the terminal speed to be 0.36 m/s when you ignore buoyancy, but you measure it to be 0.25 m/s . The buoyancy force is what fraction of the weight?