The buoyant force gets smaller.
Work Step by Step
The buoyant force on an object is the weight of the water displaced by the object. The weight of the water displaced is the volume displaced (i.e., the volume of the sphere), multiplied by the density of water, multiplied by g. With the rise in temperature, the volume of the sphere increases while the density of the water decreases (because it expands). We see that the volume expansion coefficient of water is almost 3 times larger than that of aluminum. As far as buoyant force is concerned, the percentage decrease in the water’s density plays a larger role than the percentage increase in the aluminum sphere’s volume. In other words, the volume of the sphere times the density of water becomes smaller. The buoyant force decreases.