A sphere, a cube and a thin circular plate have the same mass and are made of the same material. All of them are heated to the same temperature. The rate of cooling is

Since the material is the same, the density is the same. Since the mass is the same and density is the same, all three have the same volume. For the same volume, the surface area of the plate is the largest and of the sphere the smallest. The rate of loss of heat by radiation is proportional to the surface area.