When you label something as “metal,” “plastic,” or “ceramic,” you are referring to the general class of material involved. It won’t help you predict the performance of an object made of that material. For example, aluminum and stainless steel are both metal but can have very different densities, mechanical strength, corrosion resistance, and stiffness. Polyethylene bottles and acrylic picture frames are both made of polymer materials. It is more helpful to consider a general material class as a beginning point instead of a specific prediction.
It is a good idea to start with the general properties you might expect from each class. Metals have atoms arranged in a crystalline structure and are typically conductive and deformable. Polymers are long molecules; the way these molecules are arranged determines their mechanical strength, elasticity, temperature tolerance, and other properties. Ceramics have very strong chemical bonds that make them very hard, stiff, and resistant to high temperature, but not very deformable. Composites are mixtures of two classes of material; their strength depends on the proportion of each, the orientation of one relative to the other, and the load that is applied.
To practice this, get a metal utensil, a plastic container, a ceramic item (a mug is an ideal choice and a safe one if you are new to this), and a composite item, perhaps a tool handle. Observe each item for any differences in weight, visible signs of wear, and the ability to flex, bend, or break. Is the object cold, room temperature, or hot? (If you are not sure, allow the objects to sit in the room for at least twenty minutes.) Make sure to separate what you observed about each object from any conclusions you draw about it. For example, the metal utensil will typically feel cooler than the others, not because it is at a lower temperature, but because the metal conducts heat away from your hand quickly. The ceramic object feels rigid, but that observation alone does not tell you what force would cause it to break.
It is challenging to describe differences in objects without making claims that apply to all or almost all examples of each class. It is not true that metals are heavy, plastics are weak, ceramics are brittle, or composites are better than metals or ceramics. Material properties depend on the material’s composition, microstructure, and history, and these properties change with temperature and part geometry. For example, the heat treatment of a metal changes its hardness and toughness. Molecules within a polymer can be packed to be either very flexible or quite rigid. The amount of porosity, crystalline grain size, and heat treatment of a ceramic affect its mechanical and thermal performance. How a composite is layered determines the direction in which it resists a tensile or bending load.
A stronger comparison will use material properties to assess the service requirements of a specific design. Is this object intended to be lightweight and impact resistant in an outdoor environment? Is it intended to be resistant to cutting? Is the object to be used as an electrical insulator where the ambient temperature is high? Material families are not better than each other overall; they are better for some applications, but other classes can be better suited for others. The designer must decide what properties are required for a service environment and which limitations are acceptable.
If you consult a material property table, compare individual metals, plastics, ceramics, and composites using the same units, at the same temperature, and with the same loading conditions. Do not select from a single value for a property and label it as a generic representative of a material class; that will not make an accurate comparison. You must also ensure that you are comparing the correct property; for example, do not compare mechanical strength to thermal conductivity. Your comparison should result in a statement such as, “Material A is appropriate for this design with these requirements. If requirements change, Material B may become preferable.”