Hard refers to a concept in physics that describes the resistance of an object to a force. It is defined as the ratio of an objects force to its acceleration. The harder the material, the greater the resistance it offers to an applied force. In other words, the harder a substance is to deform or bend under an applied force, the more difficult it is to change its shape.
The concept of hardness is important in engineering and materials science. For example, when designing a bridge or building a car frame, engineers need to consider the hardness of various materials that will be used. If the material is too soft, it may not be able to withstand the stresses and strains of a large structure. On the other hand, if it is too hard and brittle, the material may not be able to bend or deform properly.
In summary, hard refers to the resistance of an object or material under a force. It is important in engineering and materials science as it helps determine the suitability of different materials for specific applications.