What should be placed between the load and the head of a jack to help prevent a load from slipping off?

Using wood as a material between the load and the head of a jack is effective for preventing the load from slipping off for several reasons. Wood offers a level of friction that helps to secure the load in place, which is essential in maintaining stability and balance while lifting. Its compressibility also allows it to conform slightly to the shape of the load, creating a more secure contact surface. Additionally, wood is commonly available and can be easily shaped or cut to fit various load sizes and shapes, making it a practical choice in many lifting operations.

Other materials, while they may have their uses, do not provide the same combination of friction and adaptability as wood. For example, metal may be too slippery, resulting in insufficient friction to keep a load stable. Rubber, although it provides some grip, can wear out or degrade over time and may not offer sufficient load-bearing capacity in certain situations. Concrete lacks the necessary properties to create a secure interface between the load and the jack, as it is heavy and immobile, which negates any potential benefits in this context. Therefore, wood is the most appropriate choice in this scenario.