A piece of ______ can be placed between the load and the head of a jack to help prevent a load from slipping off the jack.

Using a piece of wood between the load and the head of a jack is highly effective for several reasons. Wood has unique properties that provide a non-slip surface when used for this purpose. It is often textured and can conform slightly to the shape of the load, which enhances the grip and helps to stabilize the load during lifting. This firm yet flexible characteristic reduces the chances of the load slipping off, ensuring a safer lifting process.

Wood is also a common material found in various environments, making it readily available and easy to work with. It is strong enough to withstand the weight of many loads encountered in typical scenarios involving jacks. In addition, wood does not conduct electricity, making it a safe choice in environments where accidental electrical contact might be a concern.

Other materials, such as metal or plastic, may not offer the same degree of friction or grip as wood, which can lead to instability. Rubber, while it provides some grip, may not be as durable under heavy loads as wood, particularly for prolonged lifting scenarios. Therefore, wood stands out as the best option for preventing loads from slipping off jacks.