Internal forces are forces that act between different parts of a system. These forces are exerted by the bodies within the system on each other. The statement that internal forces are forces which bodies exert on each other when the bodies are part of the system is correct.

Explanation:
Internal forces are forces that act within a system and are caused by the interaction between the different parts of the system. These forces can be tension, compression, shear, or any other type of force that arises due to the interaction between the bodies within the system.

Internal forces are different from external forces, which are forces that act on the system from outside. External forces can be gravity, friction, or any other force that is applied to the system from an external source.

Internal forces play a crucial role in determining the behavior and motion of a system. They can cause the deformation or displacement of the bodies within the system. For example, in a simple mechanical system like a beam supported at both ends, the internal forces include the tension and compression forces within the beam. These internal forces are responsible for keeping the beam in equilibrium and preventing it from collapsing.

Internal forces can also be used to transfer energy within a system. For example, in a mechanical system like a bicycle, the force applied to the pedals is transferred through the chain to the rear wheel, causing it to rotate and propel the bicycle forward.

In conclusion, internal forces are forces that act between different parts of a system. These forces are exerted by the bodies within the system on each other. Understanding internal forces is essential in analyzing the behavior and motion of systems in various fields such as physics, engineering, and biology.