Definition:Internal Force

Definition

Let $B$ be a body.

An internal force is a force $\mathbf f$ exerted by one particle $P_1$ of $B$ on another particle $P_2$ of $B$, and to which there exists an equal and opposite reaction exerted by $P_2$ upon $P_1$.

Hence internal forces exist in pairs whose individual resultants are zero.

Hence the resultant of all the internal forces is also zero.

Also see

• Definition:External Force: a force which can affect a body as a whole

• Results about internal forces can be found here.

Sources

• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): internal force
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): internal force