Explain why the force of gravity will allow you to extend your forearm even if the triceps brachii is not functioning

Gravity does not depend on triceps brachii to work, it is automatic, thereby it shall enable extension of the arm.

Explanation:

Gravitational pull present on earth is a force that pushes all objects downwards.

Since it applies to all objects present on earth including human beings, it shall still act on the arm thereby enabling one to extend their arms.

The triceps shall only be needed to controll the extension in terms of distance and speed.