Final answer:
When a ball is at rest on a trapdoor, the forces acting on it are gravity and the normal force exerted by the trapdoor.
Step-by-step explanation:
When a ball is at rest on a trapdoor, there are two forces acting on it: the force due to gravity and the normal force exerted by the trapdoor.
The force of gravity acts vertically downwards, towards the center of the Earth. It gives the ball weight. The normal force is exerted by the trapdoor and acts perpendicular to the surface of the trapdoor, pointing upwards.
Both forces have equal magnitudes and opposite directions, according to Newton's third law of motion. In this case, the weight of the ball is balanced by the normal force exerted by the trapdoor, resulting in the ball remaining at rest.