Final answer:
The restoring force is negative when a pendulum bob has a positive displacement, as it acts in the opposite direction to the displacement.The correct option is option d .
Step-by-step explanation:
If a pendulum bob has a displacement that is positive, then the restoring force is negative. This is because the restoring force acts in the direction opposite to the displacement to bring the pendulum back to its equilibrium position. In physics, particularly in the context of simple harmonic motion, the restoring force is directly proportional to the displacement but acts in the opposite direction as described by Hooke's Law, F = -kx, where F is the restoring force, k is the force constant, and x is the displacement from equilibrium. The negative sign in this equation indicates that the force is opposite in direction to the displacement.
An everyday example of this would be a child's swing or an old-fashioned clock's pendulum. In such a pendulum, when the bob is pulled away from its equilibrium position and released, the restoring force works to bring it back to that position. This is why the pendulum swings back and forth, with the force always acting opposite its current motion.