Final answer:
When an elevator slows down, the force exerted by the cable and the acceleration both decrease in accordance with Newton's second law, as the tension required to decelerate the elevator is less than the force needed for upward acceleration or constant speed movement.
Step-by-step explanation:
When the elevator starts to slow down as it reaches the 20th floor, the force exerted by the elevator cable decreases in magnitude because the acceleration of the elevator is decreasing. According to Newton's second law (F = ma), if the mass of the elevator and its contents remains constant, and the acceleration decreases, then the force must also decrease. This effect occurs due to a reduction in the net force required to decelerate the elevator. Consequently, the tension in the elevator cable, which is the force exerted by the cable, is less than when the elevator was accelerating upward or moving at constant speed.