Question

Three men were being interrogated by FBI agents. The agents believed that one of them had left a federal office with 20 lb of top-secret documents under his coat, gone down the freight elevator, and slipped into a waiting getaway car. All three admitted to having stood on a scale kept in the freight elevator to weigh incoming shipments, and made the following statements:

Suspect A said, “The scale read my normal body weight only while the elevator was just starting to descend.”

Suspect B said, “I only got on the scale for a few seconds about halfway down. It seemed to read my normal body weight.”

Suspect C said, “When the elevator was coming to a stop, the scale read about 20 lb more than my normal body weight.”

If all three men are telling the truth, who should the agents arrest?

Suspect A Suspect B Suspect C None of the suspects.

Explain your reasoning.

Answer 1

Agents should arrest Suspect A.

When the elevator was about halfway down, the speed of the elevator must be nearly constant. i.e. zero acceleration. In that case, the only gravitational pull is working on the suspect. So, the weight reading must be near to normal body weight. Hence Suspect B is not guilty.

When the elevator was coming to stop, there is an upward acceleration (deceleration) working on the elevator and hence adding weight to the reading. Hence Suspect C is also not guilty.

When The elevator was just starting to descend, there is a downward acceleration working on the elevator and hence reducing weight in the reading. So, there must be less than normal reading in the scale. But Suspect A said it was normal. So, he must be carrying extra weight. So, Suspect A is guilty.