1.5 A laptop rests on a table. According to Newton's 3rd law, what is the reaction force to the weight of the laptop? A The upward force of the table on the laptop. B The upward force of the laptop on the earth. C The downward force of the Earth on the laptop. D The normal force on the laptop. A person stands on a scale in an elevator. He notices that the scale reading is low than his usual weight. Which of the following could possibly describe the motion the elevator? A It is moving down at constant speed. B It is moving down and slowing down. C It is moving up and slowing down. D It is moving up and speeding up.

A fascinating tidbit about Newton's Third Law is that it’s often phrased as "for every action, there is an equal and opposite reaction." This means that if you push against something, it pushes back with equal force. So when a laptop rests on a table, the weight of the laptop (its gravitational force acting downward) results in an equal and opposite force exerted by the table pushing upward on the laptop, which is the normal force. When the person notices a lower reading on the scale in the elevator, they are likely experiencing a change in acceleration. If the elevator is moving down and slowing down, it exerts a less upward force than usual, resulting in a lower scale reading. This is because the upward normal force from the scale is less than the gravitational force acting on the person during this period of deceleration.