1. Which pulls harder, the Moon on Earth, or Earth on the Moon?
2. A high-speed bus and an unfortunate bug have a head-on collision. The force of the bus on the bug splatters it all over the windshield. Is the corresponding force of the bug on the bus greater, less, or the same? Is the resulting deceleration of the bus greater than, less than, or the same as that of the bug?
1. Each pull is the same in magnitude. This is like asking which distance is greater, from Reno to Miami or from Miami to Reno. So we see that Earth and the Moon simultaneously pull on each other, each with the same amount of force.
2. The magnitudes of the forces are the same, for they constitute an action–reaction force pair that makes up the interaction between the bus and the bug. The accelerations, however, are remarkably different because the masses are different! The bug undergoes an enormous and lethal deceleration, while the bus undergoes a very tiny deceleration—so tiny that the very slight slowing of the bus is unnoticed by its passengers. But if the bug were more massive, as massive as another bus, for example, the slowing down would be quite apparent.