|
Good question. You accelerated the fly to 60 mph while he was on the dashboard. When he flew off the dash, there were no forces to decelerate him, so he kept going at 60. As he flew around in the cab, he was accelerating and decelerating relative to the cab, so his instantaneous speed relative to the ground varied between a little faster than sixty and a little slower than sixty. But his average velocity from the time he flew off the dash until he landed was 60 mph. Here's another thought: Suppose you were to drive your pickup off a cliff. For the purposes of our discussion you have your seat belt off. As your vehicle plummets to the ground, you, the fly, and your pickup are each subjected to the same forces: Vertically, gravity is accelerating you downward at 32 feet per second per second (32 ft/sec^2). Horizontally, there is essentially no force acting on you once your tires lose contact with the ground. Since all three objects start with the same initial velocity and are subjected to the same acceleration, everything hits the ground at the same time, going the same speed. Here's another example: You're a passenger on an airliner. The plane hits an "air pocket" (actually turbulence) and the stewardess walking down the aisle is momentarily "weightless". Does she slam against the rear bulkhead of the aircraft? Of course not! Even though the plane is flying at 400 mph, the stewardess and the plane are both going the same speed and the stewardess stays in the same horizontal position relative to the plane. (However, the plane was accelerated in the vertical axis, so the stewardess appeared to move verically relative to the plane. Actually the plane moved relative to the stewardess.)
|
