Free Falling Elevator and Floating ``` Name: Kevin B. Status: student Grade: 9-12 Location: CA Country: USA Date: Spring 2012 ``` Question: I had a debate with my uncle about what would happen to a person in a free falling elevator. I asked him if the person would float. My uncle said no. He pointed out that a ball would not float if put on the floor of the elevator and the cable was cut, so why would a person? To see what would happen to a ball in a free falling elevator, I put a ball on a folder (the folder representing the floor of an elevator), and I dropped the folder with the ball. The ball stayed on top of the folder as they both fell to the ground. This indicated that what my uncle said about a ball staying on the floor of a free falling elevator was likely correct. However, when I researched the subject on the Internet, I came across many articles that said that a person would float in a free falling elevator. So I am a bit confused. Why would a person float but a ball would not? So my question is: 1) Would a person stand on the floor of a free falling elevator or would he float above the floor in a free falling elevator? 2) If the person would stand on the floor of a free falling elevator, please explain why. If the person would float above the floor, please explain why. Replies: Since everything falls at the same rate in the absence of other forces that could intervene, (such as friction from air) there would be no difference between a ball and a human being in a free-falling elevator. Now if you were unfortunate enough to be riding in such an elevator and you happened to have a ball with you, you could release it at waste height and it would not fall to the floor. That's because both you and the ball, and everything else in the elevator, are already falling under the full effect of gravity even though you appear to be stationary from your own point of view. For the same reason, if you stepped off a stool while falling in the elevator, you would not come back down towards the floor unless you were effected by another force, for as long as the elevator remained in free fall. Jerry Gardner This is an example of reality versus theory. If the elevator accelerates only due to gravity, and if the person does not exert a force on the elevator (e.g. does not jump), then the person will stay on the floor of the elevator. The elevator and the person accelerate with gravity (which means they accelerate at the same rate), and therefore, they fall together. When two touching objects accelerate at exactly the same rate, they stay touching -- thus a person on the floor stays on the floor. That is the theory. However, in reality, the elevator is likely not exactly 'free-falling'. Its acceleration is slightly slower than gravity because there is some friction with the walls, etc. In this case, the person (who is under the force of gravity), would feel a net force toward the elevator (and therefore stay connected to the floor). In either case, if the person were to jump (and if the elevator were truly in free-fall), then the person would appear to 'float' -- or (in the case of the real elevator) at least to fall very slowly compared to normal. Hope this helps, Burr Zimmerman Hi Kevin, Your Uncle is correct for the scenario as depicted. Please consider these alterations in the scenario: If the standing person in the elevator held the ball and released the ball during the free fall, then the both would remain where it was. The ball would appear as floating. The subject would still be standing on the floor. The ball and the person are falling as fast as its surroundings. Inertia holds them there. If the person overcomes inertia with a push off the floor with their toes, they too would appear to float. Space agencies use this principle to train personnel in weightlessness. Please do not try this at home! Peter E. Hughes, Ph.D. Milford, NH First of all, I commend you for making an experiment to test this out. That is a very good thing to do. If the elevator were truly falling freely (the folder in your experiment was not), it and the ball or person inside it would both accelerate downward equally. So a ball or person on the elevator floor initially would stay on the elevator floor during the drop. However, they would not be pressed against the floor the way they are when the elevator is stationary (or, for that matter, moving at constant velocity). So, the ever-so-slightest push off the floor would make the occupant rise from the floor and not return. I suppose that is what you mean by floating. br> Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming Kevin B., If the person just stands still, that person will move with the elevator. The person will stay in contact with the floor but will feel no pressure from the floor. If the person should very gently push (a common reflex action) on the elevator floor, then that person would slowly float to the top of the elevator. Gravity accelerates both you and the free-fall elevator together. If you and the elevator are moving together when the cables are cut, you and the elevator will continue to move together. Is something pushes a little up on you or down on the elevator, such as your feet against the floor, then you slow down a little and the elevator speeds up a little. You and the elevator are no longer moving together: the elevator is moving downward a little faster than you are. You and the elevator floor move apart. Dr. Ken Mellendorf Physics Instructor Illinois Central College For your experiment, consider the effect of air resistance. For the elevator, assume it is "falling" in the vacuum of space (no air resistance). Now the person and the elevator fall at the same rate. There is nothing to make the person (or the ball) rise above the floor of the elevator since they started falling at the same rate and at the same time. However, there is nothing to hold the person (or the ball) to the floor of the elevator so a VERY small push off the floor would lift the person above the floor until the elevator stopped accelerating in free fall. Greg Bradburn Kevin, There are two issues: “free falling” and “floating.” Sometimes you can be arguing over an idea when the problem is the impreciseness of the words in your argument. What is “free falling”? Drop a feather and a rock at the same height. You might say both are “free falling,” but clearly the rock will drop at a faster rate because the drag in the air will push back on the feather and slow it. A falling elevator will most likely also experience some drag—as it falls in a shaft, it will have to push air in the shaft away; any friction from bearings or cables will also serve as a drag. Such drag will still provide small forces that keep an object “stuck” to the floor, the same as the ball on your folder. So the first problem is in the definition of “free falling.” Let us say we can eliminate all friction and drag forces from the falling elevator—we will call this “free falling.” (to be more precise, we could talk about matching the elevator’s acceleration to exactly that of earth, but we probably do not need to go so far at this point.) Now, will a ball, or a person, “float”? What do you mean by “float”? All objects “free fall” at the same rate—a ball or a person (or an elevator). Would you expect that an object would float away from the floor? Without any force on the object, there is no reason for an object an the elevator to move away from each other—they are both falling in lock step with each other. It is like two racing cars accelerating at the same rate: they stay neck and neck. Now, if there is a force—however small—that acts on the object (for example, a person pushing away from the floor), the object will move away and appear to “float.” If you sneeze in a free-falling elevator, you might be surprised that your feet are no longer on the floor. Minus, any forces however, no reason exists for a person to move away from the floor. Kyle J Bunch Click here to return to the Physics Archives

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