Originally posted by wikid_one: So my roommate asked me to look at a problem he was assigned, and for the life of me, I can't get it to make any sense. The problem is:
The upward normal force exerted by the floor is 620N on an elevator passenger who weighs 650N. What is the magnitude of acceleration?
The more I look at this, the more I think there isn't enough information given. Am I missing something? This is driving me crazy.
In the most simplistic Newtonian terms, Net F = ma. Net F = - 30N. m= mass of the passenger = 650N / (presumably) g.
(It's a poorly formulated question, one would normally provide the mass, not the "weight" of the passenger, as "weight" requires one to make an assumption regarding the environment.) If the elevator is close to the surface of the Earth, "g" is g (the accepted value for the acceleration due to Earth's gravity, near the surface.) Otherwise, you're on your own. Ignore the "minus" (the question asks for "magnitude", not direction), make a silly assumption, and plug two equations.
Since the questioner wants you to ignore frame-of-reference questions, you're probably good with ignoring coriolis effects, too, so you're off the hook there.
But it's still a poorly worded question, and no physicist would ask the question that way.
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12:06 AM
ryan.hess Member
Posts: 20784 From: Orlando, FL Registered: Dec 2002
At least its not like the stupid problem that was going around the forums a year or two ago about "if a plane was on a conveyor belt that was moving backwards, could the plane still take off" or some foolishness like that.
Tried a rubber band powered airplane on a highspeed conveyor belt myself, with the linear movement of the belt (Ft per min) running much much faster than the plane was ever capable of moving. Plane took off without a hitch. Wound the rubber band only half as tight the next time and it still went airborne with no problem.
I never did see what the big problem was understanding this.
[This message has been edited by maryjane (edited 01-29-2009).]
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08:32 AM
wikid_one Member
Posts: 2838 From: Ocean City, MD Registered: Dec 2003
I guess I was just overthinking it. I looked at it, and saw the downward force was larger than the normal force, so that means that there have to be two different accelerations. How do we know that the elevator is in freefall? Nothing in the question leads me to believe that the downward acceleration is 9.8 m/s2. Also, since the downward force is more than the normal force, that implies that the acceleration is downward, and thus the acceleration down should be greater. I started writing out my equations
Fn=620N=m*a1 Fg=650N=m*a2
I then looked at it and went... "hmmm. too many unknowns." And then, I confused myself.
The downward acceleration due to GRAVITY ONLY is approximately 9.81m/s at sea level. It changes at different altitudes, but typically the change does not show up in the three significant figures used. So GENERALLY, 9.81 m/s is used as the acceleration due to gravity in all general physics problems. It is, at least, a good first approximation.
