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One of my friends has a problem and we don't know how to get this done,

We want to to know the mass of a patient who is laying on a bed. One scales was put under the foot of a bed and weighed 232kg and the other under the head of the bed and weighed 220kg. The centre mass of the patient is 0.8 meters from the foot of the bed. What is the mass of the patient?

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The problem is formulated very oddly. "under the foot of the bed". A bed usually has four of them. Next: " under the head", what is that? One of the posts at the head end? What about the other posts when one is on the scale? Does one of the other posts "float" ? – Georg Mar 7 '11 at 10:49
@Georg: i have reworded the question. – omeid Mar 7 '11 at 12:08
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Still not clear: are those scales under the posts at the same time or in consecution? And are they placed under posts on the same side of the bed or diametrally? What about symmetry of bed/patient? – Georg Mar 7 '11 at 12:35
Constitutive and diametrically. – omeid Mar 8 '11 at 8:44

1 Answer

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Dear Omeid, this is a homework problem, right? I think we shouldn't be answering the question "explicitly".

But let me mention a hint: if you assume that the data are enough to calculate a unique answer, do you think that the information about the length of the bed may be relevant? You want to get a result in kilograms - and there are two pieces of information in kilograms and one in centimeters. To get a result in kilograms, may the figure involving centimeters matter if you require that the result has the right units?

This argument is called "dimensional analysis".

Now, imagine that you put both scales under the bed simultaneously: one of them is under the foot and one of them is under the head - at the same moment. How much do you think that those scales will show in this case? How can you determine the total ;-) mass of the bed using this measurement?

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Thanks alot for your answer Lubos, but this is not homework cause I haven't studied this kind of problems and i even don't know what they are called, can you please tell me what rules and formulas will help me with the problem, I might work it out. – omeid Mar 7 '11 at 9:44

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