This problem is mentally retarded. Please help please.

S

shazam

Newcomer
1. The problem statement, all variables and given/known data

A person is standing on a bathroom scale. Which of the following is not a force exerted on the scale?

A) A contact force due to the floor
B) A contact force due to the person's feet
C) The weight of the person
D) The weight of the scale


3. The attempt at a solution

They are check boxes. I have tried A. I have tried D. I have tried both A and D. None of those answer combinations are right.
 
Hint: this is not just a physics queston as such. It is a semantics question; you will need to look up the precise scientific definition of "weight" in order to find the correct answer.

Unless, of course, you just brute-force the damn thing by trying all 16 possible checkbox combinations in order.
 
This question all comes down to the following words: "On the scale." And Arjan's right: look up what weight means in physics terms.
 
weight = vertical force exerted by a mass as a result of gravity. So as I see it, only C counts and the rest doesn't.

BTW, what should a "contact" force mean here? I know of no such thing. Is that supposed to be the result of friction and normal force? The normal force would be the person's weight in this case, friction is irrelevant since it's all static and vertical.
 
_xxx_ said:
weight = vertical force exerted by a mass as a result of gravity. So as I see it, only C counts and the rest doesn't.
Click to expand...
Weight is the vertical force exerted on a mass as a result of gravity, not by a mass.

_xxx_ said:
BTW, what should a "contact" force mean here? I know of no such thing. Is that supposed to be the result of friction and normal force? The normal force would be the person's weight in this case, friction is irrelevant since it's all static and vertical.
Click to expand...
Contact force includes both the normal force and the frictional force.
 
_xxx_ said:
Talk to Wiki about the semantics :)
Click to expand...
Heh, well, they get it right. I rather like their quote from the National Standard of Canada:

The National Standard of Canada, CAN/CSA-Z234.1-89 Canadian Metric Practice Guide, January 1989:5.7.3 Considerable confusion exists in the use of the term "weight." In commercial and everyday use, the term "weight" nearly always means mass. In science and technology "weight" has primarily meant a force due to gravity. In scientific and technical work, the term "weight" should be replaced by the term "mass" or "force," depending on the application. 5.7.4 The use of the verb "to weigh" meaning "to determine the mass of," e.g., "I weighed this object and determined its mass to be 5 kg," is correct.
Click to expand...

Because it's a force due to gravity, the weight of an object is the force due to gravity on that object. Though it's worth going into some detail, as the Wikipedia article on weight does, as to the difference between weight and apparent weight.
 
I quoted that from Wiki, that's what I'm trying to explain... :)

But back to topic, what is the correct solution now?
 
Eh, might as well. The correct answer is C because the weight of the person is the force of gravity on that person. It is a genuine force, it's just not a force that's acting on the scale.

The forces acting on the scale are the force of the floor pushing up on it (the contact force, A), the force of the person's feet pushing down on it (B), and the force due to gravity on the scale (D).
 
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