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Find the acceleration of an elevator

  • Thread starter patrilu5
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  • #1
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Homework Statement



Find the acceleration of an elevator going upwards and downwards if your initial mass is 45 kg, when its going downwards is 36 kg, and when its going upwards is 47 kg.

Homework Equations



F=ma

The Attempt at a Solution


W=45(9.8)
W=441 N

W=47(9.8)
W=460.6 N

T=m(a+9.8)
460.6=45(a+9.8)
460.6=45a+441
19.6=45a
0.436 m/s^2= upward acceleration

Is this right???
 

Answers and Replies

  • #2
1,331
45


I don't think this question makes sense. Your mass can't change in an elevator unless it's an elevator that can take bites out of you.
 
  • #3
390
1


I assume the question was meant to say 'weight' instead of mass, in which case it should be in newtons rather than kilograms?
 
  • #4
1,331
45
I assume the question was meant to say 'weight' instead of mass, in which case it should be in newtons rather than kilograms?
Yes, and in which case this would be a tiny, tiny person.
 
  • #5
NascentOxygen
Staff Emeritus
Science Advisor
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Yes, and in which case this would be a tiny, tiny person.
45 N humans do exist. I've seen them. Perfectly formed little people, with little faces, and little smiles, wearing specially made little boots to fit their little feet. Perfect in every detail, but just tiny, tiny all over. It's just like seeing minatures of real people, you could say.

On the other hand, the question does say "your" initial mass, and I'm quite sure none of these tiny people would be reading these maths questions.

Maybe it was meant to be initial weight of 45 kg wt, not an initial mass of 45 kg.
 
Last edited:
  • #6
1,331
45
45 kg people do exist. I've seen them. Perfectly formed little people, with little faces, and little smiles, wearing specially made little shoes to fit their little feet. Perfect in every detail, but just tiny, tiny all over. It's just like seeing minatures of the real thing, you could say.
And if the units are supposed to be newtons, which is the point... have you seen a person that weighed 45 newtons before? Maybe, but the umbilical cord was literally just cut.
 
  • #7
1,331
45


Maybe it was meant to be initial weight of 45 kg wt, not an initial mass of 45 kg.
If it was worded exactly like posted on a test, I'd honestly write "Trick question, your mass doesn't change due to acceleration." or some other statement.

In fact, I'd even be annoyed if it read "45 kg wt" because kilograms simply is not a unit of weight.
 
  • #8
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,070


If it was worded exactly like posted on a test, I'd honestly write "Trick question, your mass doesn't change due to acceleration." or some other statement.
If the ride was really scary, you could lose half a kilo by vomiting. But it's hard to reconcile that with the rest of the question ....

kilograms simply is not a unit of weight.
Yes, but some teachers don't know that. You've got to give them the answer they want if you want the marks. That's the first important lesson you learn at school. o:)

Though kg wt is a perfectly valid unit of weight
 

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