Physics Review Question

Unit: electricity and magnetism
Year: 1992 Question#: 31
Question: An electric motor uses 15 amperes of current in a 440-volt circuit to raise an elevator weighing 11,000 Newtons. What is the average speed attained by the elevator?

(1 ) 0.0027 m/s
(2 ) 0.60 m/s
(3 ) 27 m/s
(4 ) 6,000 m/s

What is this question really asking?




Explanation by: Christine A

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Answer 1
0.0027 m/s

INCORRECT

When solving a problem such as this, always write out everything you know:
I=15 Amperes
V=440 Volts
F=11,000 Newtons
Now, looking at the information you have, the best formulas to use will be two variations on the power formula:
P=VI
P=Fv
Since all variations are equal, you can set the two variations equal to each other:
VI=Fv
Then, all that is left to do is to plug in your numbers and solve for the unknown variable:
VI=Fv
(15A)(440 V)=(11,000 N)(v)
6600 Watts=(11,000 N)(v)
.6 m/s=v
Thus, the average speed attained by the elevator is .6 m/s. This is not what is written for answer 1, so answer 1 must be incorrect. return to top























Answer 2
0.6 m/s

CORRECT
When solving a problem such as this, always write out everything you know:
I=15 Amperes
V=440 Volts
F=11,000 Newtons
Now, looking at the information you have, the best formulas to use will be two variations on the power formula:
P=VI
P=Fv
Since all variations are equal, you can set the two variations equal to each other:
VI=Fv
Then, all that is left to do is to plug in your numbers and solve for the unknown variable:
VI=Fv
(15A)(440 V)=(11,000 N)(v)
6600 Watts=(11,000 N)(v)
  1. 6 m/s=v
Thus, the average speed attained by the elevator is .6 m/s. This is the answer given for choice 2, so answer 2 must be correct. return to top

Answer 3
27 m/s

INCORRECT
When solving a problem such as this, always write out everything you know:
I=15 Amperes
V=440 Volts
F=11,000 Newtons
Now, looking at the information you have, the best formulas to use will be two variations on the power formula:
P=VI
P=Fv
Since all variations are equal, you can set the two variations equal to each other:
VI=Fv
Then, all that is left to do is to plug in your numbers and solve for the unknown variable:
VI=Fv
(15A)(440 V)=(11,000 N)(v)
6600 Watts=(11,000 N)(v)
  1. 6 m/s=v
Thus, the average speed attained by the elevator is 0.6 m/s. This is not the answer given, so choice 3 must be incorrect. return to top

Answer 4
6,000 m/s

INCORRECT
When solving a problem such as this, always write out everything you know:
I=15 Amperes
V=440 Volts
F=11,000 Newtons
Now, looking at the information you have, the best formulas to use will be two variations on the power formula:
P=VI
P=Fv
Since all variations are equal, you can set the two variations equal to each other:
VI=Fv
Then, all that is left to do is to plug in your numbers and solve for the unknown variable:
VI=Fv
(15A)(440 V)=(11,000 N)(v)
6600 Watts=(11,000 N)(v)
  1. 6 m/s=v
Thus, the average speed attained by the elevator is 0.6 m/s. This is not the answer give, so choice 4 must be incorrect. return to top

What's this question really asking?

Do you know how to deal with power formulas and their applications? return to top

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