Arithmetic · real student question

Use dimensional analysis to convert 92 feet per minute to meters per second.

Question

Use dimensional analysis to convert

92 ftmin92\ \frac{\text{ft}}{\text{min}}

to meters per second.

Step-by-step solution

  1. List the two conversion factors needed. The quantity has a length unit on top and a time unit underneath, so both must be replaced:

    1 ft=0.3048 m(exact by definition),1 min=60 s.1\ \text{ft}=0.3048\ \text{m}\qquad\text{(exact by definition)},\qquad 1\ \text{min}=60\ \text{s}.

  2. Write each factor as a fraction equal to 11, oriented so the unwanted unit cancels. Feet must cancel from the numerator, and minutes must cancel from the denominator:

    92 ftmin×0.3048 m1 ft×1 min60 s.92\ \frac{\cancel{\text{ft}}}{\cancel{\text{min}}}\times\frac{0.3048\ \text{m}}{1\ \cancel{\text{ft}}}\times\frac{1\ \cancel{\text{min}}}{60\ \text{s}}.

    Choosing the orientation by cancellation, rather than by guessing whether to multiply or divide, is the whole point of dimensional analysis.

  3. Multiply the numerators.

    92×0.3048=28.0416 m per minute.92\times 0.3048=28.0416\ \text{m per minute}.

    This intermediate value is a legitimate answer in m/min and worth keeping as a checkpoint.

  4. Divide by the 60 seconds in a minute.

    28.041660=0.46736 ms.\frac{28.0416}{60}=0.46736\ \frac{\text{m}}{\text{s}}.

  5. Sanity-check the magnitude. A metre is about 3.283.28 ft, so 9292 ft/min is roughly 2828 m/min, and dividing by 6060 should land just under half a metre per second — which it does. Reversing the calculation, 0.46736×60/0.3048=92.0000.46736\times 60/0.3048=92.000 ft/min exactly.

Answer

92 ft/min=0.46736 m/s92\ \text{ft/min}=0.46736\ \text{m/s}

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