Speed and Distance

[msnoeberger]

How far can a car go traveling 30 mph in 4 seconds? I think I'm getting hung up on the mph vs seconds and converting that to feet....

Thanks in advance.

Matthew

 

[lookagain]

>
>Cancel out the respective units and multiply or divide, respectively, the numbers below:

\(\displaystyle \frac{30\ \ miles}{1 \ \ hour}\cdot \frac{5280\ \ feet}{1\ \ mile}\cdot\frac{1\ \ hour}{3600 \ seconds} = \ \ how \ \ many \ \ feet \ \ per\ \ second?\)

 

[Deleted member 4993]

A good "artificial" conversion factor to remember:

\(\displaystyle 15 \ \frac{miles}{hr} \ = \ 22 \frac{ft}{sec}\)

comes handy in many dynamics problem

 

[soroban]

\(\displaystyle \text{I prefer the equivalence: }\;60\text{ mi/hr} \:=\: 88\text{ ft/sec}\)


\(\displaystyle \text{Since }\frac{88}{60} \;\approx\;\frac{90}{60} \:=\:1\tfrac{1}{2}\)

. . \(\displaystyle \text{we can get a quick approximation of feet-per-second}\)
. . \(\displaystyle \text{by multiplying by one-and-a-half.}\)


\(\displaystyle \text{Example: }\,70\text{ mph}\)

\(\displaystyle 70\text{ mph} \quad \approx \quad 70 + \text{(half of 70)} \;\;=\;\;70 + 35 \;\;=\;\;105\text{ fps.}\)