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shwatwat
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Homework Statement
A jetliner touches down at 270 km/h. The plane then decelerates (i.e., undergoes acceleration directed opposite its velocity) at 4.5 m/s2. What’s the minimum runway length on which this aircraft can land?
Homework Equations
v2fx = v2ix + 2axΔx
The Attempt at a Solution
i got the solution doing this:
(270km/hr)(1000m/km)(hr/3600s) = 75m/s
0 = (75m/s)2 + 2(-4.5m/s2)Δx
(-9m/s)Δx = 5625m2/s2
Δx = 625m
which is the correct answer.
however, i also tried to get the answer by converting everything to km/h, and i couldn't get the correct answer. i can't figure out why, and it's driving me crazy. here's what i tried:
0 = (270km/hr)2 + 2(-4.5 m/s2)Δx
9m/s2Δx = 72900km2/hr2
and then an aside for unit conversion:
(9m/s2)(km/1000m)(3600s2/hr2) = 32.4km/hr2
i plugged that into get
32.4km/hr2Δx = 72900km2/hr2
Δx = 2250 km, which is much, much more than the 625 m that i got above.
what am i doing wrong?!?
thanks for the help, i really appreciate it
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