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How many seconds will it take to drop below 8 meters per second?

 
A motor runs at 65 meters per second, and its speed is decreased by 10 meters per second every second.How many seconds will it take to drop below 8 meters per second?
 
 
A.6 seconds

B.5 seconds

C.8 seconds

D.7 seconds
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1 Answer

You could just subtract 10 over and over until it hits 8 and count the seconds.
65,    55, 45, 35, 25, 15,  5
Start   1    2    3    4    5   6
 
Or you could use a physics formula.
vf = vo  + at
8 m/s = 65 m/s + (10 m/s/s ) t
8 m/s - 65 m/s = (10 m/s/s ) t
 
8 m/s - 65 m/s  = 5.7 s
   10 m/s/s
 
So 6 s must be the answer since they only have whole seconds
and after 5 it will still be above 8 but after 6 it will be below.

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