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

Question

 

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

Answer 1

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.

v_f = v_o  + 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.