Answer: 130 m/s^2
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Work Shown:
Force = Mass*Acceleration
F = m*a
18.9 = 0.145*a
0.145*a = 18.9
a = 18.9/0.145
a = 130.344827586207
a = 130
I rounded to three sig figs because the given values are accurate to three sig figs. The units of acceleration are m/s^2.
Saying 130 m/s^2 means that every second, the baseball's velocity is increasing by 130 m/s. You can think of the velocities taking on multiples of 130 like so {130, 260, 390, 520, ...} though we're dealing with a continuous set of values rather than discrete set.
The acceleration is really large because the ball is going very fast in a short amount of time. For example, fast ball pitches can be 100 miles per hour, and the ball goes from 0 to 100 in a very short amount of time (less than 1 second).