WEBVTT
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This is College Physics Answers
with Shaun Dychko
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We begin this question by writing down
the information we've been given.
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We know that the baseball stops
in a period of 1.85 milliseconds
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and we'll convert this
milliseconds into seconds
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because it's a nice...
nice time at the beginning of a question
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when you're writing down data to take
care of any unit conversion issues
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and we know that our formulas require
meters, kilograms and seconds.
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We're always working in MKS units
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and so this is milliseconds,
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which means we have
to change it into seconds.
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And milli is the metric prefix
meaning times 10 to the minus 3.
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And so, we put a 'times 10 to the minus 3'
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in place of the milli prefix.
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And so, now, we have 1.85 times
10 to the minus 3 seconds.
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The deceleration is 2.1 times
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10 to the 4 meters per second squared.
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And I wrote a negative there because
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this deceleration is opposing
the initially positive velocity,
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which is all kind of made up in a way.
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You can decide what your
coordinate system is going to be.
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Let's assume that to the right is positive
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and that's... So, that's the
direction of the initial velocity.
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And since we've made that coordinate
system positive to the right
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and we know that the acceleration
is opposing this initial velocity
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because it has continued to slow down.
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That means the acceleration is directed to
the left and it is therefore a negative.
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Okay. And final velocity is 0
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because the ball comes
to a stop in this bit.
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And we have this formula for final
velocity equals *v naught* plus *at*
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and we'll subtract acceleration
times time from both sides
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making it disappear from the right
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and having it appear on the left.
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So, v minus *at* equals *v naught*
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and we'll switch the sides around.
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And so, we have our initial velocity.
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It is the final velocity minus
acceleration times time.
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And then, substituting in our numbers,
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we have 0 minus negative 2.1 times
10 to the 4 meters per second squared
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times 1.85 times 10 to
the minus 3 seconds,
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giving us an answer of
38.9 meters per second,
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must have been the initial velocity,
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which is about 140 kilometers an hour.