WEBVTT
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This is Giancoli Answers
with Mr. Dychko.
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This garden hose sprays water straight up
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and it's positioned 1.8 meters
above the ground
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and when the water launches from here,
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it hits the ground 2.5 seconds later.
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So the displacement of this water is
gonna be negative 1.8 meters
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if we choose upwards to be
the position direction
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and whenever in doubt about this
negative or positive
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for displacement, you can always think of
displacement as being the final position
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minus the initial position; typically,
the letter *y* is used for this,
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you know, the *y*-axis being vertical
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so *y final* minus *y initial* and we have
*y final* in this case being
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zero if this is a zero level here
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and then minus the initial positionâ€”
minus 1.8 metersâ€”above that
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giving us negative 1.8 meters
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so displacement is negative 1.8.
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We'll use this formula to solve
for the initial speed
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and we'll subtract one-half *at squared*
from both sides
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and we get this next line after we
switch the sides around
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so that the unknown *v i* is on the left
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and then divide both sides by *t*
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and we get this formula for
the initial speed.
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So we plug in our numbers;
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it's negative 1.8 meters displacement
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minus one and a half times
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negative 9.8 meters per second squared
acceleration due to gravity
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times 2.5 seconds square that
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and divide by 2.5 for a total of
12 meters per second
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it must have been the initial
speed of the water.