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Homework Help: Free-fall acceleration problem

  1. Feb 2, 2010 #1
    hello there, i had a question about a free fall question.

    Problem states that a stone is thrown vertically downward with an initial
    speed of 12.0 m/s from some tower.

    well, i was wondering if i should use 12.0m/s or if i should use gravity (9.8 m/s)

    will the rock have an initial speed of 12.0m/s then eventually become 9.8 m/s
    or will it be 12.0 m/s until the rock hits the ground.

    (all my book examples were throwing something upwards which is manageable but
    going downward gots me stumped.)

  2. jcsd
  3. Feb 2, 2010 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Hi bryans1mic, welcome to PF!

    Be careful not to confuse velocity (m/s) with acceleration (m/s2). Objects in free fall (i.e. under the influence of gravity *only*) will accelerate downward at 9.8 m/s2. What this means is that although the object will start out with a velocity (initially) of 12.0 m/s downward, gravity will accelerate the stone so that its downward velocity increases (that's what acceleration is...the rate of change of velocity). Its velocity will increase by 9.8 m/s PER second. That's where the unit of m/s2 for acceleration comes from.
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