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Homework Help: Acceleration calculation help

  1. Nov 8, 2005 #1
    A stone is dropped from a height of 32.4m above the ground. Calculate the velocity of this object when it reaches the ground.
    the step by step ans key shows that acceleration is -9.81m/s^2. y is it neg? is it bc the object is going downwards(neg direction)? if that's the case, then y in this question pos acceleration is used?:
    A large steel ball is dropped from a height of 7.00m above the floor. What is the velocity that the object will strike the floor?:confused: :confused:
  2. jcsd
  3. Nov 8, 2005 #2

    Chi Meson

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    By convention, the direction that is downward is negative. Therefore the acceleration is negative, not positive. After falling a certain distance, the velocity will be great, but also in the downward direction, therefore the velocity will be negative.
    Negative acceleration does not always mean "slowing down." If your velocity and accelration are in the same direction (have the same sign) then you speed up; if the directions of velocity and accelration are opposite then you slow down.

    FOr both problems, you assume that the initial velocity is zero. Do you have some equations that you have used so far?
  4. Nov 8, 2005 #3
    A stone is thrown vertically upwar with an initial velocity of 25.2m/s. Calculate the max displacement (height) of this stone. the ans key shows that the following is given in the prob: acceleration of neg 9.81m/s^s, 0 velocity final and the initial velocity of 25.2
    and this is what's needed to be found: distance
    this isn't needed:time

    this is where i'm having problems: why is acceleration neg? doesn't acceleration tell direction?like up or down = +or- respectively?
  5. Nov 8, 2005 #4
    Acceleration doesn't denote direction, it is simply the change in velocity. So if the velocity of your stone is upward, it is slowing down due to the acceleration of gravity until the velocity is zero, at which point it returns to earth. For these questions you can use the formulae derived specifically for constant acceleration situations, to find the distance you would want:

    vf^2 = vi^2 + 2ad
  6. Nov 9, 2005 #5

    Chi Meson

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    As Tubs said, acceleration is not velocity, but rate of change in velocity.

    If the object is going up, it is slowing down due to gravity, right? What is happening to velocity: it is getting LESS positive, that's a negative change, hterefore negative acceleration.

    IF an object is falling down, it is speeding up right? THe velocity is getting MORE negative, that is also a negative change, therefore negative acceleration.

    IF an object is falling. but a parachute suddenly pops open. NOw it is slowing down. As it slows, velocity is getting LESS negative. That's a double negative, which means a positive acceleration. Follow that?
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