1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Acceleration of Gravity

  1. Sep 21, 2012 #1
    1. The problem statement, all variables and given/known data
    A box falls from an elevator that is ascending with a velocity of 2 m/s. It strikes the bottom of the elevator shaft in 3 seconds. A) How long will it take the box to reach its maximum height?
    B) How far from the bottom of the shaft was the box when it fell off the elevator? C) What is the height of the elevator when the box is at its highest point?



    2. Relevant equations
    v(t) = v0 + at
    x(t) x0+v0t+1/2at2
    v2-v02=2aΔx



    3. The attempt at a solution

    I know that the elevator will reach a maximum height when the velocity is 0. So v = 0, v0 = 2 m/s, a = 9.8 m/s-2, t=3 sec.

    For A) (the one I'm concerned with for now), I set it up as V-V0/a = t. Plugging everything in, t = 0 - (2 m/s)/9.8 m/s-2 to get an answer of -.204 s. I know that must not be right as I don't believe t can be negative. However, since the box is falling off the elevator, shouldn't it be measured as a positive value since it's falling down?
     
  2. jcsd
  3. Sep 21, 2012 #2
    In which direction is the acceleration?
     
  4. Sep 22, 2012 #3
    In this problem, acceleration is defined as a constant, which would be gravity measuring out to be 9.8 m/s2. I know that if an object is moving "up", gravity is typically negative in that situation. However, since the box is falling off the elevator and moving towards gravity, it should have a positive value, right?
     
  5. Sep 22, 2012 #4
    OK, suppose you throw a ball up in the air. Is the sign of the acceleration negative while it is moving upwards and positive when it is moving downwards then?
     
  6. Sep 22, 2012 #5
    Oh I did not think about it in that context, so I know that the ball's velocity decreases as it's tossed up and then reaches a max velocity of zero and then it's velocity increases as it falls back to earth, which would represent a positive and increase in acceleration.
     
  7. Sep 22, 2012 #6
    For A) (the one I'm concerned with for now), I set it up as V-V0/a = t. Plugging everything in, t = 0 - (2 m/s)/9.8 m/s-2 to get an answer of -.204 s. I know that must not be right as I don't believe t can be negative. However, since the box is falling off the elevator, shouldn't it be measured as a positive value since it's falling down.
    ---------------------------------------------
    You are taking upward as positive as given to v.
    The gravity is pointing downward.
    So it must be opposite sign to v.

    You can assign a as positive and it will make v of negative sign.
     
  8. Sep 22, 2012 #7
    Well, let us take this carefully.
    1. The ball loses speed as it moves up. Thus the acceleration is in the opposite direction of the velocity, i.e. downwards. (Think of it as vectors.)
    2. The ball reaches a max height, where it has a minimum speed of zero. The acceleration continues to point in the same direction, having the same value. (Gravitation continues to exist even if you don't move.)
    3. The ball starts to move down, with increasing speed. Thus the acceleration is in the same direction as the velocity, i.e. downwards.

    Basically, the gravitation always points towards the ground. What sign does it have? It depends on your coordinate system. Say that your axis points upwards (as you have, since you say that upwards velocities are positive), then the gravitational acceleration is indeed a=-g.
     
  9. Sep 23, 2012 #8
    Ok, got it! So just all depends on how your choose to set up your coordinate system and keeping it consistent. That makes sense, I must not have been had a consistent system in that case. Thanks a lot!

     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Acceleration of Gravity
  1. Gravity and acceleration (Replies: 11)

Loading...