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Height Question: Projectile launched from a table

  1. Oct 15, 2016 #1
    1. The problem statement, all variables and given/known data
    A toy car rolls off of a table at an initial angle of 0 degrees and an initial velocity of 2.81 m/sec. The ball hits the floor 1.55 m (horizontally) from the edge of this table. How tall is the table?

    2. Relevant equations


    3. The attempt at a solution
    So far I have:
    Vo=(2.81 m/sec) * cos(0 degrees)=2.81 m/sec
    Delta x= vot + one half at^2
    1.55m=(2.81 m/sec) times t plus one half times (0) times t^2
    t=1.55 m/2.81 m/sec=0.55 sec
    Vo=(2.81 m/sec) times sin(0 degrees)=0 m/sec
    Delta x=vot plus one half at ^2
    Delta x= (0 m/sec) times (0.55) plus one half times (-9.8 m/sec) times (0.55 sec)^2=

    The result I ended up getting was -1.48225 when I was done; I feel I know I must have made a mistake at some point, as the answer shouldn't have turned up negative. Someone I asked briefly on another site came up with the answer 1.488m for the problem; I'm just not quite sure what I got off. Could someone help me figure this one out please? I feel I'm close, but off somewhat. Thanks for the help & the patience, guys.
    Really do want to learn, and whatever help you can give me in making sure I get this problem right would be very appreciated. :)
     
  2. jcsd
  3. Oct 15, 2016 #2

    haruspex

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    The formula you used for the vertical motion gives you the displacement at time t. You took up as positive, making g negative. In falling to the floor, would you expect the displacement to be positive or negative?
     
  4. Oct 17, 2016 #3
    Negative? I feel kind of ridiculous right now, but I still don't quite understand.... Do you think you could tell me what lines in my data are off and what exactly in them I need to change? I think that would help me.
     
  5. Oct 17, 2016 #4
    Was it the -9.8 in the problem that I got wrong? I just noticed that when I make that positive instead of negative, I get the positive answer.
     
  6. Oct 17, 2016 #5

    haruspex

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    You have to decide whether to make up positive or down positive. It does not matter so long as you are consistent. If up is positive then both the gravitational acceleration and the displacement are negative.
     
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