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Height and freefalling question

  1. Mar 8, 2016 #1
    There was a problem with other physics homework section so I just decided to write here.
    1. The problem statement, all variables and given/known data
    H= 270m t1= 2.45s t3=7.35 V0=0 (freefall)
    t=7.35s t2= 4.90s g=10
    We had to calculate the height what the body will be after 2.45, 4.90 and 7.35 s.
    2. Relevant equations
    h=gt2/2

    3. The attempt at a solution
    h1=30 h2= 120 h3= 270

    So this is what it was supposed to look like. But, the answers in the book say that h1=30 h2=90 and h3= 150. Our teacher told us that it might be a mistake in the task by the authors. But he said that if we find a solution to the books answers then he will give the person +2 on the test ( this was a test task btw). So i could not think of anything since it does not make sense. I tried looking on the internet and i found this forum. If you have any ideas, please share them.
     
  2. jcsd
  3. Mar 8, 2016 #2

    gneill

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    Hi Gustavas, Welcome to Physics Forums.

    None of the answers make much sense. If the body is truly in freefall from rest (zero initial velocity) then its height should be decreasing with time.
     
  4. Mar 8, 2016 #3

    PeroK

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    If it's free falling then the height will decrease with time. You seem to have an object accelerating upwards from 0.
     
  5. Mar 8, 2016 #4
    Yea its supposed to be falling, and it is, maybe you got the wrong idea, cuz of what i wrote. We have to calculate where will it be ( how much would it fall) after the 3 same periods of time ( the period is 2.45 sec). So it falls 30 m the first 2.45 sec, 120m 4.90 s, and its 270 when 7.35 s. But the answers in the book say different ( 30,90,150). If the answers in the book are impossible if it was free falling then it is impossible. The teacher himself couldnt figure it out. We get bonus if we figure it out.
     
  6. Mar 8, 2016 #5

    PeroK

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    If your teacher can't figure that one out, there must be something you're not telling us! Figure what out? That the book has given the answer to a different question, perhaps.
     
  7. Mar 8, 2016 #6

    gneill

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    The given answers would make sense if the question asked for the distance covered by the falling body between the given instants of time. So for example from time zero to time 2.45 seconds the abject falls a distance of 30 meters.
     
  8. Mar 8, 2016 #7
    Figure out why it is 30, 90 , 150 in the book but with the formulas it is 30, 120, 270. I already told you that it might be impossible, just a mistake. I would show you but you are probably not latvian. And the answers are to that task. And also if the body was freefalling it would have acceleration, so it cannot be 30, 90, 150. I do not think that this kind of mistake would be in the book. But there might be some physics things to it. I dont really know thats why im asking.
     
  9. Mar 8, 2016 #8

    gneill

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    I disagree with your conclusion. My calculations show 30, 90, and 150 meters for the given spans. You'll have to prove me wrong :smile:

    Write an expression for the distance traveled with respect to time and find the difference between two given times.
     
  10. Mar 8, 2016 #9
    Can you show me how you calculated that please? This is my calculation: gt^2/2= 10x2.45^2/2 = 30, h2= 10x4.90^2=120 , h3= 10x7.35^2=270. How can it be 30 90 and 150 for you. There would be an acceleration and the differencebetween 30, 90 and 150 is 60 meters. Please explain. I could not understand what do i have to reply because english is not my native language. ''Write an expression for the distance traveled with respect to time and find the difference between two given times.''
     
  11. Mar 8, 2016 #10

    gneill

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    Write an equation for the distance with respect to time. Use that equation to find the total distance traveled (from rest) at each of the given times. What values do you find?
     
  12. Mar 8, 2016 #11
    It is going to be hard to understand for me, since im on the verge of failing physics. The equation you mean like Distance/Time for example m/t? I also might understand something from your calculation of this. If you do not want to share it and you have the time to explain then I will stick here too.
     
  13. Mar 8, 2016 #12

    gneill

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    If a body starts from rest with an acceleration a, what is an equation that express the distance with respect to time?
     
  14. Mar 8, 2016 #13
    is it a=deltaV/t or do you mean a=v-v0/t. Im sorry if im being a bother.
     
  15. Mar 8, 2016 #14

    gneill

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    You need to review your basic kinematic equations. What is the equation of motion for a body starting from rest with a given acceleration? d = ?
     
  16. Mar 8, 2016 #15
    oh you mean d=v0t+1/2at^2? or
     
  17. Mar 8, 2016 #16

    gneill

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    Yes.

    So if it starts from rest and has an acceleration of 10 m/s2, what are the distances at 0s, 2.45s, 4.90s, and 7.35s?
     
  18. Mar 8, 2016 #17
    yes i know now g=a dont answer that. 0 would be 270, 2.45 is ~30m, the next i dont understand. if a is 10m/s2 then d= 0x4.90+10x4.90^2/2= 120 and d=0x7.35+10x7.35^2/2 = 270
    https://gyazo.com/320ae67296b95a7ec9f69170093ccf51 :H:frown:. I can't make it 30,90,150
     
  19. Mar 8, 2016 #18

    gneill

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    You're saying that at time zero the object has traveled 270 m? How is that possible if it hasn't moved yet? At time zero it has traveled zero meters. We aren't discussing height anymore, so the initial height of 270 m is irrelevant for now.

    So your times and distances are:

    t0 = 0s ; d0 = 0 m
    t1 = 2.45 s ; d1 = 30 m
    t2 = 4.90 s ; d2 = 120 m
    t3 = 7.35 s ; d3 = 270 m

    Those are the total distances (displacements) from the starting point for each of those particular times. Now, what are the distances traveled between those instants of time? For example, what is the distance covered from time t2 to time t3?
     
  20. Mar 8, 2016 #19
    Omg i now understand, its 30, 90 ,150. Well i dont understand to the fullest i might use this thread in the future. Thanks you are a great help. I lack logic.
     
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