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Homework Help: Rectilinear Motion Help please

  1. Oct 27, 2007 #1
    Hi, I recently stumbled across your forums whilst trying to find help for my homework. I have managed to do all the questions on the homework except these 2, and i cannot seem to get the right answers to them. This is because they both miss one piece of data and i would normally use the suvat equations to find the answer.

    1. The problem statement, all variables and given/known data

    A body moving with uniform acceleration covers 64.4m in 7 seconds. And 110m in 10 seconds.

    Find acceleration and initial velocity.


    2. Relevant equations

    Suvat / equations of motion

    3. The attempt at a solution

    (110 - 64.4 ) / (10- 3)

    The only other thing i could think of would be adding them and trying to work it out.. but as i dont know the start velocity i cant. ( 174.4 / 17)


    Question 2

    1. The problem statement, all variables and given/known data

    A body released from rest falls through 44.1m during the fifth second of its motion find the acceleration due to gravity.

    u= 0, s(5) - s (4) = 44.1m

    Sorry theres not much working out from me, Im just new to physics (started it about a month ago) and asides from filling in data into the suvat equations i really have no idea what im doing. could somebody point me in the right direction , thankyou
     
  2. jcsd
  3. Oct 27, 2007 #2

    learningphysics

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    Homework Helper

    For both problems, the equation you need is:

    d = v1*t + (1/2)at^2

    for the first problem... plug in your 2 sets of values... d = 64.4 t=7 and d= 110 t= 10

    2 equations.... 2 unknowns... you can solve both v1 and a.

    For the second problem, again try to use

    d = v1*t + (1/2)at^2
     
  4. Oct 27, 2007 #3
    Is the equation not d= v*t - (1/2) at^2?


    Because i used that and got 12.6 = 59.5a So a = 0.2 But the answer says a = 1.2

    But thanks for your help, maybe i did it wrong ill try again


    EDIT EDIT EDIT: WEWT. Ignore me i did a calculation wrong. :) Now all i gotta do is solve the second one

    ty btw i didnt realise i could just turn it into simultaneous equations
     
    Last edited: Oct 27, 2007
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