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Motion w/ Constant Acceleration

  1. Sep 22, 2015 #1
    1. The problem statement, all variables and given/known data
    "A jet plane is cruising at 300 m/s when suddenly the pilot turns the engines up to full throttle. After traveling 4.0 km, the jet is moving with a speed of 400 m/s."
    2. Relevant equations
    "What is the jet's acceleration, assuming it to be a constant acceleration?"
    3. The attempt at a solution

    The way I see it, the only info we have is v-initial = 300 m/s , v-final = 400 m/s , and delta-x = 4000 m.

    To find acceleration I would need delta-t. All the equations that I'm cognizant of seem to include both acceleration and delta-t, both variables that are not given. It seems like I would need the acceleration to find delta-t. I also seems I would need delta-t to find acceleration.

    This seems like a simple problem, but I don't even know how to approach it. So far this semester I've been teaching myself, because our homework and quizzes are at least 1 chapter ahead of our lectures, which means our prof. isn't teaching us jack sh!t, just (inadequately) reviewing material that we've already taught ourselves to do the homework and quizzes. This class is really making me hate physics, which I formerly loved and wanted to major in. :(

    What should my methodology be from the get-go?
     
  2. jcsd
  3. Sep 22, 2015 #2

    NascentOxygen

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    Are you sure that you haven't learnt a formula involving vi, vf, and displacement? If you don't recall such, look over the chapter in your textbook.

    Alternatively, you could involve displacement by using an equation you already know which does involve displacement, though via this route you'll need to solve a pair of simultaneous equations.
     
  4. Sep 22, 2015 #3

    SteamKing

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    Then you need to find some different equations.

    Do a little research on the web if your textbook is inadequate.

    Here is a list of various equations of motion:

    https://sentynel.com/media/old/equations.html
     
  5. Sep 22, 2015 #4
    well yes, there is [s-final = s-initial = v*delta-t] , but there is delta-t, and only one term for v. I tried using this equation, but to solve for delta-t, I would only be using one term of velocity, what value would I use?

    There is also [s-final = s-initial + v-initial*delta-t + (1/2)a(delta-t)^2], but once again, there is delta-t AND a, which are unknown, and only one value of v.

    My assumption is that I need to find delta-t to do anything here...but I don't know how to incorporate a difference of velocities into an equation requiring only 1 velocity term.
     
  6. Sep 22, 2015 #5

    NascentOxygen

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    Solving two equations simultaneously means you can start out with two unknowns. You then end up determining the values of either of those unknowns, or both if need be, by following the method you learnt in maths for solving simultaneous equations.
     
  7. Sep 22, 2015 #6
    I'm not sure what you're alluding to here, it's a little vague...My algebra isn't very good, so I'm not sure what method you're referring to that will let me solve for the variables I need without needing more information than I have.
     
  8. Sep 22, 2015 #7

    SteamKing

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    You've got the right equations here, but let's re-write them into a form which is more easily readable:

    The first equation:

    vf = vi + at

    and the second:

    sf = si + vit + (1/2)at2

    If you look at the first equation, you can still solve for t even if a remains unknown for the moment.

    The second equation can be simplified a bit by taking si = 0. You should be able to substitute t from solving the first equation into the second to find a.

    You have sufficient information to solve this problem.

    You apparently didn't bother to search the list of motion equations which was linked to, but the one you need can be derived as described above.

    A suggestion: If you plan on studying physics to any degree, you need to get better at algebra (and more). If your math is dodgy, then this will keep you from solving even the simplest problems (like this one), let alone more complex problems.
     
  9. Sep 22, 2015 #8
    How can you solve for t without a? by substituting the values for vel., I get [100 m/s = at] or [(100/t) = a]. Either way I can't solve t or a.
    I did look at the list of equations. The one you described above is one I was already using.

    Is it forum rules to not answer homework questions? So far all the help I've gotten comes in the form of more questions that don't help explain what I've actually done wrong. No one has mentioned how my thinking is incorrect, or pointed out where in my assumptions/methods I've gone wrong. Without knowing what I've done wrong, I'm no closer to advancing any skills. "You have sufficient information to solve this" isn't really advice...Its what a robot says to a human when the human asks for help.
     
  10. Sep 22, 2015 #9

    SteamKing

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    You can find an expression for t without knowing the value of a from the first equation. You substitute this expression for t into the second equation, which then you can solve for a in terms of the distance and the initial and final velocities.

    You didn't look far enough down the list I posted.

    There is an equation from which acceleration can be calculated knowing only a distance and an initial and final velocity.

    Yes, yes it is. HW helpers are allowed to provide hints and to point out errors in calculation or reasoning, but providing direct solutions is not permitted.

    No one has said you've done anything wrong. You have not, however, put forth any effort at solving your own equations to find the acceleration, even after additional hints were provided to you. Whether this is due to a lack of algebra skills, or some other reason altogether, is not clear.

    If you are dissatisfied with the help you have received, you are entitled to a full refund. :wink:
     
  11. Sep 22, 2015 #10

    SteamKing

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    You started to solve equation 1 for a and then stopped. Why? You didn't even try to substitute your expression for a into equation 2.

    You've shown two equations involving a and t, both of which are unknown. You can't find either a or t without working with both equations. This is precisely where lack of knowledge of algebra, and solving simultaneous equations in particular, is holding you back from learning physics.
     
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