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Relation between acceleration and time

  1. Jun 17, 2015 #1
    I have a problem. As I was finding the torque for an engine. I have. I get a question about the relation of acceleration and time.
    How will be the graph of varying acceleration and time if acceleration decreases?
    Will it be a straight line with negative slope or any other shape?
     
  2. jcsd
  3. Jun 17, 2015 #2

    mfb

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    This is impossible to tell without more information. It's like asking "here is a car, what will its speed be over time?"
     
  4. Jun 17, 2015 #3

    RUber

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    If you are plotting acceleration on the y axis and time on the x axis, then a horizontal line would indicate constant acceleration. Negative slope indicates decreasing acceleration, and positive slope indicates increasing acceleration.
    The question is, how is your acceleration changing? Do you have a formula for it?
     
  5. Jun 17, 2015 #4
    Most cars (including drag racers) have maximum acceleration near t=0. Then acceleration declines rather quickly before curving to a more gentle downward slope. You'll need data and curve-fitting techniques to find the formula for your application.
     
  6. Jun 18, 2015 #5

    mfb

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    Not if they are parking somewhere (a=0), or moving in traffic jams or whatever. That's the point, we have no idea what happens so it is impossible to know the acceleration.
     
  7. Jun 18, 2015 #6
    Thank you for such a response to my first question to Physics forum.
     
  8. Jun 18, 2015 #7
    Hello
    Today I get the graph of acceleration and time for my engine.
    It will have a shape of paarabola x=-y^2 in first qudrent from Max acceleration to a=0.

    Now I have a question regarding the reletion of power and acceleration & relation of torque and acceleration.
    How to get relation for my engine. having max torque 18.6 N-m @2800 rpm.
    how can I accelertion from power.
     
  9. Jun 18, 2015 #8

    RUber

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    This web page spells it out pretty simply. http://theory.uwinnipeg.ca/physics/rot/node5.html
    If you know torque, ##\tau##, mass, m, and you know the radius of the thing you are rotating, ##r##, you can relate torque to acceleration by:
    ##\tau = m r a ##
     
  10. Jun 18, 2015 #9
    I assume you mean to end up with an equation like:

    a = a0 - C*sqrt(t)

    For example, if initial acceleration were 1 m/s2 and acceleration drops to 0 after 20 seconds, you have:

    a = 1 - 0.2236*sqrt(t)

    Now, if you know the torque from the engine at a point in time, you can calculate the acceleration by calculating the force the wheels are applying to the vehicle, using F = ma. In practice, this is not possible because engine torque is not known at every point in time.

    (Are we talking about a vehicle, like a motorbike?)
     
  11. Jun 18, 2015 #10

    Thank you for response...

    But what is the C here?
    How did you get the value of C=02236 here?

    I am making an ATV car.
     
  12. Jun 18, 2015 #11
    C is a constant in the equation. It is found by putting my assumption in the example I gave that a=0 at t =20 s:

    0 = 1 - C*sqrt(20) and solve for C.
     
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