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Constant Accelaeration Problem

  1. Nov 13, 2008 #1
    1. A car initially travelling 25 m/s accelerates at a constant rate of 3 m/s squard After 5 seconds How fast is the car traveling



    2. Relevant equations



    3. The attempt at a solution
    I tried divdeing 25 m/s by 3 an then add 5 but didnt get the right answer

    Aslo i tried divding it by 25 m/s by 5 adding 3 still wrong though

    im stuck an lost help plz
     
  2. jcsd
  3. Nov 13, 2008 #2

    rl.bhat

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    Can you wright the equation of motion, which gives the relation between initial velocity, final velocity, time interval and acceleration?
     
  4. Nov 13, 2008 #3

    tiny-tim

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    Welcome to PF!

    Hi NukeNuke! Welcome to PF! :smile:

    You need one of the following standard formulas for uniform acceleration:

    v = u + at
    v2 = u2 + 2as
    s = ut + at2/2

    (You should know these by heart)

    If you don't know what they mean, you will need to look them up. :smile:
     
  5. Nov 13, 2008 #4
    Re: Welcome to PF!

    v = u + at
    v= final velocity = ?
    u = initial velocity = 25m/s
    a = acceleration = 3m/s^2
    t = time = 5sec
    Putting these values in equation:
    v = 25 + 5*3
    v = 40m/s

    So would this be right?
     
    Last edited: Nov 13, 2008
  6. Nov 13, 2008 #5

    tiny-tim

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    Yup! :smile:
     
  7. Nov 13, 2008 #6
    Don't think of it in formulas. Those formulas are just arbitrary symbols on a piece of paper meant to represent the real thing. Feynman said in his 1961 lecture that "the beauty of math is that you don't know what you're talking about. The equation is independent of what the thing really is".

    A car initially travelling 25 m/s accelerates at a constant rate of 3 m/s squard After 5 seconds How fast is the car traveling

    Think of the matrix (film). Time is almost standing still. You look at the speedometer of the car, and it says the car is moving 25 meters every second that passes. If the car did not accelerate at all, it would have moved 50 meters in any two seconds of time that passes.

    This car, however, is speeding up. For every second that passes, the car goes faster by 3 meters per second.

    After 1 second, the speed is 28 meters per second. After 2 seconds, the speed is 31 meters per second.

    If this continues for 5 seconds, how fast is that car going to be moving?
     
  8. Nov 13, 2008 #7
    Thanks,
     
  9. Nov 13, 2008 #8
    Welcome,
     
  10. Nov 13, 2008 #9

    tiny-tim

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    NukeNuke, I completely disagree that you shouldn't think of it in formulas!

    You won't pass the exams if you don't learn these three formulas …

    you don't have time to work everything out from elementary principles in the exam … and even if you did, the examiners would deduct marks for it :frown:

    though, of course, an understanding of the reality does help you to see why the equations work, and helps you to remember them. :wink:

    btw, Feynman seems to be agreeing with me … he says the equations are the important thing (he goes on to say you don't have to know what they mean in reality … but I suspect he was talking about quantum field theory rather than cars on roads). :smile:
     
  11. Nov 13, 2008 #10
    Oh I agree that for complicated stuff, you can't sit there and wrap your brain around everything that is happening at once. Engineers don't try to picture the entire skyscraper and all forces acting on it in their heads, they use formulas for components.

    BUT the formulas represent something real that you can touch or see or measure somehow. The question itself in this case was simple. What made it hard was using all the arbitrary symbols used by people that have done problems like these 1000 times. To tell someone that to find final velocity you must use a kinematic equation modeling one dimensional horizontal movement is like saying that to beat a fire dragon, use this water magic book of spells.

    I think formulas and symbols confuse the hell out of people learning physics for the first time. The hardest part of physics for me is just trying to understand what the squiggly lines on a piece of paper are supposed to represent.
     
  12. Nov 13, 2008 #11

    tiny-tim

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    That's why it's so important to practise using them, and to learn the ones that tend to come up in the exams!
     
  13. Nov 18, 2008 #12
    I might have did this wrong again aint it 25 plus 5 times 3 an the units is m/s or km/h

    cause i keep getting the answer wrong
     
  14. Nov 18, 2008 #13

    tiny-tim

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    Yes, it's 25 + (3 x 5). :smile:

    Write out "a rate of 3 m/s squared" in full …

    it really means "a rate of 3 m/s per second" …

    so the speed goes up by 3m/s for every second …

    so, for 5 seconds, the speed goes up by 15m/s. :smile:
     
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