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Please me help with this acceleration question

  1. Jan 16, 2012 #1
    1. A car accelerates from a standstill to a speed of 30 m/s in a time of 10 seconds. What is its acceleration?



    2. Equation for acceleration: a=(vf-vi)/t


    3. a= ? (it asks for acceleration)
    vf= 30 m/s
    vi= 0 m/s
    t= 10s

    30 m/s - 0 m/s / 10s = 3
    a= 3 m/s^2

    I'm not sure if I did it right.
     
    Last edited: Jan 16, 2012
  2. jcsd
  3. Jan 16, 2012 #2
    the answer is in the question... what speed is standstill - this could be your initial velocity perhaps (Vi), you already know your final velocity and now feed them into your equation

    P.S don't forget that your answer is in metres per second per second m/s^-2
     
  4. Jan 16, 2012 #3
    Since it start from standstill the initial velocity is 0m/s. the final velocity is 30m/s over a time of 10 seconds just plot those numbers in your formula and you would get your answer.
     
  5. Jan 16, 2012 #4
    I'm kinda confused. :confused:
     
  6. Jan 16, 2012 #5
    A= (30-0)/10 = 3m/s ^2
     
  7. Jan 16, 2012 #6

    Simon Bridge

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    Hi;
    there is a systematic way to do this sort of problem.
    You get the equation.
    Then you write the variables in the equation in a list, with what they are equal to.
    If you don't know one or two, just put a ? next to them.

    So your equation was: [itex]a=(v_f - v_i)/t[/itex]
    Look at what each of the symbols mean ... and relate that to the description of the problem and write the list:

    a = ?
    vf = 30m/s
    vi = "standstill" = 0m/s
    t = 10s

    Now rewrite the equation so instead of the symbols you put the numbers ... so everywhere you see a "t", you write "10".... so it looks like this: [itex]a=(30 - 0)/10[/itex] ... the last stage is to do the math.

    It works even better if you write the list first and then go look for the equation to use.
     
  8. Jan 16, 2012 #7
    It means that at first, the car is going one speed. That speed is 0 meters per second(another way of saying standing still).

    Then the car's speed starts to change. It gets faster and faster and faster until it reaches 30 meters per second.

    It takes 10 seconds for the car to go from 0 meters per second (standing still) to 30 meters per second. That means the speed is changing for those 10 seconds.

    Acceleration is the measure of the change of the speed over time. That is what your equation is saying. The subtraction in the numerator is another way of saying the "change of speed" and the total time you want to measure is in the denominator.

    So your equation is measuring the car's acceleration, which would be 30 - 0 = 30 for the numerator and 10 seconds for the denominator, that means you have 30/10, or in words, 30 meters per second divided by 10 seconds. And that division problem equals 3. So it would be called 3 meters per second per second....or 3m/s^2 which is the acceleration of the car.

    Does this make sense?
     
    Last edited: Jan 16, 2012
  9. Jan 16, 2012 #8
    CORRECT!

    m/s^2 or m/s/s. Velocity it the rate of change of position every second (m/s) and acceleration is the rate of change of velocity every second (m/s/s)
     
  10. Jan 16, 2012 #9

    berkeman

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    You MUST show your efforts on working out problems here on the PF. Please post your work on this question, or this thread will be deleted. We do not do students' homework for them here.
     
  11. Jan 16, 2012 #10
    I just edited my post. So, will this thread be deleted?
     
  12. Jan 16, 2012 #11

    berkeman

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    No, it will stand. But in the future, please follow the PF rules and show your Attempt at a Solution. In this thread others basically did your work for you. That is not permitted here.
     
  13. Jan 16, 2012 #12
    I did my work on a paper first, then I posted in here. 'Cause I'm not sure if I did it right or not. & I didn't realized we have too post a solution. Sorry bout that.
     
  14. Jan 16, 2012 #13

    berkeman

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    Staff: Mentor

    No worries. :smile:
     
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