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Newton's second law

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  1. May 28, 2015 #1
    We all know that f = m*a. But why can't we write this as f = m * s/t^2 ???
    I know this is wrong. But what is the reason?? Need opinion on this.
    Thankyou.
     
  2. jcsd
  3. May 28, 2015 #2

    SammyS

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    You can't do it because ##\displaystyle \ a\ne \frac{s}{t^2} \ ## .
     
  4. May 28, 2015 #3
    a = dv/dt. Ya now i get it.
     
  5. May 28, 2015 #4

    SammyS

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    If initial velocity is zero, and acceleration is constant, then the final velocity is v = at, but the average velocity is v/2 . The average velocity multiplied by time gives distance, so that s = (1/2)at*t = (1/2)at2, a = 2a/t2 .

    Therefore, you could write f = 2ma/t2, provided that initial velocity is zero, and acceleration is constant.
     
  6. May 28, 2015 #5
    Thanks
     
  7. May 29, 2015 #6
    if accelaration is not constant then???
     
  8. May 29, 2015 #7

    SammyS

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    Then, ... all bets are off. That's also true if the initial (or final) velocity isn't zero.
     
  9. May 30, 2015 #8
    sorry but i don't understand.
    please can u tell this mathematically. plzzx
     
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