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Stupid Question?

  1. Apr 26, 2007 #1
    Hi, I'm new here. I hope this is the right area for this question.

    I've been reading wikipedia all day trying to figure this one out, and can't so any help will be much appreciated.

    Question: If F=ma, than m=F/a, right? So logically if an object has no force being applied to it that object would have 0 mass.

    Obviously though, things which do not have any force being exerted on them do not just cease to be. So what gives?:confused:
  2. jcsd
  3. Apr 26, 2007 #2


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    Staff: Mentor

    If there's no force, there's no acceleration. So F/a = 0/0 which is mathematically undefined. You can't conclude anything about m from that.

    With a nonzero force, you do get acceleration, so if you measure both of them, you can calculate the mass of the object. If you double the force, you get double the acceleration, and you calculate the same mass.
  4. Apr 26, 2007 #3


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    Staff Emeritus
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    But if the object as no force, it also has NO ACCELERATION, which means "a" is 0 also. Then you have 0/0 condition and there's nothing you can draw out of that from just simply using that equation.

  5. Apr 26, 2007 #4
    Doh! Thanks guys!
  6. May 13, 2007 #5
    that means zero mass is accelerated....

    ok i have another tricky question in the same issue for you......

    and it tests your understanding in this subject...

    if you drive your car with constant speed 120 km/h....with zero acceleration.....

    that means you have zero force.... is not it.....

    ok if you hit a wall with that speed ....what will happen to the wall???

    from where this force comes?

    waiting your answer...

    hint: what do we really mean by force??
  7. May 13, 2007 #6
    The wall provides a force on the car (equaled by the force of the car on the wall) that quickly decelerates it from 120 to 0 km/hr.
  8. May 13, 2007 #7

    Hmmm. My guess is that from the cars vantage point the wall is accelerating towards it at 120km/h, so that's where the force comes from. It seems to be a frame of reference thing.

    Am I right?
  9. May 13, 2007 #8
    Nobody mentioned this on here

    Y'all don't understand the idea...

    What we have here are TWO closed systems.

    One, the car is travelling at a constant speed, doesn't mean zero acceleration. It could be accelerating radially, thus changing directions (such as travelling around a bend...) So I'll just assume you mean constant velocity (direction and magnitude stay constant).

    Due to static friction between the wheels of the car and the road, the car accelerates creating a force opposing this friction which happens to be equal and opposite, cancelling each other out creating a net force of 0 on the car... Translation equilibrium is the term for this. (Static friction and not kinetic friction as the contact point of the tire relative to the road is the same and yada-yada... [insert lecture here]).

    To sum, it means the sum of the forces acting on the car is ZERO.

    Now, the second situation:

    When you hit a wall, the car immediately deccelerates. We know this as impulse (J) which is defined as J=F(dT). [F times the change in time, small time, large force; large time, small force... idea behind car airbags...]

    Impulse is also equal to the momentum (P) where P=mv.

    so mv=Ft, thus mv/t = F... which turns out to be (letting v/t = a) F=ma.

    So it can travel with some speed, hit the wall for some time and create a force...

    Tadaa, the physics explanation :-o

    (Correct me if I'm wrong... which I probably am not.)

  10. May 14, 2007 #9
    Hi, a way to answer your question,
    This force that acted on the wall, it didnt come from just anywhere did it?
    No, it came from the acceleration needed to bring the car to 120km/h. So the force acted on a car for a certain amount of time. When the car hits the wall, the momentum of the car is transferred to the wall as it crashes, and the force applied is due to the momentum of the car. That force is F=ma, which is also m(dv/dt) as a = dv/dt, and m(dv)/dt is also mv/t, so you find the mass of car, the velocity, which is 120km/h, and you find the amount of time the car takes to crash (which is really short, like 0.2sec or something) and you get the force.

    Oh, anyone here taking AP Physics? I just finished mine (in Singapore)
  11. May 14, 2007 #10
    That last line, again, is impulse... So in reality, we're just looking at the impulse-momentum combination that produces this force. So, yeah.
  12. May 15, 2007 #11
    Really good answers guys! This all raises another question in my mind.

    If J=F(dT), then you cannot have zero time. Does this say something about the universe, or is it just a limitation of the math?

    I know that Newtonian physics has it's limitations, and has been proven wrong for very small, and very large things, but it seems odd to me still. Is this implying that there cannot be zero time, or just another example of the math failing for the very small?
  13. May 15, 2007 #12


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    This doesn't say anthing about having zero time-- it says that the impulse is defined as the integral of the force with respect to t. Clearly, if you are considering a time period of exactly zero seconds, then there is no time for any force to be exterted on a body, and thus the impulse is zero. I''m not sure how you arrived at your conclusion here.
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