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Help with Force?

  1. Jul 30, 2008 #1
    I'm trying to calculate the amount of force I will need to move a 9.6g object at an average speed of 450 feet/ second. So when I do this I use Force=Mass*Acceleration. Since acceleration is change in speed/change in time (both of which are initially 0) I come out with 450*9.6. This yeilds 4320. Now, either i'm doing something terrible wrong, or I just have my units incorrect. I am not even sure what unit this answer is in, so if someone could explain this to me it would be amazing... Also this is my first time ever using the forum so if I have posted it in the wrong spot could you please redirect me.

  2. jcsd
  3. Jul 30, 2008 #2
    Actually I think I might have found my mistake. I was calculating how many grams of force it would take. So if I convert grams to pounds I get 8.8890384113 pounds, which is much much more reasonable. However if someone has spotted a mistake or a math error could they please tell me.
  4. Jul 30, 2008 #3
    Applemush, there is not enough information in your problem : in empty space, the tiniest force will bring your mass to an arbitrary high speed provided you wait long enough

    Now, do you guess where you got the units wrong ?
  5. Jul 30, 2008 #4
    Well I think that I found the force it would take in grams...but thats only a guess. However, if I converted grams to pounds I get 8.8890384113 pounds of force, which is much more reasonable. Also, I'm sorry, but I dont quite understand what your saying in the previous post. I was under the assumption that this would happen in an environment that would offer resistiance (like just the normal enviroment i'm sitting in right now), but did not factor that in to my calculatoins. I would appriciate further explanation, and I think i have to do a little more research to understand what your saying. I have an extremely limmited knowledge of physics to being with.
  6. Jul 30, 2008 #5
    450*9.6 is a speed times a mass, in strange units so be it, but it is m*v, correct ?
  7. Jul 30, 2008 #6
    Well, 450 is a speed, and as far as I know is not a velocity because it lacks directoin. But will a direction change things much? And 9.6 is a mass
  8. Jul 30, 2008 #7
    Whoops. Speed is v...keep forgetting that...yes it is v*m
  9. Jul 30, 2008 #8
    Acceleration [itex]a=\frac{\Delta v}{\Delta t}[/itex]
    Here you set [itex]\Delta v = v[/itex] which is fine since you start from rest.
    Now, taking [itex]\Delta t = 1[/itex] you obtain an a force required to produce your acceleration in a unit of time in your system of unit. Once you define your system of unit, you'll get the meaning of the acceleration and how long this "unit of time" lasts :smile:

    All right, I'll finish...
    Your speed is in feet/s so you are calculating a force in g*feet/s*[how long do you want to accelerate ?]
    Last edited: Jul 30, 2008
  10. Jul 30, 2008 #9
    Alright...I'm getting that I found the force required to move the mass for one second in one second, and my answer does not make any sense because I am using more than one unit system. I thought that feet and gram were in the same unit system but I could be mistaken...?
  11. Jul 30, 2008 #10
    Personally, I use kg, m, s, and the force comes out in N. But the conversion is easy.
  12. Jul 30, 2008 #11
    Whoa, i appricate the answer, but i'm still completly lost as far as time goes. The conversion doesnt worry me that much that was just a silly mistake. But could you explain my mistake regarding time. I dont see that one yet
  13. Jul 30, 2008 #12
    K....So thats 1.3N of Force to accelerate the mass in one second (not accounting for the frictoin). This doesnt make much sense to me....I think i'm ganna go back and read up on a few things so that i can follow you a little bit more clearly.
  14. Jul 30, 2008 #13
    Why is that wrong ?
    Last edited: Jul 30, 2008
  15. Jul 30, 2008 #14
    OH! So assuming that I wanted the acceleratoin to be accomplished within 6 seconds i would divide 137m/s (~450f/s) by 6 then mutiply by .0096(9.6g in kg) to get a force of .2192N...Thats suprisingly small...Isn't that like .05 pounds....but thats without friction...so I would have to find out what air friction is and then find out how much force it would take to overcome that?
  16. Jul 30, 2008 #15
    The air friction will be proportional to the speed. What is the size and shape of your object ? It depends now whether we're talking of a sheet of paper or a steel ball.
  17. Jul 30, 2008 #16
    Ugh. once again i have dabbled in physics on a seemingly simple questoin and found myself amazed at how deep it is, physics is a truly remarkable study. I think that before i even attempt at answering your question, I should read a little about friction. When i began this thread i had no intention or need to understand friction,and now i do.
  18. Jul 30, 2008 #17


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    Then this has been a great lesson :smile: !
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