1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Force, acceleration

  1. Jul 16, 2009 #1
    1. The problem statement, all variables and given/known data
    A 0.20 kg object is struck with a stick causing it to accelerate from rest to a speed of 25 m/s over a highly polished surface for a distance of 0.75 m. What force has been exerted on the object?

    2. Relevant equations
    f=ma, v=d/t, a=v/t

    3. The attempt at a solution
    i used the formula v=d/t to find time so that i could use the formula a=v/t and plug time in there. once i found acceleration i tmultiplied that by the 0.20kg object.
  2. jcsd
  3. Jul 16, 2009 #2


    User Avatar
    Homework Helper

    Try using conservation of energy instead.
  4. Jul 16, 2009 #3
    how do i do that?
  5. Jul 16, 2009 #4
    Do we know if the stick is in contact with the object over the 0.75m?
  6. Jul 16, 2009 #5
    i dont think it is.. i think its more of just, the stick hit the object and sent it flying.
  7. Jul 17, 2009 #6
    you can't use v = d/t to find the time needed because the speed is not constant
  8. Jul 17, 2009 #7
    Have you talked about the 'kinematic equations for constant acceleration" in your class yet? If so, what are they?

    If you can use the kinematic equations to find 'a', then how would you find 'F' ?
  9. Jul 26, 2009 #8
    well im not in a class. Im taking physics 20 through the summer virtually. so i dont really have a teacher.

    how would i find time, if i cant use v=d/t formula?
  10. Jul 26, 2009 #9
    There are 3 formulas for kinematic :
    1. v = u + at ; v = final velocity ; u = initial velocity
    2. v^2 = u^2 + 2as
    3. s = ut + 1/2 at^2

    Try it ^^
  11. Jul 26, 2009 #10
    You can't use " v = d/t " because v is not constant.

    This is actually a very poorly worded, unrealistic question.

    I *think* they want you to assume that " a " is constant (as if a constant force were applied to the object for the .75 meters). There are two ways to solve the problem.

    1. For any object moving with constant acceleration, the following kinematic equations apply (you should know these for your physics class, and if you don't, memorize them now):

    [tex]\Delta[/tex]x = v0t + 1/2 at2

    v = v0 + at

    v2 = v02 + 2a[tex]\Delta[/tex]x

    [tex]\Delta[/tex]x = 1/2 (v + v0)t

    These equations apply for any situation where 'a' is constant. Assuming these equations apply, you know most of the information (v0, v, [tex]\Delta[/tex]x), and you can use them to find 'a'. Then use F = ma.

    2. The second approach is to use Conservation of Energy (as was suggested early in the thread). This is really the same as the first approach, if a and F are constant.

    The fact you need is that the CHANGE in kinetic energy is equal to the work done on an object, or

    [tex]\Delta[/tex]KE = F * [tex]\Delta[/tex]x

    What is the final KE of the object? What was the initial KE? The difference between them gives F * [tex]\Delta[/tex]x, and you know [tex]\Delta[/tex]x.
  12. Jul 26, 2009 #11
    but if i dont know the acceleration or time, how do i rearrange it to find both?
  13. Jul 26, 2009 #12
    you know initial speed, final speed, and distance...
  14. Jul 26, 2009 #13
    but with the formulas you've given, i need acceleration or time to solve it. if not i have to use v=d/t and that doesnt work as we have figured out..
  15. Jul 26, 2009 #14
    v^2 = u^2 + 2as

    With that formula you can find acceleration
  16. Jul 26, 2009 #15
    ok, i think im half retarded, because i cannt for the life of me rearrange that formula!!!!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook