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Homework Help: Force problems- help pls

  1. Oct 1, 2004 #1
    Force problems- help pls!!

    I need help with these problems:
    1. A 3.9 kg object is subjected to two forces, F1= (2.8N)i + (-4N)j and F2= (3.7N)i + (-10.4N)j. The object is at rest at the origin at time t= 0.
    a) What is the magnitude of the object's acceleration? Got That one.
    b) What is the magnitude of the velocity at t= 3.8 s? Got that one too.
    c) What is the magnitude of the object's position at t= 3.8 s? No idea how to do this one.

    2. A 1.5 kg particle starts from rest and moves a distance of 2.3 m in 2.9 s under the action of a single, constant force. Find the magnitude of the force.

    Please!! Can someone help me asap!!
  2. jcsd
  3. Oct 1, 2004 #2
    Magnitude = sqrt(i^2 + j^2) (components not just the unit vectors). Integrate the velocity function to find the position function and take the magnitude of the position vector at t = 3.8s. Shouldn't be too hard given this info.
  4. Oct 1, 2004 #3
    I am sorry, I still don't get it :cry: . What do u mean by integrate the velocity function to get position function :confused:
  5. Oct 1, 2004 #4
    How did you derive the velocity function? It should be a similar process going from velocity function to position function as acceleration to velocity function (integration)
  6. Oct 4, 2004 #5
    I got the velocity like this:
    v-v0 = at = (4.05) (3.8) = 15.39 m/s. which was correct.
    I tried to figure out the magnitude of object's position like this:
    v = change in x / change in t
    15.39 = change in x / 3.8
    change in x = 58.48 m. which was the wrong answer.
    What am I doing wrong?
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