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Work and Kinetic Energy problem

  1. Oct 26, 2008 #1
    A force Fx acts on a particle that has a mass of 1.5kg. The force is related to the position x of the particle by the formula Fx = Cx^3, where C = .50 if x is in meters and Fx is in newtons.

    a) What are the SI units of C?
    b)Find the work done by this force as the particle moves from x=3.0m to x=1.5m
    c)At x = 3.0m, the force points opposite the direction of particle's velocity(speed is 12.0m/s). What is its speed at x=1.5m?

    For A:
    Isn't C just a constant? What units is this in?

    For B:
    I have to integrate Fx and the integral of F(x)dx = x^4/4
    Since C is a constant I can put it in front of the integral so is 1/4 C x^4
    When I integrate shouldn't there be a + K? 1/4 C x^4 + K
    X1 = 3.0 and X2 = 1.5
    Plug in and solve?

    For C:
    Using answer from Part B = Work total
    Work total = 1/2 M Vf^2 - 1/2 M Vi^2
    where Vi = 12m/s?
  2. jcsd
  3. Oct 26, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Just because C is a constant doesn't mean it has no units. Hint: For any equation, the units on each side must match.

    Since you are doing a definite integral (between limits) the integration constant goes away. Just evaluate between those limits.

    Good. Use the work-energy theorem.
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