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Homework Help: Can someone helpme with this question

  1. Jan 27, 2004 #1
    The force acting on a moving charge particle with mass m and charge q in a magnetic field B is the Lorentz force F= q(v x B), where v is it's velocity. Suppose thata particle moves in the (x,y) plane with a uniform B field in the z direction. Assuming Newton' second law, mdv/dt = F, show that the forceand velocity are perpendicular, and that both have constan magnitude. Hint: Find (d/dt)(v dot v).

    I don't know where to start. How do you take the derivative of v dot v?
  2. jcsd
  3. Jan 27, 2004 #2


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    Use the product rule:

    \frac{d}{dx}( f \cdot g ) = f \cdot \frac{dg}{dx} + \frac{df}{dx} \cdot g
  4. Jan 27, 2004 #3
    still don't get it

    Can you elaborate? I dont understand.
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