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    I really need help with this!!
    The question tells us: r(t)=Rcos(wt)i+Rsin(wt)j
    a)Determine the plane polar coordinates (r,theta) of the particle in terms of R, w and t.... not sure what exactly I'm meant t write for that
    But my main problem is with the last part:
    d)Assume that the particle is a charge q with mass m subject to a magnetic force qvxB. DetermineB assuming that it is parallel to the unit vector k=ixj

    Please help! I'm extremely stuck!:confused:
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    (Original post by s4shu101)
    I really need help with this!!
    The question tells us: r(t)=Rcos(wt)i+Rsin(wt)j
    a)Determine the plane polar coordinates (r,theta) of the particle in terms of R, w and t.... not sure what exactly I'm meant t write for that
    But my main problem is with the last part:
    d)Assume that the particle is a charge q with mass m subject to a magnetic force qvxB. DetermineB assuming that it is parallel to the unit vector k=ixj

    Please help! I'm extremely stuck!:confused:
    Part a is just changing the coordinates so plug what you know into the definition of polar coords.
    part d: what is the connection between force and position?
    Spoiler:
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    Part a) as in r=Rr(hat) where theta =wt?
    Part b) I know F= q ( E+ v x B) but there's no Electric field so E=0, then we have qv x B, but I don't know how to find B

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    (Original post by s4shu101)
    I really need help with this!!
    The question tells us: r(t)=Rcos(wt)i+Rsin(wt)j
    a)Determine the plane polar coordinates (r,theta) of the particle in terms of R, w and t.... not sure what exactly I'm meant t write for that
    But my main problem is with the last part:
    d)Assume that the particle is a charge q with mass m subject to a magnetic force qvxB. DetermineB assuming that it is parallel to the unit vector k=ixj

    Please help! I'm extremely stuck!:confused:
    If \bold{r}(t) = R\cos \omega t \bold{i} + R \sin \omega t \bold{j} then

    1. since \sin, \cos eat angles, \theta(t)=\omega t is a angle that increases linearly with time.

    2. the (x,y) coords of the particle are (R\cos \theta, R \sin \theta) - so what is the locus of the particle? (Hint: draw a right angled triangle, and think about \sin^2 \theta + \cos^2 \theta)

    3. So what are the polar (i.e. (r,\theta)) coords of the particle?

    4. What is the speed of the particle? (Hint: find |\bold{v}| = |\bold{\dot{r}}(t)|)

    5. Now you know the locus of the particle, you know what force must act on it. Write this force in vector terms using \bold{\hat{r}}, and the other available quantities in the question.

    6. If \bold{B} points along \bold{k}, then what is |\bold{F}|=|q \bold{v} \times \bold{B}|, and in which direction does \bold{F} point? (Hint: it's the only force acting on the particle)

    7. Hence, write down and solve an equation for B

    By the way, it's much nicer if you use latex.
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    Thanks for the help, sorry, I'm really bad at mechanics!
    Okay... for the polar coordinates r=R and theta=wt
    The speed I obtained was wR
    I wasn't sure what you meant by the 'locus', but I assumed F=Rr(hat) <-I'm guessing that'll be wrong then (as well as in thinking F points in the k direction
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    (Original post by s4shu101)
    Thanks for the help, sorry, I'm really bad at mechanics!
    Okay... for the polar coordinates r=R and theta=wt
    The speed I obtained was wR
    I wasn't sure what you meant by the 'locus', but I assumed F=Rr(hat) <-I'm guessing that'll be wrong then (as well as in thinking F points in the k direction
    1. Please use latex. It's hard to read what you're writing otherwise.

    2. The locus is path along which the particle moves. What is this path? Describe it in words. Don't do anything else until you know the answer to this question.

    3. \bold{F} does not point in the \bold{k} direction. The cross product tells you where the force is pointing (as does an appeal to Newton II, once you've figured out the locus, which is given to you in the form of \bold{r}).

    What do you know about the cross product?
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    The path/locus is \vec{OP}: the Origin to the Particle riiight? That's defined to be \mathbf{r}, where \mathbf{r}=r\bold{\hat{r}}?

    Still not sure about (3). I know \mathbf{B} x \mathbf{k} = 0 since they're parallel
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    I've managed it now, thanks for your help

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