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Homework Help: Please help with a uniform circular motion problem

  1. Jan 13, 2005 #1
    Hey can you guys give me some help with this physics problem.. having real issues figuring it out, and ive looked all over the internet. Google'd like 100 times...

    A Particle at Constant Speed A particle P travels with constant speed on a circle of radius r = 3.35 m (Figure 5-39) and completes one revolution in 20.0 s. The particle passes through O at time t = 0. State the following vectors in magnitude-angle notation (angle relative to the positive direction of x).

    The picture of it...

    1.) With respect to O, find the particle's position vector at the time t = 5.00 s.
    answer: _________ ANGLE: 45degrees

    I figured out the angle, but i cant figure out how to get the position vector. Just so u know, in case u try the problem, ive already tried the following answers, that are apparently wrong : 1.05, 4.21, .948, 5.26, and 3.35

    the rest of the questions are similar to #1, so if u guys can help me with that one, i can figure out the rest.

  2. jcsd
  3. Jan 13, 2005 #2
    Have you drawn the picture where t=5s? In case you are looking for the magnitude of vector OP at that time, pythagoras should do it, right?
  4. Jan 13, 2005 #3
    i did draw a picture, that really didnt do much though. I got that part of the question actually, apparently i was supposed to use the equation a (position vector0 = SqRt(ax^2+ay^2). And since it went 3.35 in the x direction, and 3.35 in the y direction, at time=5s, i got an answer of 4.74

    However, I cant figure it out for when t=7.5seconds
    I figured out that the angle is 67.5, and that is correct... but i cant get the position vector. I apparently need to use the formula that i used for the 1st one, but the answers i tried didnt work, can anyone help with that?
  5. Jan 13, 2005 #4
    It's the same logic! Give a second thought to it, will ya?!

    PS. This time trigonometry. :)
    Last edited: Jan 13, 2005
  6. Jan 13, 2005 #5
    i used trig, and i tried the following answers, none of which are correct apparently:

    5.3 , 5.7 , 6.6

    do you get an answer different from those? I was sure that 5.3 was right but apparently its not
  7. Jan 13, 2005 #6
    Yeah, I get 6.19. Is it correct?

    I thought that the vector CP (where C is the center of the circle) forms a 45 degree angle with the x-axis and calculated how much longer than r the new y-component is.

    PS. The new y-component is NOT r + r/2, which you appear to have assumed... (?)
    Last edited: Jan 13, 2005
  8. Jan 13, 2005 #7
    This might be that Im stupid, but shouldnt the angle be 90?

    Just saying 20s per cycle, 4 quadrants, 20/4=5?
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