Please help with a uniform circular motion problem

atlbraves49

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...
http://www.webassign.net/hrw/166710.gif

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.

Thanks

Joel

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?

atlbraves49

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?

Joel

It's the same logic! Give a second thought to it, will ya?!

PS. This time trigonometry. :)

atlbraves49

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

Joel

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... (?)

cdhotfire

This might be that Im stupid, but shouldnt the angle be 90?

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