# Vector motion in a circle

1. Feb 24, 2012

### bobsmith76

1. The problem statement, all variables and given/known data

3. The attempt at a solution

I understand how to get the derivatives for velocity and acceleration but when the book plugs in the numbers I disagree.

velocity at pi
2 sin pi/2 = 0
2 cos pi/2 = -1j
therefore, 0 + - 1j = -1j (the books says -2i)

velocity at (3pi)/2
2 sin 3pi/2 = 0
2 cos 3pi/2 = 1i
therefore, 0 + 1i = 1i (the book says -√2i - √2j)

acc at pi
-cos pi/2 = 1/2i
-sin pi/2 = 0
therefore, 1/2i + 0 = 1/2i (the book says -1j)

acc at (3pi/2)
-cos 3pi/2 = 0
-sin 3pi/2 = 1/2j
therefore, 1/2j (the book says -√2/2i - √2/2j)

Last edited: Feb 24, 2012
2. Feb 24, 2012

### tiny-tim

hi bobsmith76!

(have a pi: π )

hmm …
sinπ/2 = 1
cosπ/2 = 0
no, you need 3π/4

get some sleep! :zzz:

3. Feb 24, 2012

### micromass

Staff Emeritus
I highly recommend you review your trig and algebra.

sin(pi/2)=1 and cos(pi/2)=0.

That's not what you need to calculate. You need to calculate -2sin(3pi/4) and 2cos(3pi/4).

cos(pi/2)=0 and sin(pi/2) = 1.

That's not what you need to calculate. You need -cos(3pi/4) and -sin(3pi/4)