differentiation of a vector r

[soccerball3211]

The position of an electron is given by vector r = 2.65t(i hat) - 5.47t^2(j hat) + 5.96(k hat), with t in seconds and vector r in meters. At t = 2.49 s, what are (a) the x-component, (b) the y-component, (c) the magnitude, and (d) the angle relative to the positive direction of the x axis, of the electron's velocity vector v ?

Ok everything I have done so far is correct and I was just wondering if someone could tell me why my part d is not correct?

My answers so far:
a.) 2.65 m/s
b.) -27.2 m/s
c.) 27.4 m/s
d.) 84.40 degrees

 

[soroban]

Hello, soccerball3211!

Your work is correct . . .did you misread the angle on your calculator?


You had: \(\displaystyle (a)\:\vec{v}(2.49)\:=\:\langle 2.65,\,10.94,\,0\rangle\:\) and \(\displaystyle \: (c)\;\left|\vec{v}(2.49)\right|\:=\:27.4\)

Then: \(\displaystyle \,\cos\alpha \:=\:\frac{2.65}{27.4}\:=\:0.096715328\)

Therefore: \(\displaystyle \,\alpha \:=\: 84.444994442^o\)

 

[mb8992]

for the angle:
arctan(y/x)