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So I'm doing a Physics problem and have run into a snag with the online homework - every answer I've tried keeps getting rejected and I'm down to my last shot and I don't know what I'm doing wrong. The problem is as follows:

A 0.36 kg particle moves in an xy plane according to x(t) = -14 + 3t - 6$t^3$ and y(t) = 16+5t-8$t^2$, with x and y in meters and t in seconds. At t = 1.9 s, what are (a) the magnitude and (b) the angle (within (-180°, 180°] interval relative to the positive direction of the x axis) of the net force on the particle, and (c) what is the angle of the particle's direction of travel?

Now I've managed to figure out a and b, but the angle I get for c (-157.716 which I rounded to -157.7 degrees) seems to be incorrect, and I can't figure out why (I took the arctan of $\frac{-25.4}{-61.98}$ from calculating the x and y components from the derivative of x(t) and y(t))

Could someone explain to me what I missed?

secondubly
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  • -157.7, do not round to +157.7 – PdotWang Feb 13 '15 at 02:43
  • Angle of particle's direction of travel is $\frac{y'(1.9)}{x'(1.9)}$ right? – Tejas Feb 13 '15 at 02:45
  • @PdotWang That was actually a typo - the original angle was 22.2843, but I subtracted 180 since the vector itself is in the third quadrant.) – secondubly Feb 13 '15 at 02:50
  • @Tejas Yes, and I got 22.2843 for that calculation, subtracted 180 from it, and got -157.716, and they want it to 0.1 sigfigs, so I'm guessing my calculation must be wrong somewhere... – secondubly Feb 13 '15 at 02:59

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