# How to plot a rotated ellipse using ParametricPlot?

I'm plotting the von Mises yield surface using CountourPlot and ParametricPlot.

With ContourPlot I get this nice rotated ellipse:

ContourPlot[Sqrt[sig1^2 + sig2^2 - sig1 sig2] - 200 == 0, {sig1, -300, 300}, {sig2, -300, 300}] Now i need to find the parametric equations to plot a rotated ellipse similar to the ellipse above, but this time using the function ParametricPlot.

I can plot an ellipse with parametric, but it's not rotated:

ParametricPlot[{200 Cos[theta], 100 Sin[theta]}, {theta, 0 , 2 Pi}]


EDIT:

If the real von Mises yield surface is ploted (using ContourPlot) and compared with the plot obtaned from ParametricPlot:

contourplot =
ContourPlot[
Sqrt[sig1^2 + sig2^2 - sig1 sig2] - 200 == 0, {sig1, -300,
300}, {sig2, -300, 300}];
gamma = Pi/4;
a = 300;
b = a/2;
pmplot = ParametricPlot[{(a Cos[theta] Cos[gamma] -
b Sin[theta] Sin[gamma]),
a Cos[theta] Sin[gamma] + b Sin[theta] Cos[gamma]}, {theta, 0 ,
2 Pi}, PlotStyle -> {Thick, Red, Dashed}];
Show[contourplot, pmplot] we can see that the plots are not the same.

How can i find the values of a and b that make the ellipses the same?

• The question added in your edit is rather different from your original question. Instead of asking multiple questions in one post, you should post a new question and (if appropriate) link back to this one for context. – Michael Seifert Jun 6 '18 at 13:12
• @Michael Seifert I have posted a new question. – Diogo Jun 6 '18 at 14:10
• Here is the link: mathematica.stackexchange.com/questions/174766/… – Diogo Jun 6 '18 at 14:10

You can use a RotationTransform to find the equation of our rotated function

RotationTransform[α][{a Cos[θ], b Sin[θ]}]

  {a Cos[α] Cos[θ] - b Sin[α] Sin[θ], a Cos[θ] Sin[α] + b Cos[α] Sin[θ]}


Now you can plot it as any other equation.

Manipulate[
ParametricPlot[
RotationTransform[α][
{
200 Cos[theta],
100 Sin[theta]
}
]
, {theta, 0, 2 Pi}
, PlotRange -> {{-300, 300}, {-300, 300}}
]
, {α, -π, π}
] 