Because the game features the online multiplayer option you must allow it to connect and receive connections and Java Applets can't do that by default. So that is only appearing to allow the multiplayer connectivity.
You most likely don't have Java or the Java web browser plugin installed. You can download and install Java here on most cases. However what you need in order to run Java Applets on your browser might be different depending on your platform, Linux users may need to install the Java plugin separately. If you are accessing this through a mobile device there may be no way to run Java Applets at all.
NO! Your soldier is NOT at the origin. Most people apparently think that when first playing the game and that is not true. The function is translated in the y axis so that it will go through your soldier at that x coordinate. Note that also does not mean that your soldier y coordinate is zero, that only means that a constant is added to your function. Look at the tutorial for an explanation in greater detail.
Look carefully at what value your function would have at the position your soldier is currently at. The limits on the x axis are -25 and 25 and your function could have a big y value at that position. When a constant is added to make it go through that point the result could be a very steep region of your function, try dividing it by some factor like 10 or 50.
Your function may explode because of two reasons, because the value at that position is undefined or because it is too long. If you try to use the function y = log(x) for example, it will explode immediately. That is because your soldier is at a negative x and log(x) is undefined for negative x values. That may also happen with square roots and a bunch of other functions, it may also happen in a division if the denominator approaches zero. The other reason your function can explode is if it is too long. If you use a high frequency sine function for example: y = sin(100*x), the graph is going to be very long and if you reach a maximum limit it is going to explode.
The angle is only used in the second order differential equation mode, changing it on other modes have no effect in the function.
That means you are probably accessing the Internet through a router. Your router is blocking incoming connections to your computer, you need to allow connections on a specific port and how to do that is specific to your brand or model. Search how to open ports on that router and you will probably find how to do it.
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