2.7 Game 2302-0370 motion -- uniform and relative velocity  (Page 6/11)

Screen output

The text shown in Figure 5 should appear in your browser window when you open the html file in your browser.

Analysis of the code

As before, the script begins by defining the getAngle function, which doesn't require further explanation.

The vectorSum function

As you can see from Listing 3 , this script contains a new function named vectorSum . The purpose of this function is to add two vectors, given the magnitude and angle (in degrees) for each vector asincoming parameters. (The function assumes that both incoming magnitude parameters are expressed in the same units.)

The need to add two vectors occurs so often in physics that I decided to encapsulate the capability into a function that we can copy into future scripts.That will relieve us of the need to rewrite the code each time we need that functionality.

Return an array object

The magnitude and the angle (in degrees) of the resultant vector is returned in a two-element array object, with the magnitude in the element at index 0 and the angle in the element at index 1.I will have a more to say about arrays shortly.

Note that this function calls the getAngle function, which must also be provided in the script.

Subtraction of vectors

The function can also be used to subtract one vector from another. To subtract one vector from another, simply add 180 degrees to the angle for the subtrahend vector before passing the angle to the function.

Adding more than two vectors

Although the function can only be used to add two vectors, it can be called repeatedly to add any number of vectors two at a time.

To use this function to add more than two vectors, add the first two vectors as normal. Then add the vector output from the function to the third vector, etc., until all of the vectors have been included in the sum.

The code in the vectorSum function

Most of the code in the function is the same as code that I have explained in earlier modules. However, there are a couple of lines of code that aredifferent.

The function begins with the following statement:

var vecResult = new Array(2);

The purpose of this statement is to create a two-element array object named vecResult .

What is an array object?

You can think of an array object as a software object containing pigeon holes into which you can store data and from which you can later retrieve that data. This particular arrayobject has two pigeon holes, and they are numbered 0 and 1.

This array object will be used as a container to return the magnitude and angle values for the resultant vector.

Can only return one item

A JavaScript function can only return one item. Up to this point in these modules, that item has consisted of a single numeric value, such as the anglevalue that is returned from the getAngle function.

However, the vectorSum function needs to return two different values. One way to do that is to put those two values in the pigeon holes of an array object andreturn that array object as the one item that can be returned.