Q: Hi, Im stuck on Step 1 in the Pendulum Puppet Challenge. I tried both for loop method and calling each limb one by one and both did not get a pass from the Grader. Example of my for loop: for(var i = 0; i < limbs.length; i++){ var limb = limbs[i]; limb.go(); } What do I have to do to get passed step 1?

The challenge (like all challenges) has a blue "Report a problem" button. Use it to complain about your issue. Then drop the intermediate, simplifying `var limb = limbs[i];` and instead use `limbs[i].go();`

Q: I have the program working; however, the grader is not accepting my answer. Can anyone look at my code and assist? It can be found here: https://www.khanacademy.org/computer-programming/pendulum-puppet-problems/5117497249103872

You don't need to declare `this.currentOrigin` or `this.even` in the constructor. `this.even` isn't needed. Don't use `this.even` or anything related to it. Just set the pendulum's position predecessor to the variable name of the same limb1

Q: I cannot get past part 1 of the next challenge (Pendulum puppet). I know I need to use a for loop, and have this as the draw function: draw = function() { background(255); // Draw the body strokeWeight(4); line(width/2-50, 110, width/2+50, 110); line(width/2, 110, width/2, 230); line(width/2-40, 230, width/2+40, 230); fill(224, 194, 134); rect(width/2-25, 39, 50, 64, 30); for(var i = 0; i <= limbs.length; i++){ limbs[i].go(); } }; All I get, however, is an error message saying "Cannot read property 'go' of undefined." I have not changed anything other than what I showed so how do I go forward? I do not see anything wrong and the error does not point to where the problem is.

I'm not sure you want "i <= limbs.length". What worked for me is doing "i < limbs.length". The problem may be something somewhere else in the code, but that's the only thing I noticed.

Question 1

Hi,
The article seems to jump from giving us the
sin(theta) = Fp/Fg formula to 
this.aAcceleration = (-1 * gravity / this.armLength) * sin(this.angle);

I understand that there is a different article that explains why to add the division by this.armLength....

However, my question now is why add the part where u multiply by -1, when the foundation/basic formula is sin(theta) = Fp/Fg formula to  which works out to
Fg * sin(theta) = Fp or Fp = Fg * sin(theta)

And in the ProcessingJS world where 0 0 normally starts on the top left of the screen, Fg should be a positive number so that the position will go down as the number gets incremented.  By multiplying it by -1, isn't it the reverse of Fg already?

Thank you....

Accepted Answer

You are very observant, but the -1 has nothing to do with the default grid layout of the Processing.js canvas.

This section makes my head explode every time I look at it. It is because the original author abandoned all the previous sections and instead concentrated on *angular* displacements, *angular* velocities and *angular* accelerations as a "simplifying" move.  (KA's only fault is republishing it...  I vented about this long ago and I agreed that there was no real choice but to keep the original text.)

A way to look at it goes all the way back to the first picture of this section.  The *angle* is the pendulum's angle relative to the *vertical* axis (and not the traditional horizontal axis).  So angle zero is  the resting position.  When the angle is positive, the sine of the angle  is positive and we want the *effects* of gravity  to "pull" the bob back towards the zero angle, so the -1 is necessary.  Similarly, when the angle is negative, its sine is negative and we still want the effects of gravity  to "pull" the bob back towards the zero angle, but now that pulling must be positive, so the -1 is necessary.  (Clear as mud, right? i tried...)

Note that the variable `gravity` is only expressing gravitational effects. With more time and possible confusion, I could make the argument that the `gravity` variable should always be a negative number, thus obviating the -1.

The other effects of basing the angle off of the vertical axis is reflected in the `display` method. There you notice that the X coordinate is computed using the sine of the angle while the Y coordinate uses the cosine. This is bass ackwards from what a casual observer (like you or me) would expect.

Bonus: Now explain the initialization of the pendulum's angle as computed by the `handleDrag` method.   Do not let your head explode.

Question 2

Regarding the Pendulum Puppet challenge:
I can't figure out how to get access to Limb1 from Limb2. If there where pointers it would be easy. 
However, when I pass the Limb1 parameter into Limb2 constructor, I just make a new copy of Limb1 and those changes made to Limb2 by the limb2.display function don't get passed into the copy limb2 makes. 
Any suggestions?

Accepted Answer

I made it so you pass either a PVector object or a Pendulum object as the origin argument to the `Pendulum` constructor function. Then, within the `display` method I immediately set a new property, `currentOrigin` which describes the pivot point of this pendulum using the Javascript `instanceof` operator: ```    if (this.origin instanceof PVector) {
        this.currentOrigin = this.origin;
    } else {
        this.currentOrigin = this.origin.position;
    }```Then I can use the `currentOrigin` property knowing it's always the pivot point of the pendulum.

Question 3

Anyone know the correct formatting to get it to accept the first step?

Accepted Answer

I think it is very sad that we are forced to solve the problems in the one way the creator thought of. I tried limbs.forEach(function(limb) { limb.go(); }); But this did not work ;/

Question 4

In the next challenge: 
I am not able to understand the concept of this code --

if( this.origin instanceof PVector){
        this.currentOrigin = this.origin;
    }else{
        this.currentOrigin = this.origin.position;
    }

Please  Help !!

Accepted Answer

Each pendulum (limb) have either a PVector as it's origin or another Pendulum. So we need to know if we just use the origin which is a PVector, or the "parent" Pendulum.position (also a PVector).

For example see how left arm is created

Upper arm have a static PVector as it's origin:
`var leftArm1  = new Pendulum(new PVector(width/2-50, 110), limbLength, random(0,360)); `

Lower arm is attached to upperArm
`var leftArm2  = new Pendulum(leftArm1, limbLength, random(0,360));`

Question 5

I'm stuck on Step 2 of the Pendulum challenge. Here is what I have:

angleMode = "radians";

var Pendulum = function(origin, armLength) {
this.origin = origin;
this.armLength = armLength;
this.position = new PVector();
this.angle = 0;

this.aVelocity = 0.0;
this.aAcceleration = 0.0;
this.damping = 0.995;
this.ballRadius = 25.0;
this.dragging = false;
};

Pendulum.prototype.go = function() {
this.update();
this.display();
};

Pendulum.prototype.update = function() {
// As long as we aren't dragging the pendulum, let it swing!
if (!this.dragging) {
// Arbitrary constant
var gravity = 0.4;
// Calculate acceleration
this.aAcceleration = (-1 * gravity / this.armLength) * sin(this.angle);
// Increment velocity
this.aVelocity += this.aAcceleration;
// Arbitrary damping
this.aVelocity *= this.damping;
// Increment angle
this.angle += this.aVelocity;
}
};

Pendulum.prototype.display = function() {
if (this.origin instanceof PVector){
this.currentOrigin = this.origin;
} else {
this.currentOrigin = this.currentOrigin.position;
}
this.position = new PVector(
this.armLength * sin(this.angle),
this.armLength * cos(this.angle));
this.position.add(this.currentOrigin);
stroke(0, 0, 0);
strokeWeight(3);
line(this.currentOrigin.x, this.currentOrigin.y, this.position.x, this.position.y);
fill(224, 194, 134);
if (this.dragging) {
fill(143, 110, 44);
}
ellipse(this.position.x, this.position.y, this.ballRadius, this.ballRadius);
};

var limbLength = 75;
var leftArm1 = new Pendulum(new PVector(width/2-50, 110));
var leftArm2 = new Pendulum(leftArm1, limbLength);
Pendulum.prototype.handleClick = function(mx, my) {
var d = dist(mx, my, this.position.x, this.position.y);
if (d < this.ballRadius) {
this.dragging = true;
}
};

Pendulum.prototype.stopDragging = function() {
this.aVelocity = 0;
this.dragging = false;
};

Pendulum.prototype.handleDrag = function(mx, my) {
if (this.dragging) {
var diff = PVector.sub(this.origin, new PVector(mx, my));
this.angle = atan2(-1*diff.y, diff.x) - radians(90);
}
};

var limbLength = 75;
var leftArm1 = new Pendulum(new PVector(width/2-50, 110), limbLength);
var leftArm2 = new Pendulum(new PVector(width/2-50, 185), limbLength);
var rightArm1 = new Pendulum(new PVector(width/2+50, 110), limbLength);
var rightArm2 = new Pendulum(new PVector(width/2+50, 185), limbLength);
var leftLeg1 = new Pendulum(new PVector(width/2+40, 230), limbLength);
var leftLeg2 = new Pendulum(new PVector(width/2+40, 305), limbLength);
var rightLeg1 = new Pendulum(new PVector(width/2-40, 230), limbLength);
var rightLeg2 = new Pendulum(new PVector(width/2-40, 305), limbLength);

var limbs = [leftLeg1, leftLeg2,
rightLeg1, rightLeg2,
leftArm1, leftArm2,
rightArm1, rightArm2];

draw = function() {
background(255);

// Draw the body
strokeWeight(7);
line(width/2-50, 110, width/2+50, 110);
line(width/2, 110, width/2, 230);
line(width/2-40, 230, width/2+40, 230);
fill(224, 194, 134);
rect(width/2-25, 39, 50, 64, 30);
fill(0, 0, 0);
strokeWeight(0.4);
ellipse(191,63,20/4,20/4);
ellipse(209,63,20/4,20/4);
fill(255, 255, 255);
ellipse(191,63,3,3);
ellipse(209,63,3,3);
fill(224, 194, 134);
strokeWeight(3);
line(191,91,209,91);
for (var i = 0; i < limbs.length; i++){
limbs[i].go();
}
};

mousePressed = function() {
for (var i = 0; i < limbs.length; i++){
limbs[i].handleClick(mouseX, mouseY);
}
};

mouseDragged = function() {
for (var i = 0; i < limbs.length; i++){
limbs[i].handleDrag(mouseX, mouseY);
}
};

mouseReleased = function() {
for (var i = 0; i < limbs.length; i++){
limbs[i].stopDragging();
}
};

The grader says: make sure you pass the top limbs as arguments to the bottom ones.
How do I that?

Accepted Answer

You did what the hint said, but in the wrong place. Look farther down and you will see a list of where the original program made the new limbs. All you need to do is replace the "new PVector(...)" in leftArm2 with leftArm1, in RightArm2 with RightArm1, etc.

Question 6

Hi,
Im stuck on Step 1 in the Pendulum Puppet Challenge.
I tried both for loop method and calling each limb one by one and both did not get a pass from the Grader.
Example of my for loop:
for(var i = 0; i < limbs.length; i++){
        var limb = limbs[i];
        limb.go();
    }

What do I have to do to get passed step 1?

Accepted Answer

The challenge (like all challenges) has a blue "Report a problem" button.  Use it to complain about your issue.  Then drop the intermediate, simplifying `var limb = limbs[i];` and instead use `limbs[i].go();`

Question 7

I have the program working; however, the grader is not accepting my answer. Can anyone look at my code and assist? It can be found here: https://www.khanacademy.org/computer-programming/pendulum-puppet-problems/5117497249103872

Accepted Answer

You don't need to declare `this.currentOrigin` or `this.even` in the constructor. `this.even` isn't needed. Don't use `this.even` or anything related to it. Just set the pendulum's position predecessor to the variable name of the same limb1

Question 8

I cannot get past part 1 of the next challenge (Pendulum puppet). I know I need to use a for loop, and have this as the draw function:

draw = function() {
    background(255);

// Draw the body
    strokeWeight(4);
    line(width/2-50, 110, width/2+50, 110);
    line(width/2, 110, width/2, 230);
    line(width/2-40, 230, width/2+40, 230);
    fill(224, 194, 134);
    rect(width/2-25, 39, 50, 64, 30);
    
    for(var i = 0; i <= limbs.length; i++){
        limbs[i].go();
    }
};

All I get, however, is an error message saying "Cannot read property 'go' of undefined."
I have not changed anything other than what I showed so how do I go forward? I do not see anything wrong and the error does not point to where the problem is.

Accepted Answer

I'm not sure you want "i <= limbs.length". What worked for me is doing 
"i < limbs.length". The problem may be something somewhere else in the code, but that's the only thing I noticed.

Computer programming

Course: Computer programming > Unit 5

Trig and forces: the pendulum

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