Specialized Javascript material developed by Christopher Orban  Published May 12, 2017
Subject Area  Mechanics 

Levels  High School and First Year 
Specialized Implementation  Javascript 
Learning Objectives 
Students who complete this exercise will be able to:
1. Create an interactive model of a ship traveling through free space that can rotate freely and apply thrusters (constant torque) to change the rotation rate by modifying the Planetoids code (Steps 1  3)
2. Demonstrate how velocity, force, acceleration and torque vectors affect the motion of the ship (Steps 13)
3. Demonstrate how the motion of the ship depends on the mass and length of the ship as well as the force of the thrusters by changing these values and trying to survive in the game as long as possible (Steps 45)

Time to Complete  60 min 
Step 0. Opening 2D Planetoids code
For convenience we have supplied an "old" version of the planetoids game so you can play it and remember the feel of the game without the craft spinning around like mad. [Click here to open the planetoids code](http://alpha.editor.p5js.org/ChrisOrban/sketches/BJzCa2fe) This version is similar to the earlier planetoids game except the name is changed and it actually has planetoids. There are also some new variables that we will use later.Very Important: Sign in to your account! Then click "Duplicate" so you can have your own version of the code!
Step 1. Think about thrusters and spin (no coding needed in this step).
Imagine that these thrusters work through clever engineering that diverts the thrust of the main engine so that half of this thrust goes to the front thruster and the other half goes to thruster at the back of the ship. ![](images/PlanetoidsTorque/torque.png "")
We can figure out the net torque on the ship by assuming that the center of mass is at the center of the ship (halfway from either end).
For counterclockwise torque: $$\tau_{\rm net} = \sum_i \tau_i = +\left( \frac{F_{\rm thrust}}{2} \right) \cdot \left(\frac{L_{\rm ship}}{2}\right) + \left( \frac{F_{\rm thrust}}{2} \right) \cdot \left(\frac{L_{\rm ship}}{2}\right) = \frac{F_{\rm thrust} L_{\rm ship}}{2} $$
For clockwise torque: $$\tau_{\rm net} = \sum_i \tau_i = \left( \frac{F_{\rm thrust}}{2} \right) \cdot \left(\frac{L_{\rm ship}}{2}\right)  \left( \frac{F_{\rm thrust}}{2} \right) \cdot \left(\frac{L_{\rm ship}}{2}\right) = \frac{F_{\rm thrust} L_{\rm ship}}{2} $$
There are no other sources of torque in the planetoids game. This means that we can use the equations above to figure out the angular acceleration ($\alpha$).
Angular acceleration: $$\alpha = \frac{\tau_{\rm net}}{I}$$
where $I$ is the moment of inertia (sometimes I call this the rotational inertia). We will talk about what to use for the moment of inertia later.
From the angular acceleration ($\alpha$) we can figure out the angular speed ($\omega$): $$\omega_f = \omega_i + \alpha \cdot \Delta t$$
Finally, from the angular speed ($\omega$) we can figure out the angle that the ship is pointing: $$\theta_f = \theta_i + \omega \cdot \Delta t $$
In what follows we will turn these equations into code.
Step 2. Make changes to the code so that the ship can spin around.
You will be given lines of code to add to your program but it is up to you to figure out the right place in torque.js to put it.
First, comment out the lines in torque.js that change theta
using //
if (keyIsDown(LEFT_ARROW)) { // theta += 0.05; } else if (keyIsDown(RIGHT_ARROW)) { // theta += 0.05; } else if (keyIsDown(UP_ARROW) ) {
Next add code so that pressing the left arrow produces a positive angular acceleration like this:
ang_accel = torque_thrusters/Iship;
and pressing the right arrow produces a negative angular acceleration like this:
ang_accel = torque_thrusters/Iship;
Now use this angular acceleration to change the angular speed (omega
) as in Eq. $\ref{eq:omega}$ by adding code like this:
omega = omega + ang_accel*dt;
In the code above, notice that the omega
on the right side is the old omega
value ($\theta_i$) and the omega
on the left is the new value of omega
(which is $\theta_f$).
Finally, add an equation so that theta
is determined from the initial angle ($\theta_i$) and the angular speed ($\omega$).
theta = theta + omega*dt;
In the above I have used the same trick where theta
on the right side is the old value ($\theta_i$) and theta
on the left side is the new value ($\theta_f$).
You also need to add something that stops the ship from rotating out of control, similar to how there is a something to stop the ship from accelerating out of contol in the x and y direction.
Your program should not work yet because you haven't specified the torque or the moment of inertia yet. If you run the code the ship will only be able to move in the forward direction and not change direction at all.
Step 3. Specify the torque and moment of inertia for the ship.
Near the beginning of the code and shortly before function setup() there are four new variables. Here they are:
ang_accel = 0; Iship = 0; Lship = 0; torque_thrusters = 0;
Currently, these variables are just set to zero but you are about to change that. Follow the syntax of other variables like Fship, mass and dt.
Set Lship
equal to 100.
Consider the comments on torque in Step. 1 and modify torque_thrusters like this:
torque_thrusters = Fthrust*Lship/2;
Does the line above make sense to you? Why is there a factor of 2? If not see Step 1.
Now consider the variable Iship, which is the moment of interita for the ship. Look up the formula for the moment of inertia of a rod of length $L_{\rm ship}$. Follow the syntax of other variables and use this formula to determine Iship
. Before you do this read through these hints:
Hint #1: use a decimal instead of a fraction in the formula for Iship
. < Very important!
Hint #2: use Lship*Lship
instead of Lship^2
.
Hint #3: Remember that in the code the mass of the ship is just the variable mass
.
If you modify all these lines of code in the right way [you should get this behavior](http://www.physics.ohiostate.edu/~orban/physics_coding/torque_files/v1/index.html)
Step 4. Add a timer and see how long you can survive.
Right after display();
, add this line:
t += dt; drawText("time = ",0.8*width,0.9*height); drawText(t,0.85*width,0.9*height);
If successful your code [should behave like this](http://www.physics.ohiostate.edu/~orban/physics_coding/torque_files/v2/index.html)
Step 5. What happens when you change Lship
, mass
and Fthrust
?
Experiment with different values of Lship
, mass
and Fthrusters
. In what you turn in for the lab, comment on the effect of changing each of these variables (make one variable larger and try the game, then change another variable, etc.) What do you think are the best values to use for surviving the longest in the game? Add comments to the end of your torque.js code or in the comments in the dropbox submission.
Extra credit: Add a projectile to the game
Option #3 In the original planetoids programming lab was to add a projectile to the ship. This projectile will travel at constant speed in whatever direction the ship is pointing when you press the spacebar. Either reuse the code you used to add a projectile to the earlier lab or develop it for the first time here. In what you turn in for this lab, indicate whether you reused old code or developed the projectile code for the first time.
Bonus Points: If the projectile hits an planetoid, the planetoid should shrink in size or split in two.
How to get full credit for this programming lab!!!
**1. Make sure the code works as intended** Make sure that your ship, once spinning, will continue spinning without speeding up or slowing down. A lot of people don't manage to do this. Common problems are that the ship only spins when you press the left and right arrow keys. Or once you press the left or right arrow keys, even if you let go of these keys, the angular acceleration is constant and it spins faster and faster. Make sure your ship just spins without speeding up or slowing down when you are not pressing the left and right arrow keys. In other words, [make sure the ship behaves like this](http://www.physics.ohiostate.edu/~orban/physics_coding/torque_files/v2/index.html) **2. Figure out the best combination of Lship, mass and Fthrust as discussed in Step 5** Make sure to include in the comment box when you submit your code the values that you think make it the easiest to survive. If you forget to do this you may lose points.
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Credits and Licensing
Christopher Orban, "Planetoids with Torque!," Published in the PICUP Collection, May 2017.
The instructor materials are ©2017 Christopher Orban.
The exercises are released under a Creative Commons AttributionNonCommercialShareAlike 4.0 license