In this exercise we will make a self-propelled car with no steering – we will make a steerable, self-propelled car in a later exercise. Don’t forget to save often whenever you work with SketchyPhysics – it is not as stable as most programs. In fact it would even be a good idea to make backup copies as you go – save ‘car_a’ and then two minutes later use ‘save as’ to save ‘car_b’ etc so that even if SketchyPhysics crashes you will not lose too much. The problem is that if you save a version that crashes when you then run it then each time you open that invalid saved file it will always crash when you run it – it may be irreparable.

1.      Make a box using the box tool in the SketchySolids toolbar. Note that you could also make the box using the rectangle and push/pull tools but you would then need to select the whole box (triple-click) and make it into a group so that SketchyPhysics can work with it. Raise the box to about the height of Susan’s head to make sure that the car will be above the ground later and not half-buried in it.

2.      Using the wheel tool , make a wheel centred on one of the bottom corners of the box, oriented like a car wheel. The wheel tool is automatically created with a hinge located where you did the first click.

3.      Place additional wheel in each of the three other corners. Try to give all the wheels the same diameter and thickness. Do NOT be tempted to copy the first wheel – copying a joint or anything containing a joint generally does not work.

4.      Use the Solid Floor tool to create a solid floor shape. Make the floor area much bigger: double-click the solid floor with the select tool to open the floor group for editing then use the push/pull tool to pull the floor sides until the floor is large.

5.      Select a wheel, open the user interface (UI), click on ‘hinge’ to see its properties and set values as follows:  Accel= 20,      Damp(ing)= 20
The final speed of the wheel can be thought of as accel divided by damp. In other words the final speed depends on both these values. Accel can be thought of as the turning force (torque) and damp(ing) can be thought of as the opposing force of friction.

Here is the finished vehicle:

Note that in this exercise we used a ‘hinge’ as a motor rather than using the ‘motor’ joint):

You should have saved you work several times already. Save your work again now and any time you are about to press the Play button for this is the moment SketchyPhysics is most likely to crash. Then press the play button in the ‘Sketchy Physics’ toolbar. The car should drop down onto the solid floor and then propel itself forwards. You can drag the car around with the mouse to stop it falling off the floor. Note that you can usually only pull things around horizontally but if you hold down SHIFT then you can pull things upwards as well as sideways (but no longer towards you or away from you horizontally). A small bug: when you start the animation you must first orbit or pan your view before you can zoom.

To stop and reset the animation, press the reset button. You can also use the play/pause button to pause the animation.

Worth trying: While the animation is running, right-click the vehicle and choose ‘camera track’ to make the camera rotate (and ONLY rotate) to follow the motion of the vehicle so the vehicle never goes out of the window. If you choose ‘camera follow’ then the camera does not rotate, it actually MOVES to keep a constant position compared to the vehicle. If you choose ‘camera clear’ then the camera is reset so that it no longer moves or rotates during the animation.

Check out the 2.5 minute video below! It's even available in HD if you click the HD button, and you can download it from www.vimeo.com if you want!