The SketchyPhysics plugin, once added to Sketchup, allows you to create objects that can be pulled around, can collide, can fall, can be driven by a motor etc. The plugin can be downloaded free from the internet. SketchyPhysics is experimental software and is not completely stable therefore you should SAVE YOUR WORK OFTEN when using SketchyPhysics.

It is very important to understand that SketchyPhysics only works with GROUPS. Only groups can move, collide etc. Objects that you have made which are not groups will not move and nothing will be able to collide with them – they will behave like ‘ghost’ objects. You can either make your own groups or use the ready-made group shapes (cuboid, cone, cylinder, sphere etc) that are available in one of the SketchyPhysics toolbars.

It's also very important to understand that SketchyPhysics can only do its complex calculations fast enough to give smooth animations if it works with simple shapes like cuboids or cylinders. Therefore if you make a more complex shape into a group SketchyPhysics will have to approximate the shape to a simpler one - the shape you see and the shape that SketchyPhysics is using may be quite different ... more on this later.

Make sure that the SketchyPhysics plugin is installed and that the four SketchyPhysics toolbars are visible (if necessary choose View>Toolbars>…  to make them visible). The four toolbars are as follows:

Sketchy Solids allows you to make standard shapes that are already groups and thus ready to take part in SketchyPhysics animations. Available shapes are box (cuboid), sphere, cylinder, cone, capsule, solid floor and three more advanced shapes. The floor shape is unlike the others since by default it will be locked in place and will not fall like the other shapes. Without a floor, all your objects will fall out of view in no time…

SketchyPhysicsJoints include hinges, springs, motors etc.
 

Sketchy Physics allows you to run your animation, to pause it (with the same run/pause button), to reset it, to display a control panel (‘user interface’ or ‘UI’ for the selected object). The UI is also called the ’inspector’. This toolbar also contains the ‘joint connector’ tool which you will use to connect joints to objects.