Okay, so we've built a tower.. Now lets do something else: destroy the tower!! We'll destroy the tower with a car with a giant catapult on it.

Lets first start by loading our previous scene, if you haven't done so yet.

Now, we start building the car. First, we use a rectangle for the base.

We zoom in, and create a circle to use as wheel.

Then, we drag that circle on the base, so that the center of the circle overlaps with the base.

And we clone the wheel.

After that, we're going to do something new: We add hinges.

To do this, select both circles, and right click on one for a menu to pop up. In this menu, go to the last option, 'geometries...'

You now see a button called "add center hinge" in that menu. Press it, and our wheels are perfectly hinged.

Start the simulation, and you'll see it will make contact with the ground just fine.

Pause the simulation again, and add a vertical thin and long bar to use for the catapult.

Move it in position

And now select the hinge tool in the tool bar.

With the hinge tool selected, just click where we want the hinge to be.

Now use the right mouse button to rotate the bar into tension position

Then we add something else that's important: the block that stops the bar from over-rotating.

So add another bar, and place it like this:

Select the fixate tool, and use it to fix the bar into place

Now right-click the bar, and go to 'collision menu...', and uncheck all checkboxes.

Then, to make it collide with the catapult-bar still, add a circle on top of the block-bar and fixate it.

Back to the catapult-bar, because we need to add a little basket to keep the projectile into place, and we need to give it force yet.

So build something to contain the projectile

And then go to the catapult-hinge, right-click it and go to 'hinges...'.

Check both the "motor" checkbox and the "reverse" checkbox, and set the following values (click on the input fields to change the values!):

motor speed = 400rpm

motor strength = 2000

As one of the last things, we add a circle to be our projectile. So zoom back in to your basket, and add a circle:

Now, you can run the simulation, but I doubt it will work :) If the projectile hasn't flown too high into the air and actually touched the building, it would just bounce off anyway!

So, we need to do one last thing: change the weights.

Now, the motor strength has been chosen to fit a certain weight of projectile.

• The projectile has a density of 100kg/m2
• The base of the car has a density of 100kg/m2

So set the weights, and check if the simulation works! If it doesn't, just play around a bit with the design and the variables like strength and mass..