Inertia Demonstrator, B3-1110.00, £16.50
This is a modern day version of a classic experiment demonstrating the principle of inertia. A plastic ball is placed on top of the small rectangular sheet of perspex. The metal spring is pulled back. When it is released the perspex shoots off and the ball falls neatly into the cup at the top of the pillar.
To add a little bit of drama to the experiment you could invite a pupil to catch the ball once it is fired!! Their surprise at the ball not travelling horizontally may help cement the concept.