Activity Based Science Learning >Why does it go up? Objective: Things move in the direction in which the potential energy decreases. Equipment: Two cycle-spokes, two plastic funnels, adhesives like fevicol or fevi-quick. Introduction:Bodies put on an inclined plane come down the plane. If it can roll it rolls down.  This can be understood in terms of potential energy. By coming down, the gravitational potential energy decreases. A system under given conservative forces moves in such a way that the potential energy decreases. This demonstration emphasizes this principle in a dramatic way as in the first look; the object seems to go up on an inclined plane. A careful analysis reveals that indeed it is decreasing its potential energy. Procedure: Join two plastic funnels at the rim with the help of an adhesive. You can even use Cello-tape to join them. This makes a double-cone.Arrange to make a rail by two spokes in such a way that the height of the rail as well as the separation between the spokes gradually increases. You can take two pieces of thermocol and push the spokes properly in them to make the structure. The plane of the spokes becomes an inclined plane. You will have to adjust the geometry by trial for the demo to work. The height of the rail increases from A to B. Place a cylindrical object like a pencil near the top of the rail and see that it comes down as expected. Now place the double cone near the top and it wont come down. Place it near the bottom. It goes up and settles near the top of the rail. Students will enjoy the scene. Guide them to the Physics of it. Because of the geometry, as the double cone goes from A to B, supposedly up the rail, more and more portion of the middle bulge goes between the spokes and the cone actually dips. You can measure the height of the straight tube part of the funnel above the table when the double- cone is near the bottom of the rail and when it is near the top of the rail. It is less in the later case showing that the double cone is actually going down while it seems to go up. Discussion:Encourage students to make their own equipment. The double-cone made by funnels has a particularly simple and effective geometry. But they can even do it with spherical balls. The only skill is in making a proper angle between the two spokes so that the object dips as it goes ahead on it. You can also discuss motion of extended bodies using this apparatus. Different parts of the double cone will move along different paths. Some portions of the cone will come up and some will go down. Gravitational potential energy of some portions will therefore increase and of others will decrease. The overall potential energy of the system is decided by the motion of the Center of Mass. If the height of the center of mass above a horizontal surface increases, the potential energy increases too. Similarly, if the height of the center of mass decreases, the potential energy also decreases. Ask the students to mentally locate and follow the motion of the center of mass as the cone goes up. Indeed these intensive discussions are to be made outside the class where students do the experiments and observe. When to use: As an interesting demo it can be shown to anyone, even children of age 5 will enjoy. However for Physics teaching you can use it when talking about the potential energy and the fact that system moves in a direction to decrease its potential energy. Most likely this is at class 11. Instrument cost and availability: Negligible cost. Available easily in any market. In place of cycle spokes, you can use the rods (called SALAI in Hindi) on which ladies knit sweaters