Balancing nails can demonstrate to students that any type of object can have a balancing point, and can seem to resist gravity. Collect 12 nails of the same size. Hammer a nail partway into a block of wood. Set the block aside. Lay another nail horizontally onto a flat surface with the point facing to the left. Place nails across the horizontal nail with their heads close the the horizontal nail, four on one side of the nail and five on the other, alternating sides. After you have placed the nine nails along the horizontal nail, place the last nail on top of the cross-nails horizontally with the point facing to the right. Holding on to the two horizontal nails, lift the entire structure, then place the bottom nail, centered, on the top of the nail that has been hammered into the wood block. The entire structure will be able to balance without falling.
A pendulum works with the force of gravity to swing to and fro. Poke two holes under the rim of a paper cup. Tie two 24-inch pieces of twine onto the cup. Tie the other end of the pieces of twine together. Tie a third 24-inch piece of twine onto the top of the other two, and attach it to the middle of a door frame using duct tape. Fill the paper cup with soil. Swing the cup through the door frame. The pendulum will swing in and out of the door frame by itself for several seconds.
You can demonstrate centripetal force to students by showing them how a penny can spin for a long time inside a balloon. Insert a penny in a clear deflated balloon. Blow up the balloon, then knot the end. Hold the balloon with the tied end toward the ceiling and your hand over that end, then twirl the balloon so that the penny spins around inside. Then place a finger of your other hand onto the bottom of the balloon to keep it still, and the penny will continue to travel around the inside of the balloon for a long time.