A pendulum swings because the force of gravity pulls down on it once it has been released from a higher position beyond its at-rest position. Momentum keeps the pendulum moving past the at-rest hanging point. Gravity then acts again to pull it back toward the hanging point. Prove that when a weight tied to a string is heavier than a lighter weight tied to a string of equal length, and both are left to swing like a pendulum, the heavier weight will swing longer.
Gravity exerts an equal force on all objects the same distance from the earth's surface. Momentum is the product of force times mass. Drop a table tennis ball and a golf ball from the same height into a mound of soft modeling clay. The force of gravity pulling down on both balls is the same, but the mass (weight) is different. Momentum will be stronger on the golf ball when it hits the clay; you will see the evidence of this greater momentum in the larger impression it makes.
Centripetal force that acts on an object moving in a circular path, with acceleration toward the center point. This force opposes centrifugal force, which pulls a moving object outward from a circular center point. Using a remote-control model racing car, run the car at a speed high enough for it to skid as it makes a turn. Here, centripetal force is the resistance from friction between the tires and the road surface. When the model car's speed becomes high enough that the centripetal force can no longer push it around at that speed, it skids.
Demonstrate that energy can be transferred from one object to another while also teaching the concepts of kinetic and potential energy and their relationships to gravity. Lay three or four marbles in a row on a flat surface. Set one end of an empty paper towel roll up on several books, making a ramp. Position the bottom end of the tube against the first marble in the row. Place a marble between your fingers, and hold it near the top end of the tube. At this point, the marble is said to have "potential energy." The marble is positioned above the surface of the earth, and it has the potential to do work thanks to the force of gravity pulling down on it. Let go of the marble. As it rolls down the tube, work is taking place, called "kinetic energy." When the marble reaches the bottom, it hits the row of marbles and they begin to roll. Some of the kinetic energy in the rolling marble becomes transferred to the row of marbles.