Conversion of Gravitational Potential Energy into Mechanical Energy

The Idea

Energy can neither be created nor destroyed. One form of energy, however, can be changed into another form of energy. For instance, the combustion that occurs in a car engine produces heat energy, which, in turn, is converted to mechanical motion by the motor. In this project, you convert gravitational potential energy into mechanical energy.

What You Need

• wheels: old CDs or DVDs
• post to hold the weight: a dowel 1 m in length and ½ to 1 inch in diameter
• 1 kg weight
• string
• axles (dowels work well)
• materials to build the body of the car
• plumbing fitting or a block of wood to attach the post to the car body
• tape measure
• stopwatch

Method

1. Assemble the car. Use Figure 55-1 as a guide, but feel free to develop and build your own concept.
2. The basic design criteria for the car includes:
   • The CD (or equivalent) used for wheels should turn freely.
   • The descending mass should freely turn an axle to produce forward motion of the car.
   • The car should be balanced with the mass in both elevated and falling positions. (Remember, a 1 kg mass raised 1 meter above the ground can exert a lot of torque that could topple the car.)
   • The mass should fall onto the car after it descends (rather than dragging along the ground, which can limit how far it goes).
   • There should be enough symmetry between left and right, so the car moves forward, rather than turning.



3. Wrap the string around one of the axles and attach the other end to the weight. Decide if you want front-wheel or rear-wheel drive and wrap accordingly.
4. "Arm" the car by raising the mass and leaving some of string still wrapped around the axle it will turn.
5. Orient the car on a designated course, and then release the mass.
6. Measure how far it goes.
7. If several individuals or groups are involved, it may be fun to do this as a race to see which car goes the farthest or which car crosses the finish line first.

Expected Results

It might take a few hours to build the car(s), depending on what materials are available. If built correctly, the car should move as the mass descends. Once the mass falls and comes to rest (somewhere on the car), the car has enough momentum to keep moving.

Why It Works

A mass at a height contributes an amount of energy equal to its mass times the height above the ground times gravitational acceleration. In this case, 1 kg dropping a distance of 1 meter will contribute 9.8 joules of energy. (One joule is about the amount of energy needed to lift an apple 1 meter.)

Other Things to Try

A variation of this is to use a mousetrap to supply the energy. The mousetrap is started in the open position. As with the previous kilogram car, the potential energy stored in the spring of the mousetrap is transferred to the forward motion of the car.

The Point

Energy is conserved. The potential energy you start with equals the kinetic energy given to the car (plus any energy lost to friction).

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