Читать книгу Janice VanCleave's Physics for Every Kid - Janice VanCleave - Страница 11

1 Energy Conservation

The Law of Conservation of Energy states that energy is neither created nor destroyed. Instead, energy can be converted, or changed, into another form of energy. In this activity, the mechanical energy of a pendulum will be investigated. Mechanical energy is the summation of an object's potential energy (stored energy) and kinetic energy (energy of moving objects). A pendulum is a weight, called a bob, hung from a fixed point so that it can freely move backward and forward. Each swing of the bob, from one side to the other forms an arc, as shown in Figure 1. Work is done on the pendulum when it is raised to position A. This means that energy is being transferred to the pendulum. When raised, the pendulum gains gravitational potential energy, which is stored energy due to an object's height. When released, gravity pulls the pendulum down and its gravitational potential energy is converted into kinetic energy.

Gravity is the force of attraction between any two objects with mass in the Universe. The greater the mass, the greater is an object's gravitational force. Earth is very massive; thus, it attracts objects near or on its surface in a direction toward its center. At position A, the pendulum has maximum gravitational potential energy, which is changed to kinetic energy as the pendulum swings down to position B. From position B to C, the pendulum is moving against the downward force of gravity; thus, it slows. During this upward part of the swing, the pendulum's kinetic energy changes into gravitational potential energy again.

FIG 1

The mechanical energy of a pendulum involves the transfer of kinetic energy into potential energy and back to kinetic energy, and so on. It is important to note that the amount of potential energy at position C is less than it was when the pendulum was first lifted to position A (Figure 1). This means the pendulum loses mechanical energy with each swing and each swing is lower and lower until it finally stops. This lost energy was changed into another form of energy, such as heat or sound (air vibration).

See for Yourself

Materials

 string, 8 inch (20 cm)

 tape

 washer with a hole or any comparable weight

What to Do

1 Tie one end of the string to the washer.

2 Tape the free end of the string to the edge of a table (Figure 2).FIG 2

3 Pull the pendulum to the side a short distance and release. It should swing back and forth. Observe the movement of the pendulum. Make note of the pendulum's height during each swing.

What Happened?

In Figure 1 the pendulum is first held stationary at position A, which is higher than position C. This means work has been done on the pendulum by lifting it, giving the pendulum gravitational potential energy. When the pendulum is released, the force of gravity acts on the pendulum pulling it downward. When moving, the pendulum has kinetic energy. Halfway between A and B, half of the mechanical energy is divided between potential energy and kinetic energy. At position B, the potential energy of the pendulum is zero and the kinetic energy is at its maximum. This kinetic energy decreases as the pendulum moves toward position C. Halfway between B and C, mechanical energy is again divided between potential energy and kinetic energy. Finally, the pendulum rises slightly below position C. In this position, its potential energy is less than at the start of the swing. This reduction of mechanical energy decreases incrementally until the pendulum stops moving and hangs vertically at a standstill at position B.