Energy Conversion in the Pole Vault

The pole vault is a difficult track and field event that requires a combination of speed, strength, timing, and energy conversion. In the pole vault, an athlete uses a flexible pole to propel himself over a high bar. Look at the pole-vaulter's jump in Figure 14 and think about how energy changes during the jump.

In order to start the jump with as much kinetic energy as possible, the pole-vaulter sprints down the runway as fast as he can. At the end of his sprint, he plants the end of a long pole at the base of the high bar and propels himself into the air. The pole-vaulter's kinetic energy is partially converted into elastic potential energy as the pole bends. The pole springs back into shape, propelling the pole-vaulter upward, hopefully high enough to clear the bar.

As the pole-vaulter soars, his kinetic energy decreases while he gains gravitational potential energy. Once the highest point has been reached, his gravitational potential energy begins to convert back to kinetic energy. The pole-vaulter picks up speed as he falls back to the ground.

Energy Conversion Calculations

When friction is small enough to be ignored, and no mechanical energy is added to a system, then the system's mechanical energy does not change. Recall that mechanical energy is the total kinetic and potential energy of an object.

$\mathrm{Mechanical} \mathrm{energy} = \mathrm{KE} + \mathrm{PE}$

You can apply the law of conservation of energy to any mechanical process. A mechanical process can be any action, such as the motion of a pendulum, water falling in a waterfall, or a diver propelled by a diving board. In all of these processes, if friction can be neglected, the mechanical energy at the beginning equals the mechanical energy at the end. That is, total mechanical energy remains constant. This equality can be stated as follows.

Conservation of Mechanical Energy

$(\mathrm{KE} + \mathrm{PE}{)}_{\mathrm{beginning}} = (\mathrm{KE} + \mathrm{PE}{)}_{\mathrm{end}}$

The Math Skills box on the following page shows how the conservation of mechanical energy equation can be used.

Figure 14 A pole-vaulter converts kinetic energy into potential energy in order to propel himself high into the air. Applying Concepts At which point does a pole-vaulter have the most gravitational potential energy?

---

End ofPage 457

Table of Contents