Question

To start a pendulum swinging, usually you pull it slightly to one side and release.

• What kind of energy is transferred to the mass in doing this?
• Describe the energy changes that occur when the mass is released.
• Is/are there any other way/ways to start the oscillations of a pendulum? Which energy is supplied in this case/cases?

Answer 1

• Pulling the bob of a basic pendulum to one side raises it slightly higher. Consequently, it obtains gravitational potential energy.
• When the bob is released, it performs simple harmonic motion (SHM) in a vertical plane. During this motion, energy continuously changes between gravitational potential energy and kinetic energy. At the extreme position, the bob has maximum potential energy and zero kinetic energy. As it moves towards the mean position, potential energy changes into kinetic energy. At the mean position, the kinetic energy is maximum and the potential energy is minimum. As the bob moves towards the opposite extreme, kinetic energy is again converted into potential energy. In undamped SHM, this continuous exchange of energy repeats without any loss of total mechanical energy.
• The bob can be set into motion either from its equilibrium position or from any other point along its path. If it starts from the equilibrium position, its initial energy is entirely kinetic energy. If it starts from another position, its initial energy is shared between kinetic energy and gravitational potential energy.