A mass M is hung from a spring. It is then pulled down slightly and allowed to vibrate vertically between P and Q.

Which is correct?

Energy at point P | Energy at point Q | |

A | kinetic | Kinetic |

B | Kinetic | Potential |

C | Potential | Kinetic |

D | Potential | Potential |

The answer is D. Here’s the explanation:

When the spring is pulled, the force exerted to pull the spring and the weight of the mass M transfer energy to the spring. The spring gains in elastic potential energy stored in the spring. When the mass is released and allowed to vibrate between P and Q, the stored elastic potential energy in the spring is converted to kinetic energy as the mass moves up and down.

At the centre of vibration, the kinetic energy is at its maximum. However, beyond the centre of vibration, the spring is either compressed or stretched. Whenever the spring is compressed or stretched, there is stored elastic potential energy. This elastic potential energy is converted from the kinetic energy.

At point P and Q, all kinetic energies are converted to elastic potential energy. Hence at point P and Q, the spring has stored elastic potential energies and zero kinetic energies. As the system is above ground, at point P and Q, the system has gravitational potential energies.

Hence, the answer is D, there are potential energies at both Point P and Q.

The above is a suggested solution for GCE O Level Physics, Year 2011 November, Multiple Choice Question 11 in Paper 1.

Spring icon from flaticon.