Friday, 5 July 2013

Feynman on kinetic model

This week I am doing work and energy with my Y3 students. We started with the concept of work and went on to the activity bouncing ball activity shown below.

Bouncing Ball Activity

  • Describe the conversion of energies during the motion from point of release to just before hitting the surface 
  • Explain why the returned height after each bounce is less

Students' explanation on why the ball has a lower height after each bounce

It is a rather common for learners to associate any lost in mechanical energy as "being converted to heat". It is probably because of the fact that they have experienced the warmness due to phenomena such as rubbing their palms, hitting a nail, etc.

Qualitative understanding of the energy exchange during the bounce

What really happens requires the understanding of the following concepts:

  • Temperature is a macroscopic observable/measurable of the average kinetic energy of random molecular motion
  • The contact between the ball and the floor increases the random molecular motion at the expense of the ordered mechanical energy. This causes the temperature of the ball and floor to rise or we say that the internal energy of the system increases.
  • Assuming that the ball and floor is at thermal equilibrium to the surrounding, the increase in their temperatures result in a net transfer of heat to the surrounding, i.e. the warmness we feel if we are to touch them (the temperature difference is likely to be less than what we can perceive in reality).
In addition, we can also apply the same thinking to conclude that the movement of the ball through the air also increases the internal energy of both the ball and the surrounding air.

Feynman's explanation below (from about 1min45sec to 3min15sec) really sums it for learners and most importantly encourages them to enjoy thinking about science.