Lesson 7: Mass and Energy
In the last lesson, we saw that at relativistic speeds, we needed to redefine momentum to preserve the law of conservation of momentum. In the same way, we need to redefine energy preserve the the law of conservation of energy. This is why we need to define a relativistic energy for an object with a rest mass of m moving at speed v relative to an inertial reference frame. In the case where the object is not moving, Einstein derived his famous E=mc^2, which states that there may be forces in nature that can transform mass into energy and vice versa. While classical mechanics conserves mass and energy separately, these laws are combines in special relativity to obtain the conservation of mass-energy.
The central concept of this chapter is to understand that a relativistic energy exists and that it is possible to transform mass into energy and vice versa according to the law of conservation of mass-energy.
- Understanding that rest mass is the mass of an object at rest and that it is the only mass used in relativistic calculations.
- Knowing when and how to use Einstein's mass-energy equivalence equation and the equation for the total energy of a particle.
- Realizing that rest mass is a type of energy that can be converted into other forms of usable energy.
To commence the lesson, the teacher could ask the class:
Then have students watch the following video:
The following video explains Einstein's famous equation E=mc^2 in a very fun and interactive manner.
A video discussing how to solve a problem that takes relativistic energy into consideration.
You might wish to do the following for this particular topic:
- Go to the computer lab and have students explore the different applications of Einstein’s E=mc^2 equation and the idea of mass-energy equivalence.
- Have students jot these applications down and choose an application to research further.
- Inform students that they will be sharing their application within their groups next class and that you will be sitting in on each group and listening to the presentations.
- Hirsch, A., Stewart, C., Martindale, D, & Barry, M. (2011). Nelson Physics 12. Toronto: Nelson Thomson Learning