You are here:

Physics/Energy Mass relationship


We understand that energy (in Joules) equals a certain amount of mass (in kilograms). We also know from an electronics aspect that Joules can be measured in watts/second. I have a question however. There is something called peak power. If I have a 1 watt power pulse with a width of 1 femtosecond, though the average power is 1 watt (1 joule), the peak power would be around 9x10^14. This 1 joule of power reflects a certain mass. When I get peak power, does the mass per pulse change, or is the mass constant?

No, joules=watts*seconds, not the other way around.  A Joule/second is a Watt.  And if you have a joule in  a femtosecond, you have 10^15 Watts of average power.  Mass and energy are related by E=mc^2, where c is the speed of light (3*10^8 m/s).  I don't know where you got that number, it makes no sense.


All Answers

Answers by Expert:

Ask Experts


Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

©2017 All rights reserved.