What is the difference between "mass" and also "rest mass?" does this typical that massive is not always the same?

Nuclear fragment tracks in the ten-inch balloon chamber an installed inside a superconducting magnet at... <+> Argonne display what taken place to two an adverse K mesons that gotten in the bubble chamber from Argonne"s ZGS. C.1966 picture credit: united state Department the Energy, publicly domain

This is a an excellent question, and likewise a good reminder to be mindful with one’s language once writing about physics!

Most that the time, if you’re analysis an article, what we mean by mass and also rest mass is precisely the same. Rest mass is a slightly an ext precise hatchet in the phrasing, meaning specifically the massive of that object as soon as it is in ~ rest, relative to the person measuring its mass. This is almost always how we measure mass. If you’re in a rap (or a kitchen), and you measure up an object’s load on a scale, that object is not relocating at any kind of speed. If it is moving at a speed, you must probably catch it, because it’s roll off your scale.

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If you’re on Earth, you take the weight -- i beg your pardon is the pressure with which that object is pushing on your scale, in ~ our Earth"s gravitational field --and have the right to then convert it into a mass. This mass you’ve measured is the "rest mass", since nothing’s moving in this scenario. Typically this number -- the rest mass -- is the most helpful metric of exactly how much material is assembled into everything you’ve measured, so “rest mass” is regularly abbreviated right into just consistent old “mass”.

However, over there is a means to make an object sort of behave together if it has a much bigger mass than its rest mass, and also given my pointing out how nothing’s relocating in the an interpretation of remainder mass, you will rightly guess the it has actually something to execute with motion. However, it’s not just any motion -- one egg rojo off your kitchen scale isn’t any heavier than the egg that managed to continue to be balanced.

You’d need to accelerate that egg come a far-ranging fraction the the speed of light prior to the egg started behaving together though that were heavier (and if you have the right to do that without crushing the egg, you’ve won the world’s most daunting Egg drop Challenge). It’s an essential to keep in mind that the egg itself is no intrinsically heavier, and so we’re mindful to keep “rest mass” different from this relativistic speed effect. However, since the egg now has actually so much kinetic power from relocating so fast, and there is an equivalence between mass and energy (hello E=mc²), the energy of the object have the right to masquerade as additional mass by including to the object’s momentum.

A Newton"s Cradle toy in Darlington-Park in Mülheim one der Ruhr. Photo credit: frank Vincentz, CC BY... <+> SA 3.0

The faster our relativistic egg is moving, the bigger this energy-mass bonus gets, and also the egg gains more and an ext momentum. The relevant attribute of momentum below is that it resists transforms to its speed. In particular, for our relativistic egg, the faster and also faster the goes, the more momentum that has, therefore the harder it is to rate it up any more --you must start expending truly ludicrous quantities of energy to speed it up also a small bit. This is one fifty percent of the factor it’s physically impossible to accelerate an item all the method to the rate of light -- the closer you get, the closer to infinite energy you require to continue speeding the up.

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comparison of Newtonian and relativistic momentum. 1 on the x axis is the speed of light. As you... <+> method the speed of light, in a relativistic framework, inert rapidly philosophies infinity. Picture credit: Wikimedia user D.H, CC BY-SA 3.0

So you have the right to safely substitute “mass” anytime you see anyone cite a “rest mass,” but there are cases in which an object might behave together though it has more mass 보다 its “rest mass” -- this are, however, limited to objects traveling a far-ranging fraction of the rate of light.


I am an Assistant Professor in Physics & Astronomy in ~ Oberlin College. I obtained my Ph.D in 2014 native the university of Victoria, researching the interactions between…

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I to be an Assistant Professor in Physics & Astronomy at Oberlin College. I got my Ph.D in 2014 from the university of Victoria, researching the interactions in between galaxies in the nearby Universe. I have been running an outreach blog called Astroquizzical since 2013, i beg your pardon solicits questions around outer an are from the general public. Astroquizzical tackles all kinds the questions, and also is meant as a ar for friend to get answers to those nagging concerns you’ve never had the opportunity to ask. You can discover me ~ above Twitter