- Home
- Reference
- Education
- Directories
- Science eStore
- Job Board
- Ask The Experts
- eGreetings
- Fun
- About Us

Welcome to PhysLink.com - Your physics and astronomy online portal. Stay a while! Check out our extensive library of educational and reference materials. Also, check out our fun section!

QuestionWhat is rest mass? How to explain it especially to the high school students? Asked by: Alper Akyurek AnswerOne of the consequences of Einstein's special theory of relativity (1905) is that the mass of an object increases with its velocity relative to the observer. When an object is at rest (relative to the observer), it has the usual (inertial = tendency to resist an applied force) mass that we are all familiar with. This is called the 'rest mass' of the object. As the speed of the object is increased, the inertial mass of the object also increases. For speeds significantly less than the speed of light, the increase in mass is nearly imperceptible, but as the speed of light is approached, the mass starts to increase very rapidly toward infinity. Theoretically, the mass would become infinite if the object could be accelerated all the way to the speed of light. However, because the acceleration of an object in response to a given force is inversely proportional to its inertial mass, as the speed of light is approached the force required actually to reach the speed of light also becomes infinite. Therefore, it is impossible actually to accelerate an object with non-zero rest mass to the speed of light. Another way of expressing the fact that a massive object cannot be accelerated to the speed of light is through the concept of energy. That is, an infinite amount of energy would have to be expended, via the accelerating force, to reach the speed of light. So, quite simply, the 'rest mass' of an object is the inertial mass that an object has when it is
at rest.Answered by: Warren Davis, Ph.D., President, Davis Associates, Inc., Newton, MA USA Mass measures the amount of inertia an object has, with inertia defined as the resistance the object offers to a change in its state of motion. While we usually think of mass as being constant for an object, Relativity tells us that energy and mass are interchangeable. Any object in motion has, by definition, Kinetic Energy. That energy has the effect of increasing the object's mass, so the MINIMUM mass of an object is its mass when stationary, or its
REST MASS.
Mathematically, an object's true mass is given by the expression: where m is its mass at rest, _{0}v is its velocity, c is the speed of light.
You can see that when v=0, square root of (1-0) equals 1 and m = m. As velocity increases,
(1-v_{0}^{2}/c^{2}) becomes less than 1, so square root of (1-v^{2}/c^{2}) also becomes less than 1, and m becomes
greater than m. In fact, as _{0}v gets close to c, the square root of (1-v^{2}/c^{2}) value becomes so small
that when divided into m, _{0}m represents an inertia too great to overcome and actually
attaining a velocity equal to c becomes impossible. At that point, dividing m by 0 gives
an infinite mass.
_{0}Since c is over 186,000 miles/second, we are not accustomed to dealing with objects travelling at velocities high enough to have a mass noticeably greater than their rest mass. Experiments with subatomic particles, however, confirm Relativity's prediction of increasing mass at speeds near c. Devices from particle accelerators to TV picture tubes must account for mass increase in order to work properly. Answered by: Paul Walorski, B.A. Part-time Physics Instructor |

Loading

a FREE ride with Uber!

sign up for Birchbox

To the Master's honor all must turn,

Each in its track, without sound,

Forever tracing Newton's ground.'

(

10 Ways to Keep Your Kids Interested In Science

Young children are natural scientists: they ask questions, pick up sticks and bugs outside, and are curious about the world around them. But as they get a bit older, many kids gradually lose their interest in science. They might see it as just another task at school, something that doesn't apply to their lives. Of course nothing could be further from the truth, so here are ten ways you can remind your kids that science is everywhere. Most of these are fun for adults, too! Continue reading ...

Here are our physics & astronomy bestsellers:

KonusScience 5 Way Microscope Kit

Mini Plasma Ball

3D Magnetic Field Tube

Scorpion, Ant, Wasp and Flower Bug

Alnico Bar Magnet - 6 inch Long

Solar Radiometer

Weather Station 4M Kit

Cherry Wood Levitron

12 inch Galileo Thermometer

Revolving Multi-Color Fiberoptic Light