# Q & A: mass and speed

Most recent answer: 10/22/2007
Q:
Hello, I wish to have somthing cleared up. I was wondering why an object composed of fermions increases in mass the faster they are going. My hypothisis is that as they go faster they gain more kinetic energy and since mass is equivalent to energy they weigh more. Am I right or completly wrong
- jake (age 13)
bishop pinkham jr.high, calgary
A:
You're right.

The mass of an object which consists of many pieces stuck together depends on the state of motion of the pieces. If you heat up an object, its atoms jiggle around faster. Metals even let electrons move freely inside, and a hot metal will have electrons moving more quickly. Because the component pieces have more energy when they are moving faster, the total energy of the object is greater. The total energy may include that of photons too, and so it isn't just the fermions which contribute. The mass of an object at rest is its total energy divided by the square of the speed of light. The object will weigh more on a scale if it has more mass.

If the entire object is moving, it's hard to put it on a scale in order to weigh it. If the Earth moves along with the object (or equivalently, a stationary object on a stationary Earth as viewed by a moving observer), then the amount of force a scale reads remains the same. If the object flies past the Earth at high speed, then the force due to the earth's gravity on the object does increase with the speed of the object, so one can say that in this case it weighs more too.

Mike W. and Tom J.

(published on 10/22/2007)