# What is relativistic mass and rest mass

1. Apr 23, 2010

### Rico L

what is "relativistic mass" and "rest mass"..

as far as i know the rest mass is the mass which measured by an observer who is at rest relative to the object (please correct me if this is not right), if this is right.... what if when the object is travelling at the exactly same velocity of the observer.. does "rest mass" also applies to this situation?

relativistic mass is the mass of an object measured by an observer who is moving relative to the object or the mass of an object moving relative to an observer who is measuring the mass entitled to regard himself as stationary. i mean if this is right, in which way, how can the mass be measured if the object is moving.. ?

i am so confused...

thanks

2. Apr 23, 2010

### mgb_phys

Re: what is "relativistic mass" and "rest mass"..

With small particles where their energy when they are moving is larger than their rest mass we can measure their relativistic mass.

So an electron has a rest mass of 511Kev (we even measure rest mass in units of energy) at high speed we can determine it's mass by the momentum effects when it hits something.

But yes it's diffcult to put an electron moving near the speed of light on a set of scales!

3. Apr 23, 2010

### LostConjugate

Re: what is "relativistic mass" and "rest mass"..

This is the same thing as being at rest relative to the observer.

It could be measured by applying a small perpendicular force to the object. As it travels by you can measuring by how much it's path changes direction. mass resists force.

It can also be calculated because 2E/v^2 = m so the connections between Velocity and Mass are clear.

4. Apr 23, 2010

### Janus

Staff Emeritus
Re: what is "relativistic mass" and "rest mass"..

Yes, all that counts is the relative velocity between the object and measurer. Either one's relative motion relative to any other reference doesn't matter.
By measuring how much effort it takes to change the velocity of the object.

5. Apr 23, 2010

### Rico L

Re: what is "relativistic mass" and "rest mass"..

thanks a lot guys ! :)

6. Apr 23, 2010

### starthaus

Re: what is "relativistic mass" and "rest mass"..

Yes, the rest mass can be inferred from deflection experiments :

$$m_0=\frac{qB}{\omega \gamma(v_0)}$$

or

$$m_0=\frac{qBr}{v_0 \gamma(v_0)}$$

where:

B=magnetic induction
q=particle charge
$$\omega=2 \pi/T$$