Understanding Relativity: An Introduction for A-Level Students

[Al3ks]

I'm no scientist but I am learning A-level Maths and Physics, so if there is mathematics involved try not to make it too advanced. Thank You!

If you have a mass (A) in one frame of reference (1) and it is staionary in this frame and is weighed and has a certain mass (x). There is another frame of reference (2) and frame 1 is moving relative to frame 2. If mass A is slowed down to being stationary in frame 2 and is then weighed to give a mass (y) is mass x larger than mass y due to the kinetic energy adding mass? (assume accurate enough instruments or large enough velocities, etc. to make this noticeable)

Thanks
Alex

 

[Nabeshin]

x=y.

That is, the mass as measured when the object is stationary in their reference frame is the same. This mass is known as the rest mass of the object (For obvious reasons).

 

[Al3ks]

OK thanks and if mass y were measured from frame 1 x < y? and vice versa?

 

[Nabeshin]

That's right. The "mass" of the object measured when it is in a different inertial frame is always larger than the rest mass.

Note that physicists generally don't like to say the mass has changed at all, as it is quite confusing. Usually, we refer only to the rest mass of the object and we say that the energy has changed.