if a body accelerates, does the weight or mass change at all (compared to when the body is at rest)?

Answer 1

The rest mass, m0, doesn't change. But at high speeds (achieved after acceleration, of course!) , there is a measurable (and CONFIRMED!) Special Relativistic increase in what physicists call the "effective mass." That is given by:

m_eff = m0 / [sqrt (1 - (v/c)^2)].

What they mean by that is that just as there is an energy equivalence of rest mass, m0, and rest-mass energy, m0 c^2, there is an energy equivalence between m_eff and m_eff c^2. It looks like this:

m_eff c^2 = m0 c^2 / [sqrt (1 - (v/c)^2)] = m0 c^2 + (1/2) m_0 v^2 + ... O[(v/c)^4].

Notce that this says that

m_eff c^2 = (rest-mass energy) + Newtonian K.E. + higher order terms.

Newtonian Kinetic Energy (K.E.) is given a new significance by this result.

Similarly, in relativistic dynamics the Newtonian momentum pN = m0 v is replaced by m_eff v. AS v --> c, the momentum increases WITHOUT LIMIT. This means that it gets harder and harder to accelerate something, as it approaches the speed of light. In fact it does that in such a way that a particle of FINITE MASS can NEVER reach the speed of light.

I have dealt here with the concept of mass. Weight is another matter, yet it's also true that what you might call "weight" ALSO increases, in General Relativity. That tells one that not only does pure rest-mass have weight, but that sheer energy is also acted on by gravity, as well, according to its mass-equivalent. That's one way of understanding why and how the path of light is "bent" in a gravitational field. Similarly, inside so-called relativistic stars, the kinetic energy that helps provide the pressure tp resist the star's own gravity, also CONTRIBUTES to the mass and therefore the gravitational attraction of the star.

Isn't that simply mind-blowing ?!

Live long and prosper.

Answer 2

Compared to a body at rest, as a body of mass approachs the speed of light it becomes infinitly more dense. I believe this is covered in the Theory of Relativity, although I could be way off.

Answer 3

It increases, very slightly at first, then more and more, as the energy that is used in its acceleration is converted to mass, in accordance with Einstein's famous equation, e = mc^2.

Answer 4

Think about this: As a body's velocity approaches the speed of light, it's mass increases to infinity.

Wrong
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Answer 5

no, acceleration and mass are not conncted at all. acceleration relates to speed and direccton