Why does mass change with speed?

when a body moves with a velocity comparable to the speed of light, then its mass is given by the expression m=m (normal) divided by root over (1-(v*v/c*c)) where c is the speed of light. I feel that the mass changes because if v is very high then the body' surface scrapes off thus reducing the mass.

mass and energy are interchangable, this means that one can be converted to the other according to the famous Einstiens equation E=MC^2

Mass should not change as speed change. The energy of the mass changes as the kinetic energy changes. Keep in mind that speed is a relative. At different reference point, the energy level is different, but the mass will always stays the same. The only areas that I think mass will change is nuclear reaction or inside the black hole where mass becomes energy.

Because of the equation: E=mc^2, and with a complicated calculations, we get the next: mr = m0 /sqrt(1 - v^2/c^2) where v is the vilocity of the body. c is the speed of light. m0 the initial mass(with no movement). mr the motion-based mass. so that with a very big velocity the mass will change.

http://en.wikipedia.org/wiki/Rest_mass

E=mc^2 is probably not the proper equation for this. The combined (potential + kinetic) energy for a given mass DOES NOT CHANGE, unless the mass changes. Nor does the mass of an object with an increase in speed. The increase in mass is simply an illusion created by the FORCE of an object. Since Instantaneous Force= Mass x Instantaneous Velocity, the speed of an object will affect its weight at that particular instant. A perfect way to test this is to stand on a scale, record your weight, then jump slightly, landing back on the scale, and notice for a brief second your weight will be significantly higher, which is an indication of the force exerted on the scale at the precise time.

In more modern notation, the rest mass of an object does not change with its speed. What changes is the energy. The relativistic energy is mc^2/sqrt[1-v^2/c^2] where m is the rest mass. The confusion arose because most people were accustomed to writing momentum as "mv" whereas the relativistic momentum is mv/sqrt(1-v^2/c^2). Back in the day, people often attatched the denominator to the m, that is, they thought momentum=Mv, M=m/sqrt(1-v^2/c^2), energy=Mc^2. Hence the "changing mass." But there are good reasons to think of the rest mass as unchanged--you stated some of them in your question. In special relativity, mass is the invariant length (eg the same for all observers) of something called the "energy-momentum 4-vector." That is, there is a relationship between energy and momentum: sqrt(E^2-P^2c^2)=mc^2 which is fundamentally different from the Newtonian E=(P^2)/2m. This difference ultimately comes from the difference between the geometry of spacetime itself in the Newtonian and relativistic descriptions.

F=M/c2 M=F/c2 Since c2 is a constant ,then if F increases,The mass increases.

The mass is constant, always, it's the inertia that changes. The inertia is in direct porportion with mass.