How can an object lose energy but still move at the same speed with constant momentum? I'm studying collisions, and while doing it, the conservation of momentum seems to strike me in a weird way.
If the momentum in a system is constant but energy can still be lost through thermal radiation, wouldn't this mean that a particle can lose most of its energy while still moving at the same speed?
Is the momentum accounting for some emission of a heat particle that I forgot the name (some less-used brother of the photon)?
Or maybe conservation of energy and momentum are unrelated?
 A: Temperature is a thermodynamic variable , i.e. it depends on a large number of particles because it can be measured in matter, and thermodynamics as a theory developed long before the particle nature of matter was studied. When  Newtonian mechanics was developed, it was shown that the theory of thermodynamics emerged from statistical mechanics .
In statistical mechanics temperature is :
$$T_{kinetic}=\frac 2{3k}\left[\,\,\overline{ \frac 1 2 m v^2 }\,\,\right]=\frac 2{3k}KE_{avg}$$
related to the average kinetic energy, and through that to the average momentum.
You ask

If the momentum in a system is constant

The average momentum can be considered constant,

but you could still lose your energy by thermal radiation,

thermal radiation is electromagnetic waves, and they do carry energy and momentum , that is how black body radiation leaves a massive object.

wouldn't this mean that a particle can lose most of its energy while still moving at the same speed?

Now you are confusing statistical averages with individual interactions. As kinetic energy and momentum are related functionally, a particle cannot lose kinetic energy and keep the same speed.

Is the momentum accounting for some emission of a heat particle

the changes in momentum due to collisions and interactions do come from the radiated eletromagnetic radiation, as the object cools.

that i forgot the name(some less used brother of the photon)?

there is no brother of the photon, photons are what electromagnetic radiation is made up of.
