Final answer:
The motions of the particles in an object remain constant if the temperature of that object is constant, meaning that the average kinetic energy and the speed of the particles do not change, according to the kinetic-molecular theory.
Step-by-step explanation:
When the temperature of an object remains constant, the motions of the particles that make up the object also remain constant. This means that the average kinetic energy of the particles does not change. At a microscopic level, matter is composed of atoms and molecules that are in continuous motion. In solids, these particles vibrate back and forth; in liquids, they slide past one another; and in gases, they fly about freely.
According to the kinetic-molecular theory, temperature is a measure of the average kinetic energy of the particles in a substance. Therefore, if an object's temperature is constant, the vibrating quickly of the particles within the solid or the rapid translations within the liquids and gases do not become more vigorous, and the distribution of kinetic energies among the particles remains relatively unchanged. The speed of the particles, their masses, and the distribution of their energies will all be consistent with that particular temperature.
Thus, even as particles are always in motion at the microscopic level, without a change in temperature, there is no increase in their average kinetic energy, no discernible change in their speed, and the masses of the particles do not change. What we perceive as a change in temperature in everyday experiences, such as holding a cold ice cube, is the transfer of kinetic energy from our warmer hand to the colder ice cube.