# What causes increment in volume when compressive forces are applied on an object

Doubt

When compressive forces are applied on a body, what causes increment in volume. According to me the volume should remain constant since the mass and density of body are constant.

• Why do you assume that the density remains constant when a pressure is applied? Doesn't it strongly depend on the container of the object, if any? – AccidentalBismuthTransform May 8 at 7:46

Compactifying volume means that we increase object's density, because density is inversely proportional to objects volume: $$\rho = \frac mV$$. So neither density stays constant.
If you have A LOT of compression power, then atoms of object can become so close to each other that electron capture reaction can begin : $$p + e^- \to n + \nu_e$$ Proton captures electron and converts into neutron emitting neutrino in the process. If you do the math correctly you will see that $$m_n - (m_p + m_e) = 0.7823 \,\,\left[\frac{\text{MeV}}{c^2}\right]$$ So in the end matter becomes more massive in the process of neutron accumulation. By rough analogies of mass-energy equivalence, $$E=mc^2$$, this means that you are accumulating/converting your compression energy into object mass, so object becomes heavier in the process. Thus mass too isn't constant under high compression. This process can be seen in neutron star formation. In Earth we don't have such huge pressure conditions, but astronomers can observe them in high-density star formation. Only in that case, pressure role is taken by gravity forces.