"relentless force" is correct and usable in written English.
It typically refers to an unstoppable power or drive that is almost overwhelming. For example: "The relentless force of the hurricane destroyed everything in its path."
In this multilayered retelling of Oedipus and Jocasta's incestuous marriage, the chorus explodes with relentless force.
Mr. Petterson convincingly portrays a character untethered by the relentless force of grief and guilt.
"But from that time on, he was a relentless force for the programs at Sundance".
The group's relentless force recalled hard-core punk, with melody and grace augmenting pure adrenaline.
Then we begin to feel something of that relentless force by which nature controls her own.
They lack the relentless force of physics, but exert something totally nonphysical: cunning.
The Ohio State defense also was relentless, forcing 18 turnovers with 13 steals.
They were loving, patient and inspiring, but they were also relentless, forcing me to work hard, to look in that mirror and believe in myself.
But the caste system, weakening for a half-century in India, is withering here under the relentless forces of assimilation and modernity.
The odors of the driver; the crippled urchins knocking at the windows; the million honking horns; the smog of unmoving traffic, these are the relentless forces of the nation seeking to breach the walls of the elite.
Those who make the pilgrimage may be witnessing the dawn of a new, ecstatic religion — that or the co-opting of spiritual yearning by the relentless forces of capitalist consumerism.
