“Momentum” is a word that we don’t usually think of as having a technical origin, even though we hear it used a lot by reporters during election season. A typical use would be something like “Senator Belfry’s campaign seems to have gained momentum following his recent speech to the Bloom County Chamber of Commerce.” There seems to be nothing technical about that.
But the Oxford English Dictionary notes an early print use of “momentum” from 1735 describing the movement of particles in Newtonian physics. The Online Etymology Dictionary places its origins as a technical term a little earlier, in the 1690s, with its metaphorical uses following about a century later.
In either case, the term would have been used to describe something both observable and somewhat mysterious: the degree or force of motion. And this may explain momentum’s appeal to the reporter. She should be committed to being objective, so she uses a technical term to point to a tenuous relationship, one that requires something subjective, an interpretation she’d like to mask.
If we go back to our previous example, we can see this at work. After Senator Belfry’s speech, perhaps his poll ratings improved. To say the speech was the cause of that requires speculation. Reporters should not speculate, so the reporter deploys “momentum” to cloak the speculation: the senator’s poll ratings improved from a force that is at once measurable and invisible.
Relying on the term “momentum” also lets the reporter suggest a narrative that may or may not actually exist; by throwing in a hint of action--like a gratuitous car chase in an action movie—she keeps our interest up while the plot takes a rest, and reality, with its own dull movement, takes its course.