I want a word that could mean to either speed up or slow down something. Accelerate doesn't quite work, because although it could theoretically apply to both, the default is positive.

Ideally, the default will be neutral.

By moving this lever, you can ____ the vehicle.", "Both car pedals are designed to ____ the car, but in opposite directions.

  • I think change the velocity of is the closest there is. – Davo Jun 22 '17 at 16:30
  • ... though 'adjust the speed of' might sell more cars. – Edwin Ashworth Jun 22 '17 at 16:36
  • "propel" fits the example sentences . With no propulsion you have no movement. – Tom22 Jun 22 '17 at 16:45
  • 1
    ...but then so does 'move'. – Edwin Ashworth Jun 22 '17 at 16:55
  • 3
    You can control or change speed of the vehicle. – NVZ Jun 22 '17 at 16:59


exert a modifying or controlling influence on

or "Regulate"

control or maintain the rate or speed of (a machine or process) so that it operates properly.

are both neutral, but you will need to qualify them with e.g. the word "speed."

I think your second sentence about the pedals working in opposite directions is false. Speed is not a vector, and brakes cannot not reverse direction.

  • Trouble with the second of these two is that it implies both an upper and lower limit. That isn't my interpretation of the OP's request. – Carl Witthoft Jun 22 '17 at 18:13

The obvious term would be accelerate in the phyics sense of

undergo or cause to undergo a change in velocity

This would work for your second example, but not so well for the first. Outside of the physics community, accelerate is used to denote an increase in speed, while decelerate is used in connection with a decrease in speed.

  • 2
    +1 Rick suggests: "Deceleration" is not actually a term in physics. It is only an English word used in common speech, meaning "a decrease in speed." The problem with this word is that some students mistake it to mean the same as negative acceleration. But since the negative sign just defines a direction in a particular problem, relative to a chosen axis, an object's speed would actually INCREASE if it has a negative velocity as well as a negative acceleration.. See lhup.edu/~dsimanek/scenario/physlang.htm – NVZ Jun 22 '17 at 17:02

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