Can a constant force cause variable acceleration? I think that the answer is no. How can a force cause variable acceleration?

  • 4
    $\begingroup$ Maybe you should link the article? $\endgroup$
    – Puk
    Jun 15, 2020 at 1:03
  • $\begingroup$ As Puk suggested, unless you can recapitulate the rationale or link to the article, this question doesn't make much sense as it stands. Voting to close, notify me if you edit the question to include more substance. $\endgroup$
    – user87745
    Jun 15, 2020 at 1:05
  • 3
    $\begingroup$ $F=ma$ If $F$ is constant and $m$ is constant, the $a$ must be constant. Sometimes $m$ isn't constant, such as when a rocket becomes lighter as it uses fuel. Is that what you are thinking? $\endgroup$
    – mmesser314
    Jun 15, 2020 at 1:17
  • $\begingroup$ Dvij D.C I was unable to link the article so i have removed that part. $\endgroup$
    – royboy
    Jun 16, 2020 at 1:20

1 Answer 1


Only if the mass of the accelerating body is changing; or in the theory of relativity, depending on how you define acceleration.

Newton's second law reads $\vec{F}=m\vec{a}$, so in Newtonian mechanics, if the force is constant, mass must change if acceleration is to change. As an example, consider a rocket propelled by a constant force in space, expending fuel as this happens. Even though the force is the same, the rocket gets lighter over time, and accelerates faster and faster.

In special relativity, when we speak of uniform acceleration, we normally mean constant acceleration with respect to an inertial frame in which the accelerating body is instantaneously at rest. This means the accelerating body always "feels" the same force. However, the so-called three-acceleration of a uniformly accelerating body measured by an observer in an inertial frame changes over time. This is most notable when the accelerating body approaches the speed of light with respect to the observer: it cannot exceed the speed of light, so the magnitude of the acceleration measured by the inertial observer eventually decreases.


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