How to change speed into force

Perhaps a more direct concept of changing speed into force is the idea of electromagnetic induction, the principle operating within an electric motor. The motion (speed?) of an electric charge creates a magnetic field, which in turn applies a force to a magnet.

In the mechanical world in general, speed (velocity, really) and force directly imply each other. Intuitively we know how force changes into speed: the classic equation F=ma (force equals mass times acceleration). Intuitively, this means that a force, applied to an object with mass, will accelerate that mass, i.e., change its velocity. So we say that the force causes the acceleration: force becomes speed.

But this causation is not actually in the equation ... indeed, this equation is reversible like most physics relationships. So we can also interpret it as follows: any change in velocity (an acceleration) implies that there must have been a force.

Can we conclude, therefore, that the change in velocity caused the force? That speed "changed into" force? For electromagnetism, that's a reasonable interpretation. For electrostatic forces (like those car crashes in other answers), probably not.

I suppose one could say that the kinetic energy (speed) of the quarks in protons and neutrons deform spacetime, "changing" into gravitational force, but that feels like a stretch (as does relativistic frame dragging).


You can put the thing that has some speed in a viscous fluid (damper). Multiplication is force. Of course you will dissipate energy but feel force
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