# B What Does It Mean For A Force To Act On A Particle?

#### iScience

I know this is kind of a dumb question but please forgive me it's been awhile.

Is it enough for a particle to merely "feel" an external force F to state that "F is acting on the particle"?
ie if the particle was confined in a potential well and experiences F but does not move.

or does $$\vec{F} \cdot d\vec{s}$$ have to be non-zero? ie does force have to do work on the particle to state that "F is acting on the particle"?

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#### Drakkith

Staff Emeritus
2018 Award
Yes, I would say that a force 'acts' on a particle even if it doesn't move. It still affects the equilibrium state, meaning that the energy level, orbit, or whatever of the particle and its surrounding environment is different when the force is applied versus when it is not applied. Force example, if you place a hydrogen atom in a magnetic field the atomic energy levels shift slightly even though the electron is still in a stationary state and isn't being ripped from its atom and accelerated away.

#### iScience

Yes, I would say that a force 'acts' on a particle even if it doesn't move. It still affects the equilibrium state, meaning that the energy level, orbit, or whatever of the particle and its surrounding environment is different when the force is applied versus when it is not applied. Force example, if you place a hydrogen atom in a magnetic field the atomic energy levels shift slightly even though the electron is still in a stationary state and isn't being ripped from its atom and accelerated away.
Thank you!

#### lightarrow

Another simple example: hang a heavy body at the roof with an elastic thread; at equilibrium you can verify and measure the existence of the thread's force on the body with its elongation; cut the thread and the body falls down.

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lightarrow

"What Does It Mean For A Force To Act On A Particle?"

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