| 11. | :: : The weak equivalence principle only applies to point particles in a gravitational field.
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| 12. | As far as we know, electrons are point particles with no intrinsic size at all.
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| 13. | Nevertheless, there is good reason that an elementary particle is often called a point particle.
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| 14. | Given some molecule, you are free to consider it as a system of point particles.
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| 15. | It is often a hypothetical simplified point particle with no properties other than mass and charge.
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| 16. | Likewise, in electromagnetism, physicists discuss a "'point charge "', a point particle with a nonzero charge.
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| 17. | This implies that the point particle would have infinite inertia, making it unable to be accelerated.
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| 18. | String theory extends ordinary particle physics by replacing zero-dimensional point particles by one-dimensional objects called strings.
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| 19. | In general relativity, geodesics describe the motion of point particles under the influence of gravity alone.
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| 20. | However unlike point particles, point mass can only apply to an object that is infinitely small.
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