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Show that a force that does no work must...

Show that a force that does no work must be a velocity dependent force.

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As work done by force is zero,
`thereforedW=vecF*vec(dl)=0`
`thereforevecF*(vec(dl))/(dt)*dt`
`thereforevecF*(vecv*vec(dl))=0" "(becausevecv=vec(dl)/(dt))`
`thereforevecF*vecv=0,dl ne0`
`thereforeFvcostheta=0" "thereforetheta=90^(@)`
If v changes direction then to make `theta=90`, F must charne angle according to v. So, F is dependent on v to make work done zero.
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