Why does (dr/dt)*θ = 0 when deriving Velocity?

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SUMMARY

The discussion centers on the derivation of instantaneous velocity in circular motion, specifically addressing the equation V_{t} = (dθ/dt)*r + (dr/dt)*θ. It concludes that when the radius (r) is constant, the term (dr/dt)*θ equals zero, simplifying the equation to V_{t} = (dθ/dt)*r. This is a direct application of the product rule in calculus, confirming that the angular velocity (ω) multiplied by the radius yields the instantaneous velocity.

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  • Understanding of calculus, specifically the product rule.
  • Familiarity with angular motion concepts, including angular displacement (θ) and angular velocity (dθ/dt).
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  • Basic grasp of derivatives and their physical interpretations in motion.
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LearninDaMath
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Homework Statement


A derivation of Instantaneous Velocity

Arc length = s = θ * r

ds/dt = d(θr)/dt = Instantaneous V

Product rule:

V_{t} = (dθ/dt)*r + (dr/dt)*θ

V_{t} = (dθ/dt)*r + 0

V_{t} = ωr




Question:

Why does dr/dt*θ = 0 ?
 
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Since r is constant, the rate of change of the radius with respect to time (or anything for that matter) is going to be 0.
 
ah makes perfect sense, thanks.
 
LearninDaMath said:
ah makes perfect sense, thanks.

No problem, sir. Good luck.
 

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