Driving an RC car in a vertical loop, What formula to use?

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Discussion Overview

The discussion revolves around the physics of driving an RC car through a vertical loop, specifically focusing on the formulas needed to calculate the minimum speed required for the car to successfully complete the loop. The conversation touches on concepts of centripetal force and gravitational force, as well as the implications of mass in these calculations.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes a practical scenario involving an RC car in a vertical loop and asks for the formula to determine the minimum speed required.
  • Another participant proposes that the force of gravity must be less than the centripetal force for the car to complete the loop, leading to the inequality Mg < MV²/r.
  • It is noted that mass cancels out in the equation, suggesting that it does not affect the minimum speed required.
  • A participant questions whether the derived formula is applicable to a simple planetary orbit, indicating a potential misunderstanding of the context.
  • Another participant clarifies that the formula for gravitational force at the surface of a planet differs from that used in orbital mechanics, introducing the gravitational constant and the concept of the center of mass in multi-body systems.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the formulas discussed, particularly regarding the relevance of mass and the context of gravitational forces in different scenarios. There is no consensus on the correct application of the formulas to the scenario of the RC car versus planetary orbits.

Contextual Notes

There are unresolved assumptions regarding the conditions under which the formulas apply, including the specific context of gravitational forces and the simplifications made in the analysis of the RC car's motion.

FreeForAll
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I once drove my RC car in a vertical loop in a concrete drain pipe.

If you know the diameter of the pipe, and the weight of the vehicle, what is the formula used to determine the minimum speed required for it to complete the loop?
 
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At the top you want the force of gravity Mg to be less than the centripetal force needed to move in a circle radius r at velocity V. So...

Mg < MV2/r

Mass M cancels

Rearrange it to find V.
 
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Wow I never would have thought that mass is irrelevant, but it makes sense. I thought it would have been way more complicated than that, taking into account the amount of time "at the top" and mass.

Would this formula be for a simple planetary orbit of constant speed and radius? Mg = MV2/r
 
No because gm is only valid at the surface of a planet. You would need to use GmM/r2 instead. There is also the complication that both planets orbit about their common centre of mass (eg the moon does not orbit about the centre of the earth).

In case you haven't studied this: m and M are the masses of the two bodies. G isn't the same as g. "Big G" is the gravitational constant whereas "little g" is the gravitational acceleration on earth.
 
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