Calculating total kinetic energy of a car at a given velocity

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Discussion Overview

The discussion revolves around the calculation of total kinetic energy of a car at a given velocity, specifically addressing whether to include the rotational kinetic energy (RKE) of the wheels and other components such as the flywheel and pistons. The scope includes theoretical considerations and practical implications of kinetic energy in automotive contexts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether the rotational kinetic energy of the wheels should be included in the total kinetic energy calculation.
  • Another participant affirms that RKE contributes to the total kinetic energy.
  • A later reply suggests that the significance of the RKE compared to translational kinetic energy may vary, proposing a calculation for a solid disk to illustrate this point.
  • Another participant introduces the idea that the RKE of the flywheel in older diesel engines could also be significant, especially under certain conditions, and shares a personal anecdote regarding their boat's engine dynamics.
  • One participant raises the question of whether the kinetic energy of the pistons should also be considered in the overall calculation.

Areas of Agreement / Disagreement

Participants express differing views on the significance of various forms of kinetic energy, indicating that multiple competing perspectives remain on the importance of including RKE and other components in the total kinetic energy calculation.

Contextual Notes

The discussion includes assumptions about the conditions under which different forms of kinetic energy may be significant, such as vehicle speed and engine operation, but these assumptions are not universally agreed upon.

Who May Find This Useful

Individuals interested in automotive physics, engineering students, and those exploring the dynamics of kinetic energy in vehicles may find this discussion relevant.

Phunee
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When calculating the total kinetic energy do i have to take into account the rotational kinetic energy of the wheels?
 
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Yes. It's kinetic energy so it contributes to the total kinetic energy.
 
Thanks
 
But is the RKE of the wheels significant compared to the translational KE of the car? It might make an instructive exercise to calculate the RKE of e.g. a solid disk of mass M rotating with angular speed ω = v/R where v is the linear speed of the car and R is the radius of the wheel.
 
For some old diesel engines, the RKE of the flywheel could be significant, too. If you ran the engine at full speed, with the car stationary and then lifted the valves and let the clutch in, the car would move forward at 'significant speed', implying that the RKE of the flywheel could be significant at low car speeds.
I can't do that experiment with my 3.5 ton boat because I can only engage drive with the engine on tick over but, as KE is proportional to v2 and the maximum speed is less than 10 km/hr, the flywheel energy must be significant. I must do the sums one day.

They used to quote the instantaneous horse power of traction engines for when you would use the flywheel to provide some high power in bursts.
 
What about the KE of the pistons?
 

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