Car acceleration if resistance forces don't exist

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Discussion Overview

The discussion revolves around the acceleration of a car under the hypothetical scenario where resistance forces, such as aerodynamic drag and rolling resistance, are neglected. Participants explore the implications of constant power output from the engine on acceleration rates at different speed intervals, particularly comparing the acceleration from 0-100 km/h to 100-200 km/h.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants argue that if the change in velocity (ΔV) is the same for both intervals, then the acceleration and force should also remain constant, suggesting that the car would accelerate from 100-200 km/h in the same time and fuel consumption as from 0-100 km/h.
  • Others contend that constant power output means that thrust decreases with increasing speed, leading to a longer time and greater fuel consumption for the second interval.
  • Some participants introduce the idea that a real engine does not deliver power linearly across a broad range, suggesting that an electric motor might provide a clearer example.
  • There is a discussion about the role of resistance forces, with some stating that without any resistance, the car would not accelerate at all, while others propose that rolling resistance could be negligible for the sake of the argument.
  • Several participants highlight the relationship between kinetic energy and speed, noting that more energy is required for each increment of speed as the car accelerates, which complicates the assumption of constant acceleration from constant power.
  • Some participants emphasize the importance of the reference frame in analyzing acceleration, arguing that the energy required to accelerate is frame-dependent and that the car's perspective changes the interpretation of speed and energy requirements.

Areas of Agreement / Disagreement

Participants do not reach a consensus. There are competing views on whether the car can maintain the same acceleration and fuel consumption across different speed intervals when resistance forces are neglected. The discussion includes various interpretations of the physics involved, leading to unresolved disagreements.

Contextual Notes

Limitations include assumptions about neglecting resistance forces, the dependence on the type of engine, and the complexities of energy requirements at different speeds. The discussion also highlights the need for clarity in the problem statement and the implications of different reference frames.

  • #91
jbriggs444 said:
You showed a 50 N weight on a string pulling a cart. That is a constant force device. It is not a constant power device.
No this not string pulling cart.
Engine/gearbox stay at place and winding wheel lift up weight with rope like cran that has I.C. engine for lifting.Weight is accelerate up.
 
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  • #92
Jurgen M said:
No this not string pulling cart.
Engine/gearbox stay at place and winding wheel lift up weight with rope like cran that has I.C. engine for lifting.Weight is accelerate up.
If you have constant 50 N force, it is a constant force device. Period. End Of Story.

Maybe if it was a 5 kg mass, it could be constant power. But it's not 5 kg. It's labeled as 50 N.
 
  • #93
jbriggs444 said:
If you have constant 50 N force, it is a constant force device. Period. End Of Story.

Maybe if it was a 5 kg mass, it could be constant power. But it's not 5 kg. It's labeled as 50 N.
It has mass of 5kg, on the Earth this aprox 50N, and when accelerate up tension in rope is bigger then 50N...
 
  • #94
PeroK said:
I suspect that he doesn't ride a bicycle!
Indeed bike can travel very fast if reduce aero drag, 183mph (295km/h) !

 
  • #95
phinds said:
A car can't have ANY acceleration if there is no resistance anywhere including the tire/road resistance because the wheels would just spin and the car would go nowhere.

What was it Meatloaf said - you took the words right out of my mouth (the rest doesn't apply here :wink:). Seriously the only reason a car can move is the resistance between the tyres and the road. If there is none all the wheels would do is spin - as you mentioned. I seem to recall Feynman discussing it somewhere in his lectures, which of course anyone interested in applied math/physics should read (but not after a usual course in physics without going into why).

Thanks
Bill
 
  • #96
Jurgen M said:
Problem is that some members at stackechange who comment at special relativity tags, has different comment,answers then here, so it makes even more confusion for me.

With my mentor's hat on first, this is a thread about classical Newtonian mechanics; relativistic considerations are not appropriate. But different answers to the same question can be confusing; so just a passing comment that there are different approaches to relativity that can lead to different ways of explaining things. I think these days the explanation based on the symmetries of an inertial frame gets to the heart of the matter best:
http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

If you would like to discuss the matter further please start a thread in the relativity forum.

Thanks
Bill
 

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