Why higher speeds need more power if the backward force is the same?

  • Context: High School 
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Discussion Overview

The discussion revolves around the relationship between power, force, and speed, particularly in the context of a horse pulling a load and vehicles accelerating. Participants explore why higher speeds require more power even when the backward force remains constant, examining concepts of work, time, and energy transfer in various scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that power is defined as the product of force and speed, leading to the conclusion that higher speeds necessitate greater power even if the force remains constant.
  • One participant highlights the distinction between work done and the time taken to do that work, suggesting that less time at higher speeds requires greater power.
  • Another participant draws an analogy with a car, noting that even with constant drag force, a car traveling at higher speeds requires more power due to the increased rate of energy transfer.
  • Some participants discuss the implications of constant force in scenarios like falling objects, where constant force leads to increasing power as speed increases.
  • A participant provides an example comparing a weightlifter and a college student moving boxes, illustrating that the same work done at different speeds results in different power outputs.
  • Concerns are raised about the intuition surrounding applied force, with some participants questioning the assumption that force remains constant as speed increases.
  • One participant mentions the complexity of bicycle mechanics, noting that maintaining force on the pedals becomes more challenging at higher speeds.
  • Another participant emphasizes that as speed increases, resistance forces also tend to increase, complicating the relationship between applied force and acceleration.
  • Mathematical examples are provided to illustrate how power calculations change with varying speeds, reinforcing the idea that higher speeds lead to higher power requirements.

Areas of Agreement / Disagreement

Participants express a range of views, with no clear consensus on the underlying principles governing the relationship between power, force, and speed. Some agree on the necessity of increased power at higher speeds, while others challenge the assumptions about constant force and resistance.

Contextual Notes

Participants acknowledge various complexities, such as the effects of resistance forces, the role of gear ratios in bicycles, and the nuances of energy transfer in different scenarios. These factors contribute to the ongoing debate without resolving the fundamental questions raised.

  • #31
Dale said:
Yes, but sometimes there are shortcuts.
What are shortcuts in case of engine?

So engine that have more area under power curve in operating rpm interval where you shift gears will have higher acceleration even if both have same peak power, because it produce more average power in that rpm interval?
 
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  • #32
gen x said:
...why then horse need to be more powerfull to pull at higher speed even if backward force at him(rope tension) stay the same?
Do you understand why a static force doesn't require any energy? Lean a stick against a wall, and it can stay there exerting a force forever, without consuming any energy.

So the energy / time (= power) required for exerting a force must depend on the velocity the force is applied at, not just on the force.
 
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  • #33
A.T. said:
Do you understand why a static force doesn't require any energy? Lean a stick against a wall, and it can stay there exerting a force forever, without consuming any energy.
But is this 100% correct? If you hold 20kg weight in one position, you consume more energy than if you dont lift anything?
 
  • #34
gen x said:
But is this 100% correct? If you hold 20kg weight in one position, you consume more energy than if you dont lift anything?
Just because you can have an inefficient system, that wastes energy on something, doesn't mean that energy is required to achieve that something.

Put the weight on the ground, and it will be supported by a force, without consuming any energy.
 
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  • #35
This is also why talking about biomechanics in energy discussions is usually not helpful. Life is messy.
 
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  • #36
A.T. said:
Put the weight on the ground, and it will be supported by a force, without consuming any energy.
correct
 
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  • #37
gen x said:
but don't understand why is harder to pull at higher speed if weight(backward force) is the same
Depends on what you mean by "harder". The force exerted is the same but the rate at which energy is expended is greater.
 
  • #38
A.T. said:
Put the weight on the ground, and it will be supported by a force, without consuming any energy.
gen x said:
correct
I hope from this example it is more intuitive, that transferring energy is not just a matter of applying a force, but also of the distance it is applied over:

work = force * distance.

Power is just the time derivative of that work, so for constant force:

power = force * velocity
 
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  • #39
jack action said:
Say, your horse needs to eat 1 kg of hay. . . . etc.
And there was I, thinking you were a petrolhead. What a quaint experiment - the price of horses being what it is. It would do better with oats, they say.
 

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