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

[gen x]

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?

 

[A.T.]

...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.

 

[gen x]

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?

 

[A.T.]

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.

 

[Dale]

This is also why talking about biomechanics in energy discussions is usually not helpful. Life is messy.

 

[gen x]

Put the weight on the ground, and it will be supported by a force, without consuming any energy.

correct

 

[Herman Trivilino]

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.

 

[A.T.]

Put the weight on the ground, and it will be supported by a force, without consuming any energy.

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