Power Calc: Net vs Driving Force

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SUMMARY

The discussion clarifies the application of the power formula P=Fv in the context of a cyclist and a car. It emphasizes that the force used in the equation should correspond to the specific scenario of power being calculated, whether it is the net force or the driving force. For example, the power delivered by a cyclist's foot is calculated using the force exerted on the pedal and its tangential velocity, while the power produced by the tire on the road considers the frictional force and the bicycle's road speed. The relationship between these powers demonstrates that due to losses, the power at the tire/road interface is always less than the power delivered by the cyclist.

PREREQUISITES
  • Understanding of basic physics concepts such as force, velocity, and power.
  • Familiarity with the equation P=Fv and its components.
  • Knowledge of mechanical systems, particularly in cycling or automotive contexts.
  • Basic grasp of energy losses in mechanical systems.
NEXT STEPS
  • Study the implications of energy losses in mechanical systems.
  • Learn about the differences between net force and driving force in physics.
  • Explore real-world applications of the power equation in various mechanical systems.
  • Investigate how to optimize power output in cycling and automotive performance.
USEFUL FOR

This discussion is beneficial for physics students, mechanical engineers, cyclists, and automotive enthusiasts seeking to understand the dynamics of power generation and loss in mechanical systems.

Janiceleong26
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Homework Statement


P=Fv,
I am confuse with which force to substitute into the equation. Is it the net force? or the driving force? (if it is a car) ..
 
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Janiceleong26 said:

Homework Statement


P=Fv,
I am confuse with which force to substitute into the equation. Is it the net force? or the driving force? (if it is a car) ..
It depends on what power is to be determined, but whichever force you pick it must be a velocity that the force travels at.
E.g. a cyclist pushes with a force Fp on a pedal, and the pedal moves at tangential velocity vp. The power delivered by the foot is Pp=Fpvp. The frictional force between the rear tyre and the road is Fr. The bicycle advances at roadspeed vr. The power the tyre/road produces is Pr=Frvr. Because of losses, Pr<Pp.
 
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haruspex said:
It depends on what power is to be determined, but whichever force you pick it must be a velocity that the force travels at.
E.g. a cyclist pushes with a force Fp on a pedal, and the pedal moves at tangential velocity vp. The power delivered by the foot is Pp=Fpvp. The frictional force between the rear tyre and the road is Fr. The bicycle advances at roadspeed vr. The power the tyre/road produces is Pr=Frvr. Because of losses, Pr<Pp.

Thanks! I got it now
 

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