[H2 Physics: WEP]

[u/Hot_Confusion5229]

Hi sorry Ik that this qn is on proportionality but I'm so confused sorry like why they used P_out for the ans key when they should be using P _ in...can someone please help me explain what is going on here

Also love yall I'm so happy I can do 5 phy tys topical topics within 17day ik it seems v long but I'm working ft and suffering from phone addiction lol I'm gonna quit soon n really appreciate the help here ( saying cus I'm clearing a lot of misconceptions and ppl r really nice and patient )

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[u/GammaRayBurst25]

What do you mean by they used P_out for the ans [sic] key when they should be using P_in? The answer key didn't label any quantity as P_in or P_out. What's more, the answer key's steps are all explicitly written and the reasoning is clear (at least it's clearer than a text where every other word is abbreviated for no good reason).

The force is proportional to the square of the speed, so the power is proportional to the cube of the speed. Increasing the speed by a factor of 2 increases the power by a factor of 2^3=8. Since 8*23=184, the power required is 184kW.

[u/Hot_Confusion5229 → u/GammaRayBurst25]

Sorry if I wasn't clear but what I was trying to ask is that why they were using power output when we need to find power input ie power car requires to drive at constant speed but they used output power ie including power lost via friction

As you can see from my working I do understand till P1/P2 what I do not understand is the values of power they used

Also sorry but about the abbreviations that's how the people I interact with and I text on the daily I believe that it's very efficient though I tried to make everything full form here

[u/GammaRayBurst25 → u/Hot_Confusion5229]

The car is moving at a constant speed, so the power input matches the power output. That's also written in the answer key.

[u/Hot_Confusion5229 → u/GammaRayBurst25]

But that's 100% efficiency why would that happen when there is power lost via friction like P=E/t and there is energy lost due to frictional force and that can't be negligible since if so friction will become negligible

[u/GammaRayBurst25 → u/Hot_Confusion5229]

That's not only true at 100% efficiency. It's true at any nonzero efficiency.

Suppose the efficiency is η, the power output needed to keep the vehicle steady is still 184kW. This is a consequence of Newton's laws of motion, so this does not depend on the efficiency. The total power expanded is (184kW)/η, with 184kW of useful power and (184kW)(1-η)/η of loss.

[u/Hot_Confusion5229 → u/GammaRayBurst25]

Ah I'm sorry I see I think I understand now thank you