Lesson 5

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Lesson 5

Post by Lionroar » Mon Oct 09, 2017 1:59 pm

In Lesson 5, Problem-solving Cooldown, Q19, Bryan solved for final velocity using conservation of energy. However, isn't the much simpler method to solve this is to disregard the mass and use the kinematics formula v(f)^2 = v(i)^2 + 2*a*d? We were given the height, and we know acceleration is 10 m/s^2. Final velocity shouldn't depend on mass.
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Re: Lesson 5

Post by NS_Tutor_Andrew » Mon Oct 09, 2017 10:15 pm

Hi Lionroar,

Excellent point! In my experience, the question of whether it feels more elegant to use kinematics equations or conservation of energy to solve questions like this is largely a matter of personal preference. If the kinematics solution seems more straightforward to you, then go for it! In a situation like this where there are multiple ways to get the right answer, the only thing that matters on Test Day is that you pick a correct method and execute it.

The only other thing I'd point out, though, is that using ΔKE = ΔPE, you actually could cancel out mass: (1/2)mv^2 = mgh → (1/2)v^2 = gh. Again, whether you do that or first calculate a numerical PE value (as Bryan did in the video) is largely a matter of preference.

Best of luck studying and please feel free to keep asking questions :).
Andrew D.
Content Manager, Next Step Test Prep.
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