AAMC Fizzicz Self-Assessment #100

[bear2roo]

Ok I got confused by half this question. I got it right by POE and common sense but I don't understand the AAMC Explaination.

"Given that the friction between the bearings and axle will slightly increase, which of the following changes would most likely be true of the motion of the loop and shaft if a solid disk of metal were attached to the end of the shaft? The loop and shaft would accelerate more:

a) slowly and reach a lower maximum angular speed
b)slowly but reach a higher maximum angular speed
c)quickly but reach a lower max angular speed
d) quickly and reach a higher max angular speed

Answer is A

I uploaded the diagram and the answer explanation screenshot. I get the first part about rotational inertia and how it's affected by mass added. I don't understand anything starting from the sentence "Second, the amount of power supplied....."

Honestly, I used intuition and my weak knowledge in rotational stuff.

ALSO: Can someone kindly please explain the big concepts to look out for when we see electric motor apparatus (combining torque, electrostatics/circuits, and rotational motion) on a passage? I am really weak on rotational stuff but I know the basics. I also saw a similar machine on a TBR test I thinkk.

Thanks!!

Edit: Darn, i need to resize the images...hold on

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[PlumbLine]

Similar to a scenario where you are dragging an object by a rope: If you want to drag at a constant speed, your force pulling the object must equal the friction force that opposes it. With those forces being equal, there will be no acceleration and velocity will stay constant. Adding mass to that object and using the same amount of power, it will accelerate slower.

In this problem they are also pointing out that a battery has an upper limit to its power, and it always operates at that upper limit. Say you are dragging an object but can only exert a certain amount of power, if you add mass to that object you increase friction, thus increase the amount of force necessary to pull. Given your limited power, you will never reach the same velocity that you reached when the object was lighter.

Hope this helps and that I interpreted the question correctly.

 

[TheJourney]

Bump. If anyone can further elucidate the problem and explanation I would appreciate it.