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S2w4 FC Rotational motion

Last updated 3 months ago
22 questions
1
Review time!
is an equation that tells you that the __________ is equal to __________times __________
2
Equations of motion are


Where v=__________, a=__________, t=__________, v0=__________. The 0 in v0 stand for __________ d=__________
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p=mv tells you that __________ is equal to the __________times__________
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has units that are __________ because work is a __________
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Tells you that the __________ of an object is half its__________ times its __________
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you are spinning on a playground merry-go-round. you are hanging on for dear life as you spin, body entirely off the disk of safety. is your body

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Explain why this is what happens on a merry-go-round

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Rotational motion is very similar to translational motion, and all the equations of translational motion and properties of translational motion have their rotational equivalents. displacement might be the only hard one - is it theta? or s? Which do you think it should be?

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oh, remember when you were creating your spreadsheets and had to decompose the vector for the initial velocity into horizontal and vertical using sine and cosine? and for some reason sheets doesn't like degrees, so we had to convert degrees to radians. I explained at the time that radians was just a different unit of measure for angles, like feet and meters. the conversion looks like this:
except, you would never write radians like that. Radians are considered a dimensionless unit. so using the equation
find the arc length when the ball has gone 360°. remember, you have to use radians, not degrees.

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There will be a lot of talk of what is linear or translational motion and what is rotational motion

  • a bike frame moving down the street
  • the bike wheel rolling down the street
  • a kid on a merry go round
  • a kid flying off a merry go round
  • a rock inside a slingshot
  • a rock released from a slingshot
  • translational
  • rotational
  • both
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This is an instance where the angular displacement is a better equivalent for displacement than arc length. Notice - if two kids are on a merry go round. One is sitting on an edge, the other is sitting closer to the middle. the __________ has the larger radius, therefore after three turns, he has a larger arclength displacement. but the two kids have __________ angular velocity.
1

Oh hey! this is what Ben mentioned in class. I had actually totally forgotten that this is used in angular motion, because I am completely comfortable making myself the center of the universe and discussing clockwise and counterclockwise. It's a pretty good idea to use the right hand rule here, though, since we will be seeing that again in the emag section, and, to be fair, it is doing pretty much the exact same thing in that scenario.
Anyway - give me an example of what he means by clockwise and counter-clockwise are observer dependent.

1

What do you think angular acceleration means?

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We now have several equations to consider for angular motion. Match the angular version to the translational version

Draggable itemCorresponding Item


1

Consider the movement of the kid on the merry go round. If the kid suddenly lets go, describe his motion

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the arc length of the path of the scared merry-go-round kid near the center is different for the kid sitting on the edge of the merry go round.

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Because the two kids go around the whole circle in the same time, but one travels a longer arclength, the kid on the edge has a higher angular velocity than the kid near the center.

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Mass has a rotational equivalent too - it is called moment of inertia. Remember that mass is just the measure of inertia, think of that as measure of TRANSLATIONAL inertia. if the translational inertia can be described by the amount of stuff something is made of, what do are you guessing moment of inertia will involve?

1

Axis of rotation means the point around which everything spins. What is the axis of rotation of a door?

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That's right, to find the rotational inertia of a system you have to find the moment of inertia of every tiny piece of your object. Making this easier, lets assume with have a tiny mass of 1kg is rotating around a stick. the equation for that situation would be
What kind of relationship is this?

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what do you think

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what is the center of mass?

We will be keeping the center of mass at the center of rotation for a bit before we delve into that.