AP Physics C: Rotational Dynamics Review - 2 of 2 (Mechanics)

[@jlongworth3879]

This channel is one of the most helpful youtube channels i've ever seen, thank you so much for taking your time to help thousands of lost students :)

[@FlippingPhysics]

You are welcome. Thank you for your lovely comment!

[@Yashodhan_B]

You deserve millions of views and subscribers! I am cramming before the AP exam tomorrow! Best review ever, I don’t have to even look through my notes to review! I hope your channel prospers and gets a lot of views!

[@FlippingPhysics]

Wow, thank you!

[@-SamarJyotiDeka]

Very good channel 🎉

[@john.dough.]

thank you so much

[@charlottelaww]

i'm here again and just wanted to comment to support the channel! the work you do is amazing and keep it up!!

[@FlippingPhysics]

Thanks for the love!

[@qilinxue989]

Can anyone explain why at 14:52 theta is 90 degrees? How does it have to do with tangential velocity?

[@qilinxue989]

I'm mostly confused about what the final theta represents...

[@FlippingPhysics]

At 10:58 the animation shows this pretty well. The final position is after the collision when the gum is stuck to the cylinder and is therefore an object moving along an arc. The gum's final velocity is a tangential velocity which, by definition, is tangent to the radius.

[@cp0bo593]

Thank you for the video, what happened to torque by gravitation acting on gum in rotational momentum example ?

[@aartisangwan3116]

It sure was a great idea to include the conservation of angular momentum problem. During the review my students almost forgot that we did SOMETHING like that.... spent almost week there.

[@FlippingPhysics]

Yes. Absolutely had to do an example. It's kind of all just gibberish without a concrete example. Thanks for checking it!

[@michaelwinters327]

thank you for your help, i love the format with three students as it is insightful and funny. great job explaining!

[@FlippingPhysics]

You are welcome!

[@mirabelcoverston]

you are my lord and savior

[@artsydai4818]

I’m working on an inquiry lab and it has to do with angular momentum lab and I was wondering what happens to angular momentum equation if gravity is involved because in my lab I created a rotating beam which rotates if a force acts upon it perpendicularly but it’s not on the floor it’s like in the air ??

[@danielkwon3062]

Can L=rmvsin(x) be used in AP Physic C E&M Question about particles, if any, like 'protons' and 'electrons' ? Is the example problem come out (normally/typically) in FRQ not MCQ? of if comes out in MCQ, is it way simpler? Thank you!

[@FlippingPhysics]

If an object mass mass and is moving, it can have angular momentum, regardless of the size of the object. So yes, protons and electrons can have angular momentum.

[@RickTrajan]

do you have a video about the basics of all this variables?

[@FlippingPhysics]

I have many in-class lecture videos which cover the basics of this video. flippingphysics.com/calculus.html

[@EaszyInitials]

W

[@rodneyd.6195]

Hey thanks a lot man. These two videos on rotational dynamics are extremely helpful. Great job mr physics man.

[@FlippingPhysics]

You are very welcome. I have 40 topic specific videos on Rotational Dynamics which you might find helpful as well. www.flippingphysics.com/algebra.html#rd

[@Ved3sten]

But isn't the gum exerting a horizontal force on the cylinder, based on the diagram, which is causing the cylinder to rotate? I don't understand how Newton's third law applies here, if the force from the gum is horizontal.

[@FlippingPhysics]

The force from the gum is not horizontal. The force from the gum on the cylinder is inward and the force from the cylinder on the gum is outward.

[@Ved3sten]

Flipping Physics Can you please tell me why it’s not horizontal. Isn’t it’s line of action horizontal?

[@FlippingPhysics]

The force from the cylinder on the gum is a normal force and is therefore perpendicular to the surface of the cylinder which means the force will be outward.

[@ampa4989]

If anyone else is wondering, the gum is moving (at constant speed) but exerting no force. This is simply a conservation of momentum problem. Some of the (angular) momentum of the gum has transferred to the cylinder so caused it to spin (presumably very slowly).

[@-andreiDNA]

Awesome channel and very informative! If it wasn't for you I'd fail all my exams lol

[@FlippingPhysics]

No, if it wasn't for you putting in the hard work, you would fail your exams. Give yourself the credit; you deserve it.

[@aedanmckee8698]

This is an amazing video, really helpful.

[@FlippingPhysics]

Thanks!

[@Mainline100]

For the matrix why is it minus j hat?

[@FlippingPhysics]

That is really a math question. Honestly I don't really remember why. The positives and negatives alternate. If there were a fourth term there would be a negative in front of it.

[@carultch]

Construct a copy of the matrix to the left, and a copy of the matrix to the right. Consider the three unit vectors in the middle, as we start with each of these to form the terms of the cross product. Ignore the top row in the other two copies of the matrix. Diagonals that are arranged top-left to bottom-right, I call positive diagonals. Diagonals that are arranged top-right to bottom-left, I call the negative diagonals. Think about our sign convention that right is positive, and which way a ball would roll along a ramp on one of these diagonals, to remember which diagonals are positive, and which are negative. Multiply the groups of three terms along positive diagonals, each starting with one of the unit vectors in the middle matrix. Then multiply the groups of three terms along negative diagonals, each starting with one of the unit vectors in the middle matrix. Assign a negative sign to each of them. Add up all of the above, and that is how you keep track of the cross product visually. This has a lot in common with the linear algebra concept of a determinant, as it is the determinant of a matrix formed from the three unit vectors in the top row, the first vector in the second row, and the second vector in the third row.

[@abhisheaksharma3765]

Amazing!

[@katehdy7164]

Really this class is amazing. Thank you so much for all of these courses!

[@FlippingPhysics]

You are absolutely welcome!