This is a perfect video in so many ways and extremely relevant to our lesson for the day!!

So we’ve all used the idiom “it’s just like riding a bike” in regards to an action that has become second nature to us. But our primary mode of recreational transport as young people exemplifies one of my favorite physical principles, Pure Roll.

Pure Roll is commonly referred to as “rolling without slipping.” In this situation we see a specific relationship develop between speed, v, of the CoM and the angular speed, w, of the wheel’s rotation, v = rw. The top of the bicycle wheel and the bottom are going this same speed, v = rw, but in opposite directions. Now, if we add in the velocity of the CoM, we find that, at any instant, the top of the wheel is going twice as fast as it’s center and the bottom of the wheel is at rest with respect to the ground! Whoa!! Here, the force of friction does no work because there is no relative motion!! But friction does ensure that a portion of the total kinetic energy is rotational kinetic energy. Now, please enjoy this edition of Darren and the Tiger concerning Pure Roll.

Ok, so I would like you to have a go at this scenario from the workbook in the next couple days. It is a great example of crossover concepts!! If you use Conservation of Energy, Rotational Inertia and your qualitative reasoning skills you should do great!!

Also, use your same skills to answer this question: If my Beatles record has a mass of 125g, a radius of 30cm and spins up to 33 1/3 rpm in .80s, what is the power delivered to the turntable??

Thanks so much everyone. It was so awesome to see you these past couple days. I will be doing more problems during class time using the screen share so let’s keep the momentum going!!

Mr. C