8.01x - Lect 24 - Rolling Motion, Gyroscopes, VERY NON-INTUITIVE

What people said

Recommends JustBeingHelpful · 2020-09 · Physics Stack Exchange · on “Why does torque point perpendicular to direction of the motion?”

Why does torque point perpendicular to direction of the motion? Physicists often say they use first principles thinking, but in order to come up with those first principles, they always use real world observations and create equation(s) to explain the observation, but sometimes cannot explain the fundamental reason behind the observation which gave rise to the equation. In three dimensional space, there are two possible orthogonal vectors (torque vector in this example) relative to another plane (rotating object in this example). The torque vector could potentially be in either direction, thus it's counterintutitive. Our universe (from what humans observe from Earth and outer space in our so

Recommends Aseem Mittal · 2021-02 · Physics Stack Exchange · on “What is the motion of two charges constrained by a bar in a magnetic field?”

First of all I want to say that i also don't know exactly how the rod will move, but i can say for sure that it will not be the way you are saying. Now before you read this any further, i suggest you have a look at this [this video] (part relevant to your question starts at about 14:06. Also i suggest you watch the whole video from there as there will be clearer demonstrations after he derives the equations.). So, if you have watched the video, i guess you can tell where the problem is. The angular momentum that you gave to the rod ( which I am assuming is along y-axis ) is along the x-axis whereas the torque due to magnetic field will be along z-axis. So, the rod will try and realign itself in s

Cites andrehgomes · 2017-11 · Physics Stack Exchange · on “Mass and Newton's Second Law”

Force could be treated as a vector even if mass behaved different in different directions. In this case, mass would not be a simple scalar, but it would be what is called a second rank tensor. Treating vectors as column $3 \times 1$ matrices, force and acceleration would be $ \mathcal{F}=\left( \begin{matrix} F_x\\ F_y\\ F_z\\ \end{matrix} \right) \quad \text{and} \quad \mathcal{A}=\left( \begin{matrix} a_x\\ a_y\\ a_z\\ \end{matrix} \right); $ the second rank tensor representing mass would be simply a $3 \times 3$ matrix: $ \mathcal{M}= \left( \begin{matrix} m_{xx} & m_{xy} & m_{xz}\\ m_{yx} & m_{yy} & m_{yz}\\ m_{zx} & m_{zy} & m_{zz}\\ \end{matrix} \right); $ and Newton's second law would

Cites Rory Cornish · 2019-12 · Physics Stack Exchange · on “An expert opinion on controversial video appearing to defy Newton's Laws”

Thanks for your comments. Regarding the transfer of momentum comment. I think I know what you are referring to (see [this video] ) at 23:22. Here angular momentum is being transferred to prof Lewin. But this is pure torque, and there is no net force.

Cites Chaitanya Garg · 2020-10 · Physics Stack Exchange · on “Torque in a rotating wheel about a axis”

[LINK] In the given situation there wheel is rotating about its own axis as well as about the axis PQ. Now my Query is that which torque of which force is responsible for the rotation of the wheel about axis PQ. I could not find any external torque on the rod plus wheel system which is in the vertically upward direction .Pls help me out in this. I hope my question is clear. If it is not please do watch the video the link of which I have attached. [this video] (Start from 24:30 minutes)

Cites Adrian Howard · 2020-10 · Physics Stack Exchange · on “Why under pure roll solid cylinder rolls down faster than hollow cylinder? Should they not be the same?”

Here is the video, hollow verses solid starts at 10:10 [this video] but he says nothing about friction being a factor, which it would not.

Cites Linkin · 2020-11 · Physics Stack Exchange · on “Two Cylinders on Ramp”

Just to give strength to user25's answer here is a link to a proof and application of this phenomenon [this video]

Cites Sophile · 2021-08 · Physics Stack Exchange · on “Derivation of precession rate of a wheel gyroscope”

I searched on the web and all I could find is the precession rate of a spinning top. But what I want is the derivation of the precession rate of a wheel hanging from a rope, as shown below: which is taken from this video [LINK] . Professor Walter Lewin says in his video [this video] that $$\omega_{\text{precession}} = \frac{\tau}{L_{\text{spin}}} $$ But the derivation is not given in that video. So it would be much appreciated if someone will give me the derivation.

Cites Rav Raider · 2022-05 · Physics Stack Exchange · on “Will a spacecraft with angular momentum have to course correct if it thrusts along the same axis as its rotation?”

For the sake of illustration, let's say you have a cylindrical spacecraft, sitting "at rest" in space, relatively speaking. It has a thruster on one end. If it fires the thruster, it goes forward in a straight line, in the direction opposite the thruster. [LINK] But what happens if that same cylindrical spacecraft is spinning before you fire the thruster? I have learned from this lecture [this video] that if you apply torque to an object with angular momentum, the object will tilt, as the spin angular momentum moves in the direction of the torque. But I'm not clear on if this thruster would be applying torque or not. Would this spacecraft have the same trajectory in both cases, or would the spinn

Cites WilliamHarvey · 2023-08 · Physics Stack Exchange · on “Why don't bike wheels behave like a gyroscope when you turn them?”

Let's say we have a bike pointing forward in the $+z$ direction into the screen. It's moving in the $+z$ direction hence the wheels have angular momentum in the -x direction ,which is towards the left of your screen. Now when we turn the wheel right we apply a torque in the -y direction. As the change in angular momentum must be in the same direction as the torque why doesn't the wheel's angular momentum "follow" the direction of torque. Kind of like in the following video [this video] at time 19:17.