Laws & Causes

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Strong rec user68014 · 2019-11 · Physics Stack Exchange · on “Rotational Mechanics: Is Angular Acceleration Possible without any External Torque?”

When you pull your arms in you aren't pulling them directly towards the centre, because you're rotating as you're pulling them in. This is where the force comes from that actually makes you spin faster. You should definitely watch this video [this video] where he explains exactly this. Skip to 10m in if you're in a hurry but the whole video is well worth watching. I know what you mean that "conservation of angular momentum" explanations can feel like they're hiding the details of the actual forces and torques going on. You can make a case that the conservation laws are actually more fundamental but either way both explanations are always possible and always give the same result.

Strong rec naturallyInconsistent · 2024-04 · Physics Stack Exchange · on “How can tangential acceleration from a radial force be explained?”

VSauce has a video on this very topic, that is done very well. The answers below are fantastic too, but the video presents a different view that is worth watching [this video]

Recommends Devansh Mittal · 2019-12 · Physics Stack Exchange · on “Rotational Mechanics: Is Angular Acceleration Possible without any External Torque?”

Angular Acceleration is not the same as Translation Acceleration. Translational Acceleration in an object cannot come without any external force, but the angular acceleration in an object may come without any external torque. External torque is NOT the only source of angular acceleration. Angular acceleration in a rotating body may come even when the moment of inertia of the body is changing. Let's consider a simple case in which a car is moving on a straight road and an observer is seeing the car from a distance. In this case, the car will have a variable angular velocity w.r.t. the observer and hence an angular acceleration as well. In this case, we do not ask the question that which torqu

Recommends Manish S · 2021-05 · Physics Stack Exchange · on “What force changes the speed of the mass in circular motion as the radius of the circle decreases?”

When the radius is decreased, the particle will somewhat spiral inwards. This causes a part of the tension to be along the velocity. This causes the tangential acceleration of the particle, increasing its speed. There is a video by VSause, that explains this well. Here is the link: [this video]

Cites user249968 · 2020-01 · Physics Stack Exchange · on “Conservation of angular momentum - linear velocity”

Angular momentum $\mathbf L$ is given by: $$\mathbf L = \mathbf r \times \mathbf P$$ Now the torque $\mathbf {\tau} $ acting on an object is given by: $$\boldsymbol {\tau } = \frac {d \mathbf L}{dt}$$ $$\Rightarrow \boldsymbol {\tau } = \frac {d (\mathbf r \times \mathbf P) }{dt}$$ $$\Rightarrow \boldsymbol {\tau } = \mathbf r \times \mathbf F + \mathbf v \times \mathbf P$$ Since $\mathbf v $ and $\mathbf P$ are in the same direction therefore $$\mathbf v \times \mathbf P = 0$$ i.e., $$\Rightarrow \boldsymbol {\tau } = \mathbf r \times \mathbf F $$ Now as you correctly say that $\mathbf L$ is constant therefore $$\boldsymbol {\tau } = \mathbf r \times \mathbf F =0$$ Clearly since $\mathbf r

Cites Cleonis · 2023-01 · Physics Stack Exchange · on “Do hurricanes spin?”

You are framing your question in terms of 'what would a satellite observer in space see'. I assume that what you are asking is: To an observer who is not co-rotating with the Earth, how does air move as a hurricane starts to form'. I propose to phrase the question as follows: We define the following coordinate system: the non-rotating coordinate system that is co-moving with the center of mass of the Earth. In order to have a short name I will refer to this coordinate system as 'the inertial coordinate system' (To refer to it each time as the comoving-with-the-Earth-center-of-mass-inertial-coordinate-system is too many words, so I use the shortened 'the inertial coordinate system.) We can ha

Cites Ani · 2023-01 · Physics Stack Exchange · on “What exactly causes the velocity to change in conservation of angular momentum when an external force is applied?”

Suppose an object is rotating in a circular motion, and we change increase its radius pulling it outwards in such a way that the net torque is always zero. In such a case, the angular momentum will be conserved so the velocity and the omega of the object will decrease. I wanted to know the exact mechanical reason behind this thing, that which force exactly slows the velocity of the object while it moves from smaller radius to the bigger one. And I got the answer from a video on YouTube: [this video] (just skip to 12:00 to get what I am talking about) The part of the video basically says that the centripetal force starts decelerating the object when it jumps from one orbit to another (following a

Critical naturallyInconsistent · 2026-01 · Physics Stack Exchange · on “The Orthogonality Paradox: Why the Work-Energy Theorem ($W=0$) forbids Tangential Acceleration in Radial Force Systems?”

Edit: My answer is superseded by J.G.'s much more proper answer, so please instead refer to that. Your question is just showing how contradictory that you are. While the radial component of work $(\int F_r\,\mathrm dr)$ accounts for changes in radial kinetic energy We basically always consider the quasistatic limit for the radius change. In this very reasonable approximation, radial KE = 0 for both initial and final states, or your can take a constant radial speed if you so want, and that means $\Delta KE_\text{radial}=0$ no matter what. The fact of the matter is that there is no such decomposition of the energy transfer that you think you can do. The kinetic energy is one whole thing, and t