A tag for questions about the mechanical interactions of rotating objects, including torque and angular momentum.

learn more… | top users | synonyms

4
votes
4answers
2k views

Why do rolling disc (coin) move in circular path?

We have a coin that is rolled such that it's tilted at an small angle $ \theta $. Question:: What turns around rolling disc so that it traces circular motion (spiral as it's speed decreses)? ...
4
votes
2answers
1k views

Question about moment of inertia and velocity

First off, I swear this is not homework. I'm doing some practice problems because I got an exam coming up. I'm stuck on this one: I figured I would use energy conservation for this problem. So since ...
4
votes
1answer
584 views

If a bullet is fired vertically upwards, when it comes back does it fall to the same spot? [duplicate]

What I'm basically asking is that if a body is projected with sufficiently high velocity so that it doesn't escape from the earth's gravitational field but reaches an appreciable height with respect ...
4
votes
1answer
336 views

Why is it difficult to ride a bicycle with a partially inflated or deflated tyre?

It is a common observation that riding a bicycle with an inflated tyre is easier than riding one with a deflated tyre but why is it so? As per my knowledge in an ideal case of no deformation in ...
4
votes
3answers
7k views

Why does the coriolis effect dissapear at the equator?

I'm studying from the book "Classical Mechanics" by Goldstein and from a coursebook my Professor provided me. In the coursebook, it says that "the Coriolis effect disappears at the equator (Where ...
4
votes
3answers
5k views

Which force makes a wheel roll down the hill? What causes friction?

A wheel rolling down a hill has two axis of rotation. One is where the center or mass is and the other is the point of contact with the surface which acts as a fulcrum. I was trying to understand ...
4
votes
4answers
908 views

Wheel locks and spinout

Imagine driving in a straight line on a ice lake, when you hit the brakes, if your goal is to stay in straight path with no spinout, which wheels would you choose to have locked: front or rear? ...
4
votes
3answers
349 views

What is an intuitive explanation using forces for the equatorial bulge?

The earth is not a sphere, because it bulges at the equator. I tried fiddling with centripetal force equations and gravity, but I couldn't derive why this bulge occurs. Is there (a) a ...
4
votes
4answers
14k views

Static as opposed to Kinetic Friction in Rolling Motion

During analysis of rolling motion, why do we consider coefficient of friction as that of static friction and not kinetic friction?
4
votes
4answers
3k views

Outcomes of earth slowing down spinning on its own axis

What are the possible outcomes if earth slows down spinning on its own axis? To be specific: Can the decrease in the internal centrifugal (or centripetal) force due to slowing down earth's spin: ...
4
votes
1answer
6k views

Ball Rolling Down An Inclined Plane - Where does the torque come from?

There is a ball rolling down an incline, with no slipping. If we consider the point of contact between the ball and the inclined plane to be the pivot point (for our torque calculations), then I have ...
4
votes
1answer
471 views

How to simulate rotational instability?

I'm trying to simulate (for an educational game) the well-known effect that rotating objects with three nonequal moments of inertia are unstable when rotated around the middle axis. Some explanations ...
4
votes
2answers
3k views

Angular momentum in a rod rotating around one end?

Sorry if I can't get straight to the point, I have to give a lot of details before I actually state the question. The formula for angular momentum is $L=I \omega$. If we look up $I$ for a thin rod ...
4
votes
1answer
97 views

Total angular momentum of earth

I have problem understanding total angular momentum of earth about center of sun. Consider that earth has an orbital angular velocity of $\omega_0 \hat z$ and a spin angular velocity of $\omega_s \hat ...
4
votes
2answers
150 views

Can net torque $\sum_i\mathbf r_i\times\mathbf F_i$ be expessed as $\mathbf r\times$ (net force) for some $\mathbf r$?

Let $\mathbf F_i$ be forces each of which is applied on $\mathbf r_i$ of a rigid body. Then is there a position vector $\mathbf r$ that satisfies $$\displaystyle\sum_i\mathbf r_i\times\mathbf ...
4
votes
3answers
290 views

Why doesn't the ball have rotational energy after it leaves the ramp?

I am having trouble solving #13 from the 2010 F=MA contest: A ball of mass $M$ and radius $R$ has a moment of inertia of $I = \frac{2}{5}MR^2$. The ball is released from rest and rolls down the ...
4
votes
5answers
682 views

What happens to angular momentum when matter is converted to energy?

Let's say a spinning star radiates mass-energy only from it's pole regions. How does the loss of mass-energy effect the angular momentum of the star?
4
votes
1answer
142 views

In 2-dimensional and 3-dimensional universes, stellar systems and galaxies are flat and disky. But what about in 4-dimensional universes?

I just watched that interesting video: https://www.youtube.com/watch?v=tmNXKqeUtJM In 2 dimensions a cloud of particles rotating in a plane is flat by definition since it's in 2 dimensions. ...
4
votes
2answers
985 views

Why does pitch in a helicopter take effect 90 degrees later?

In a helicopter if you want to give it a forward pitch, you change the angle of the blades when it is in this position ---- So the two blades experience unequal lift and because o gyroscopic ...
4
votes
1answer
267 views

Understanding gyroscopes

Considering the typical situation of a rotating bicycle wheel held by one end of its axle by a rope tied to the ceiling: gravity torque is the time derivative of the angular momentum, and in this case ...
4
votes
2answers
534 views

Thrust center in space

I have this dilemma: Suppose you have a space ship somewhere in deep space, where there is no drag force or substantial gravity. If the ship has a single engine situated in such a way that the center ...
4
votes
1answer
264 views

Stability of a rotating ring of multiple electrons at relativistic speeds

There was a time when physicists where concerned about electron internal structure. The rotating ring model was one of the proposals to explain how a charge density could become stable against ...
4
votes
1answer
351 views

How do I visualize the non-coaxial rotation of this device?

The picture below shows an isolated system with a fairly massive wheel at one end, attached via its axle to a long shaft, like a bike tire on a bike frame, but the bike frame is merely a low mass ...
4
votes
1answer
290 views

Does the Moon's orbital inclination follow the wobble in the Earth's rotation?

As the Earth wobbles during rotation, does the higher gravity at the equator tend to pull the moon toward an equatorial orbit even as the earth does that thousands of years wobble cycle? It would ...
4
votes
1answer
99 views

Optimal size for a bike tire

While I was riding my bike funnily enough I wondered if there was an optimal size for a bike wheel. So I suppose that is my question, What is the optimal size for a bike wheel? If the bike accelarates ...
4
votes
1answer
228 views

Sum of energy for 2 solids in rotation

I would like to compute the sum of energy of the following case: Two solids are turning (disks). Yellow solid is turning at $w_1$ radians per second around its center of gravity and blue solid is ...
4
votes
1answer
273 views

Sum of forces with liquid in rotation

It's not homework (I'm teacher). I would like to compute sum of forces on this study : The shape is symmetrical like that I'm sure the center of gravity is in the center of the shape. I compute ...
4
votes
1answer
66 views

Taking pivot about an accelerating point

Given this question: A small ball of mass $m$ and radius $r$ rolls without slipping on the inside surface of a fixed hemispherical bowl of radius $R>r$. What is the frequency of small ...
3
votes
3answers
92k views

Linear acceleration vs angular acceleration equation

I'm learning about angular velocity, momentum, etc. and how all the equations are parallel to linear equations such as velocity or momentum. However, I'm having trouble comparing angular acceleration ...
3
votes
3answers
880 views

Is a spinning object heavier than a non-spinning object?

Does spinning an object make it heavier? A real-world example: I was mowing the lawn in front of my house, a lawn that tends to have some steep inclines. I realized that the lawn-mower was easier ...
3
votes
2answers
281 views

Rotation matrix of Euler's equations of rotation relative to inertial reference frame

I was playing with simulation of Euler's equations of rotation in MATLAB, $$ I_1\dot{\omega}_1 + (I_3 - I_2)\omega_2\omega_3 = M_1, $$ $$ I_2\dot{\omega}_2 + (I_1 - I_3)\omega_3\omega_1 = M_2, $$ ...
3
votes
2answers
189 views

Conceptual rolling

If you could roll a ball down a slope with friction, we would all know which direction the ball is rolling. However, say we have a frictionless slope and we roll the ball in the opposite direction and ...
3
votes
5answers
2k views

Veritasium - Firing bullet in block - along center and away from center

This question is about this video on YouTube, in which a bullet is fired vertically into the center of a wooden block from below, sending the block up into the air. Next, a bullet is fired vertically ...
3
votes
1answer
1k views

how does international space station change its orientation?

In what way does the rotating gyroscope help in changing the orientation of the international space station in space ? Is there any other way to change the the orientation of ISS ?
3
votes
2answers
361 views

Astronaut on a rotating asteroid

This might sound like the silliest question ever, but can someone shed some light on what is actually happening in the following situation: There is an asteroid in outer space what is rotating with ...
3
votes
2answers
1k views

Can moment of inertia be negative? [closed]

Q: Find out the moment of inertia of a uniform circular disc of radius $r$ & mass $M$ & the axis passes through a point on the circumference. My attempt: Let the axis passes through $O$ on ...
3
votes
3answers
3k views

Why doesn't static friction decelerate a rolling body?

I know that static friction isn't the cause of deceleration of a rolling body. But if static friction is the only force in the horizontal direction, then shouldn't there be some acceleration produced ...
3
votes
4answers
22k views

Relationship between torque and angular momentum

I want to know how to deduce the equation $\vec{\tau}=\vec{\omega} \times \vec{L}$, where $\vec{\tau}$ is the moment of force (also known as torque), $\vec{L}$ is the angular momentum, ...
3
votes
2answers
2k views

Force applied off center on an object

Assume there is a rigid body in deep space with mass $m$ and moment of inertia $I$. A force that varies with time, $F(t)$, is applied to the body off-center at a distance $r$ from its center of mass. ...
3
votes
3answers
2k views

Does a toy top weigh less when it is spinning?

I am under the understanding that a toy top will weigh less when it is spinning. The Russians made a spinning type transport back in the 70s to lessen its payload over the tundra. Is this an effective ...
3
votes
3answers
629 views

what make the bottom portion of a wheel in rolling motion move?

As I just learn about the rolling motion which is the combination of pure translation and pure rotation. The top portion of the rolling body has the speed of double speed at the center of the object ...
3
votes
2answers
214 views

Maximum permissible speed while going down a ramp

So, I was playing hill climb racing and I noticed that if we move with high speeds towards a ramp going down we just jump it off. While lower speeds, help us to stay in contact with the ramp. ...
3
votes
1answer
451 views

What happens to a rotating rod that breaks in two?

I know that the approximation for the moment of inertia of an infinitely thin rod of mass $m$ and length $L$ spinning around an axis perpendicular to its own axis at its center is $\frac{mL^2}{3}$: ...
3
votes
1answer
108 views

Why isn't kinetic energy conserved in this rotational dynamics problem?

Consider a uniform rod which is spinning about an axis that goes through its centre, perpendicular to the rod itself. Two small rings are attached on the rod at equal distances from the centre. As the ...
3
votes
1answer
68 views

What would a star look like that was the most oblate possible?

This question about why is the Sun so spherical and it's corresponding answer made me wonder: If some younger stars that are rotating faster than the sun are more oblate, then how oblate are they? ...
3
votes
2answers
213 views

Free rotation of a rigid body

So I am currently reading Fowles and Cassidy and there is something I'm confused about in the section about geometric description of free rotation of a rigid body. I will present the stuff first that ...
3
votes
2answers
525 views

can we calculate velocity of a spinning apparatus by just analysing the frequency of sound emitted by it

I have used a new term spinning apparatus as I was unable to name it. I have tied a thread to a stone and was spinning it and I heard a sound something like that of a rotating propellor of a ...
3
votes
2answers
6k views

Moment of inertia of disc with a hole

Suppose we have a disc with a hole, when computing moment of inertia of this about the disc's centre. Why do we subtract the moment of inertia of the removed part from the moment of inertia of ...
3
votes
1answer
874 views

Why does dust stick so well to fan blades? [duplicate]

After reading and understanding the reasons why dust stick to rotating fan propeller, I am interested to find out why the dust particles stick so well. Spraying powerful jets of water does not ...
3
votes
2answers
1k views

Ice skater increase of energy

This may be a very basic question but I am not seeing how it works. Consider the standard example of an ice skate rotating about his/her center of mass and pulling in his/her arms. The torque is zero ...