A tag for questions about rotational motion, including angular velocity and angular acceleration.
9
votes
5answers
936 views
How is it that angular velocities are vectors, while rotations aren't?
Does anyone have an intuitive explanation of why this is the case?
7
votes
4answers
417 views
Why does a ping pong ball change direction when I spin it on a table?
When I spin a ping pong ball on the table, it rolls forward in the opposite direction of the spin, and then eventually changes direction and rolls backward.
Here's a video demonstrating the effect.
...
5
votes
3answers
265 views
Do rotating bodies emit gravitational waves?
Suppose we have a cylinder of mass $m$, radius $R$ and height $h$ in rotation with speed $\omega$ around its symmetry axis with no friction (ideal situation).
I'd expect this cylinder to emit ...
5
votes
2answers
251 views
Do we have any effect on the rotation of he earth?
We walk or ride on our vehicles to our destinations daily. Does our movement have any effect on the rotation of the earth according to Newton's law? What will be the effect if we move all the peoples ...
5
votes
2answers
249 views
Earth moves how much under my feet when I jump?
If I'm standing at the equator, jump, and land 1 second later, the
Earth does NOT move 1000mph (or .28 miles per second) relative to me,
since my velocity while jumping is also 1000mph.
However, ...
5
votes
1answer
106 views
In tennis, why does topspin serve bounce higher than flat serve?
When receiving servers (while playing tennis), I've noticed that the tennis ball seems to bounce up higher on me when the server uses a topspin serve than when the server hits a flat serve. Why is ...
4
votes
4answers
978 views
Two axes for rotational motion
I understand that angular momentum is a vector, etc..
But, what really happens when some object, say a ball for example, is set to rotate along two axes? What would the resulting motion look like?
4
votes
1answer
135 views
Creating a fair 3 sided coin
I want to make a cylindrical three sided fair coin, with sides: heads, tails, and edge.
What should the area of the edge be in relation to the area of the head of the coin?
Assume it is all made of ...
4
votes
1answer
106 views
Rotation of Spacetime => Change in orbit/path
Along the idea of frame-dragging;
Will the rotation of a black hole, which has some velocity v and angular momentum, influence its path in 3D space?
I've seen the fact that depending on the ...
3
votes
3answers
86 views
Origins of Moment of Inertia
Where exactly does the equation $MR^2$ for moment of inertia come from? The quantity itself seems fairly arbitrary.
3
votes
1answer
231 views
Why is the velocity on the top of a wheel twice the velocity of its axle?
When a wheel is rolling, not skidding, and its axle moves at velocity $\vec{v}$, then a point on the top of its circumference will move at velocity $2\vec{v}$, shown below.
I really don't ...
3
votes
1answer
220 views
Definition of angular velocity
I'm trying to follow the Feynman lectures and I'm stuck on a particular piece. Let me frame it for you before I ask my question. Nobody does it better than the author himself,
we must find out how ...
2
votes
2answers
220 views
Piston movements in four stroke cycle?
I was reading about a four stroke cycle. Here's what I understood:
In the first stroke, the piston starts at the top and moves down.
In the second stroke, the piston moves upwards.
In the third ...
2
votes
1answer
1k views
Merry go round physics problem?
So I have the following statement.
"A merry-go-round is spinning with a fixed angular speed. As a person is walking towards the edge, the force of static friction must increase in order for the ...
2
votes
3answers
1k views
Angular momentum equations
I do not understand this because angular momentum is $L=I\omega$ ($I$ is moment of inertia;$\omega$ is angular velocity) but it I have also seen equations where $L= rmv\sin(x)$. I do not understand ...
2
votes
1answer
249 views
How does weight/mass affect angular momentum?
How does weight/mass affect angular momentum?
For my 8th gr science fair project I have to do an experiment on angular momentum. My problem is that we have not been taught any of that in physics yet, ...
2
votes
1answer
212 views
Euler angle: space-fixed vs body-fixed axes
I am sooo confused!! Between active and passive, intrinsic and extrinsic, vectors and basis ....
Stipulate that we stick to active rotations only. Then
Standard derivation of $R(\alpha, ...
2
votes
1answer
153 views
Relating angular and linear kinematics
In my physics book "University Physics", there is a chapter on relating linear and angular kinematics.
I understand the parts where it shows $v = r\omega$ and $a_{\text{tan}} = r\alpha$.
However in ...
2
votes
1answer
1k views
What's the right way to calculate the principal moment of inertia?
I am writing a program that incorporates calculating the principal moment of inertia for a protein residue based on its component atom XYZ coordinates. I am exceedingly confused about which formulas ...
2
votes
1answer
312 views
Equations of motion for an inertial computer mouse
I'm struggling with a seemingly simple problem in 2D motion. It has been many years since I looked at basic kinematics and my mind seems to be too slow and confused now. Imagine we want to build a ...
2
votes
1answer
245 views
Paradox of the Relativistic Record Player [duplicate]
Possible Duplicate:
Invariant spacetime - distance - Circular Motion
This is a question that I thought up a few years ago when I was taking mechanics. I asked the professor but didn't ...
1
vote
5answers
312 views
Why can mass not be considered concentrated at CM (center of mass) for rotational motion?
Could anyone explain the following expression: Why can mass not be considered concentrated at CM (center of mass) for rotational motion?
1
vote
2answers
3k views
What is the difference between angular speed and tangential speed in a circular motion?
I was looking a long time for the way the equations of this two speeds are obtained, and i found pretty much nothing important, so can someone explain how are those obtained, and which is the ...
1
vote
2answers
67 views
force applied not on the center of mass
when applying a force not on the center of mass of the body, the body will get both linear and angular mumentom (right?).
does the linear velocety from this force (that is applied not on the center of ...
1
vote
2answers
295 views
3D: Get linear velocity from position and angular velocity
I want to find out the linear velocity of a point in 3D space, (Euclidean), given:
Its position
Its angular velocity
The point it's rotating around (fulcrum)
(This is a problem I need to solve ...
1
vote
1answer
32 views
Determine the velocity and acceleration of the vertex $B$
1) The bent rod $ABCD$ rotates about the line $AD$ whit a constant angular velocity of $90 rad / s$. Determine the velocity and acceleration of the vertex $B$ when the rod is in the position shown in ...
1
vote
2answers
423 views
Applying angular velocity to a rotation matrix
I have a very simple question. In our project we store an object's orientation as a 3x3 matrix which holds the orthonormal base of that object's local space. For instance if the object is aligned with ...
1
vote
2answers
239 views
Extracting acceleration vector from rotated aircraft
Suppose we have an aircraft with accelerometer measuring accelerations along each axis. It is mounted in a way so it is perpendicular to the plane in all axes (that should be obvious). We also have ...
1
vote
2answers
506 views
What are the expressions for rotational and translational kinetic energies of a system of point particles?
Consider a system of point particles , where the mass of particle $i$ is $\mu_i$ and its position vector is $r_i$. What are the expressions for translational kinetic energy and rotational kinetic ...
1
vote
1answer
599 views
Hollow Wheels Down a Hill
Lets say I have a hoop of mass $M$ and radius $R$. It is rolling down a hill without slipping, so I don't need to worry about friction doing work on it. Lets say the angular speed is $\omega = ...
1
vote
1answer
43 views
Problem with a rotating frame of reference on the South pole
Consider this problem:
A high-speed train is traveling at a constant 150 m/s (about 300 mph)
on a straight horizontal track across the south pole. Find the angle
between a plumb line suspended ...
1
vote
1answer
28 views
Asking about centrepetal acceleration
please look at the fig first
1) How can you claim that the triangle ABC is same as the triangle PQR?
2) How can you claim that the angle between V1 and V2 is same as the angle between AC and AB?
...
1
vote
2answers
144 views
Kinetic energy of a cylinder
It is a long cylinder (you can approx $R=0$), and it has a fixed point in one os its ending points, it's rotating on a plane and I have to calculate the kinetic energy from reference systems situated ...
1
vote
1answer
198 views
Non-commutative property of rotation
Addition of angles are non-commutative in three dimensions. Hence some other angular vector quantities like angular velocity, momentum become non-commutative. What is the physical significance of this ...
1
vote
2answers
321 views
motion in the body-fixed frame?
This is really basic, I'm sure: For rigid body motion, Euler's equations refer to $L_i$ and $\omega_i$ as measured in the fixed-body frame. But that frame is just that: fixed in the body. So how ...
1
vote
2answers
269 views
Spin angular momentum of a system of particles : Is there any energy associated with it?
Consider a system of point particles , where the mass of particle $i$ is $μ_i$ and its position vector is $\vec{r}_i$. Let $\vec{r}_\text{cm}$ is the position vector of the center of mass of the ...
1
vote
1answer
167 views
Finding stopping time when only given initial angular velocity and an expression for angular acceleration?
Question:
A wheel starts is spinning at $27\text{ rad/s}$ but is slowing with an angular
acceleration that has a magnitude given by $\alpha(t) = (3.0\;\mathrm{rad/s^4})t^2$. It stops in a time ...
1
vote
1answer
117 views
Double Compound Pendulum: why use inertia about the center of mass for bottom pendulum?
I'm trying to wrap my head around the kinetic energy of a double compound pendulum, like the one shown in the Wikipedia article on double pendulums.
I know for computing the kinetic energy of the ...
1
vote
1answer
42 views
interpreting aspects of rotational motion conceptually [closed]
Level - First Year Physics University
I don't understand the concept of angular momentum, conceptually. What is it? if I were to explain it how would I go about doing that? without having to explain ...
1
vote
0answers
36 views
Calculate Rotational Intertia
If a can of soup, and a can of beans (tightly packed), are set in a race down a rough hill (has friction), the soup wins, because the inside of the can (soup) is not drawing energy from the system.
...
0
votes
2answers
214 views
Combined Translation and Rotation of a disk possible? material and references?
I am considering building a robot that can rotate and move at the same time. Since it's just a theoretical idea at the time and I need read up material, I thought I would ask here.
I am thinking of a ...
0
votes
2answers
300 views
Vector Nature Of Angular Velocity
I am currently reading about angular position, angular velocity, and angular acceleration. I came across this paragraph that was particularly confusing, and was wondering if someone could perhaps help ...
0
votes
1answer
148 views
How is torque equal to moment of inertia times angular acceleration divided by g?
How is the following relation true
$$\tau = \large\frac{I}{g} \times \alpha$$
where $\tau$ is torque,
$I$ is moment of inertia,
$g= 9.8ms^{-2}$,
and $\alpha=$ angular acceleration.
0
votes
1answer
448 views
What is negative angular acceleration?
I was doing an exercise in physics and I noted that the angular acceleration was negative. What does that mean?
0
votes
1answer
154 views
Two Different Sorts of Inertia: Inertial Mass and Moment of Inertia
There are two different sorts of inertia: inertial mass and moment of inertia.
I am currently reading about moment of inertia. Now, I know inertia is an important concept; with it, we can determine ...
0
votes
1answer
320 views
Finding Constant Angular Acceleration
The question is, "A centrifuge in a medical laboratory rotates at an angular speed of 3500 rev/min. When switched off, it rotates 46.0 times before coming to rest. Find the constant angular ...
0
votes
3answers
2k views
Finding Angular Acceleration of rod given radius and angle
A uniform rod is 2.0 m long. The rod is pivoted about a horizontal, frictionless pin through one end. The rod is released from rest at an angle of 30° above the horizontal. What is the angular ...
0
votes
2answers
32 views
Expected behavior of the gravity under some experiment [duplicate]
I would like to know the expected behavior of the Gravity under the following mentioned imaginary experiment:
What if we dig a well or a boar or a straight hole (say, its diameter is 100 meter) ...
0
votes
2answers
27 views
3D Vector Rotation of Point Mass
Triangle defined by points OA, OB and OC : (-i,3 j,-4 k), (i,2 j,2 k) and (3 i,7 j,- k) where i, j, k are unit vectors along x,y,z axis. Point mass is placed at OA. Triangle rotates with angular ...
0
votes
2answers
99 views
Internal/Rotational angular momentum
I have some difficulties to understand the relation between the internal and the rotational angular momentum of a rigid body which is also known as König's theorem, so what physical intuition lies ...
