All Questions
10,535 questions
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What method should I use to solve for the final acceleration of a projectile being launched from the earth's surface?
What method should I use to solve for the final acceleration of a projectile being launched from the earth's surface?
The question I am working on is:
A projectile is launched vertically from the ...
- 237
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2
answers
2k
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How is angular momentum conserved when a spinning top finally stops spinning?
Where does the top's angular momentum get transferred to? Does it very slightly change the angular momentum of the table, and then the angular momentum of the Earth?
- 3,662
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2
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Essential background for QFT study
The preface to Mark Srednicki's "Quantum Field Theory" says that to be prepared for the book, one must recognize and understand the following equations:
$$\frac{d\sigma}{d\Omega} = |f(\theta,\phi)|^2,...
- 453
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Lagrange's equations: What is $\dot{q}_j$?
I'm looking at the solutions to a problem about a uniform thin disk. For the sake of this question, I start with
$$L=\frac{1}{2}m\left( r\omega \right)^2$$
Then we plug it into Lagrange's equations:
$...
- 1,483
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D'Alembert's Principle: Where does $-Q_j$ come from?
This is a follow-up question to D'Alembert's Principle and the term containing the reversed effective force.
From the second term of Eq. (1.45)
$$\begin{align*}
\sum_i{\dot{\mathbf{p}}_i \...
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1
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D'Alembert's Principle and the term containing the reversed effective force
For our Classical Mechanics class, I'm reading Chapter 1 of Goldstein, et al. Now I come across Eq. (1.50). To put it in context:
$$\begin{align*}
\sum_i{\dot{\mathbf{p}_i} \cdot \delta\mathbf{r}...
- 1,483
4
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2
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554
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Coincidence detectors in Bell tests: How close is close enough?
When is a coincidence a coincidence? We know that to identify entangled photons, the electronics is set to look for simultaneous clicks at opposite detectors. The size of the window is to some degree ...
- 4,219
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320
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Why isn't pressure used for flight?
Why isn't pressure used as flight?
I've heard that 2L bottles can hold a pressure of up to 90 PSI safely. Since $F = PA$, if the nozzle of a pressure rocket is about 4 inches squared in area, that ...
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1k
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Non-Linear Density Shell Problem
I'm trying to understand Newton's Shell Theorem (Third)
http://en.wikipedia.org/wiki/Shell_theorem
However this applies to a sphere of constant density. How is this formulated for sphere of varying ...
- 1,538
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Is it theoretically possible for the orientation angle of a projectile to remain exactly equal to the orientation of velocity?
This question is sparked by my answer to this question: Is this simulation following real physics?
After examining the math, I don't see how it is theoretically possible for the situation simulated ...
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6
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When is the Hamiltonian of a system not equal to its total energy?
I thought the Hamiltonian was always equal to the total energy of a system but have read that this isn't always true. Is there an example of this and does the Hamiltonian have a physical ...
- 1,820
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Invariance of Lagrange on addition of total time derivative of a function of coordiantes and time
My question is in reference to Landau's Vol. 1 Classical Mechanics. On Page 6, the starting paragraph of Article no. 4, these lines are given:
If an inertial frame $К$ is moving with an ...
user4235
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2
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How could pinion in automatic quartz watch be rotated at 100K RPM?
Wikipedia article on automatic quartz watch describes the watch mechanism as follows: a rotating pendulum is attached to a pinion and when the wearer moves his hand the pinion is rotated at up to 100 ...
- 831
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9k
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A spinning bullet
I know the rifling in a gun or rifle puts a spin on the bullet along the axis of trajectory. Now I don’t understand exactly why does it make the trajectory more stable and allow for greater travel?
- 734
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Invariance and forms of the Lagrangian
I have been reading the 1st chapter of Landau & Lifshitz Mechanics, and due to its concise style been facing a few problems. I hope you can help me out here somehow.
Does the "homogeneity of ...
- 103
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what is uniform velocity?
i have a very basic question from school days. what does it mean to say an object is moving with uniform speed? it seems to me now that it should be an unit dependent concept.
for example if speed is ...
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In a gas of particles, how is the displacement vector related to the number density?
Suppose I have a gas of particles that is initially uniformly distributed so that the number density is $n_0$ (number of particles per unit volume), and then I displace the particles by the vector ...
- 852
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2
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4k
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Stress vs Strain for mild steel
After Yield point on stress strain diagram the under curve occurs what does it mean what will happen for the mild steel at that particular time and again why the curve goes to up and reaches ultimate ...
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1k
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Collision between a rod and a bullet
There lies a homogeneous rigid rod of mass $M$ and of length $H$ on a frictionless table at rest. A small bullet of mass $m$ moves toward the rod with velocity $v_0$, perpendicular to the rod and ...
- 2,717
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What are the properties of two bodies for their collision to be elastic?
For example, must the shock wave in each body be of a particular form which influences the shape and material properties of the bodies?
I suspect part of the the answer is that the objects must be ...
- 5,398
3
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2
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694
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Wave equations & propagation theories
I'm interrested in making computer simulation but I've run into rather physics oriented problem. I have to choose how to propagate my wave. Though I've found technique called FDTD (finite-difference ...
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2
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351
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Church–Turing Thesis [closed]
Can the Church–Turing Thesis be proved assuming classical mechanics, how is the proof or disproof?
Edited: I was looking for a proof of "everything computable by a device obeying CM is computable by ...
- 5,301
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Spinning bucket of water in zero gravity
Everyone knows how the surface of a spinning bucket of water would look like on earth - parabolic. But what if we turned off gravity (for instance by doing the experiment in a freely falling lift)? ...
- 1,977
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Why are generalized positions and generalized velocities considered as independent of each other? [duplicate]
I'm confused how
$$\dot{\mathbf{r}}_{j}=\sum_{k}\frac{\partial\mathbf{r}_{j}}{\partial q_{k}}\dot{q}_k+\frac{\partial\mathbf{r}_{j}}{\partial t}$$
leads to the relation,
$$\frac{\partial\dot{\...
- 1,977
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0
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264
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Displacement due to sinusoidal load on a finite strip in an infinite plane
From a paper on tunnel design I've been reading: (http://www.sciencedirect.com/science/article/pii/0886779887900113)
In the present application, the solu-
tion corresponding to a sinusoidal load
...
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Artificial Gravity - Spinning Station Questions II
In an answer to Artificial Gravity - Spinning Station Questions Vintage wrote:
A theoretical space station of radius $900\ \mathrm m$, doing a complete rotation every $60\ \mathrm s$ (in order to ...
- 734
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3
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333
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Constrained particles under distance dependent force
This question is from the 1975 Canadian Association of Physicists Exam. No solutions are posted and I am quite lost on how to proceed with it.
A particle is constrained to move along the x-axis of a ...
- 1,294
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Invariance of Lagrangian in Noether's theorem
Often in textbooks Noether's theorem is stated with the assumption that the Lagrangian needs to be invariant $\delta L=0$.
However, given a lagrangian $L$, we know that the Lagrangians $\alpha L$ (...
- 2,930
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How do you find conserved quantities for linear second order ODEs?
I have a differential equation of the form
$ \frac{d^2 y}{dt^2} + f(t) \frac{dy}{dt} + g(t) y = 0 $
where $f$ and $g$ are known functions of time.
Is there a systematic (or otherwise) way of ...
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4
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What do we consider "Perpetual Motion"
I know this is a bad question to most serious Physics but I have a question about what is considered “Perpetual motion.” The Foucault pendulum in the UN consists of sphere that passes directly over a ...
- 734
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318
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a question on Lagrange's equation when the time derivative of the generalized co-ordinates is constant
Consider a system whose generalized co-ordinates are $q_i$ and is under the constraints $\dot{q_i} = K_i \forall i = 1,2,3,...$ where $K_i$ are constants. I have a problem in writing the Lagrange's ...
- 2,172
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2
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Do we know exactly how fast we are going relative to the center of the galaxy
I mean total inertial on my body siting here at my computer "California US." Ok so the earth is rotating on its axis and in turn around the sun and the sun around the galaxy. The object the question ...
- 734
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Sum of angular momentum of all electrons in a magnet
Can the sum of angular momentum of all rotating electrons in all the aligned atoms in a permanent magnet have a significant contribution to the macro angular momentum of the magnet? If yes, why does ...
- 977
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Analyzing the motion of a ball rolling without slipping inside a hemispherical bowl
Consider a solid ball of radius $r$ and mass $m$ rolling without slipping in a hemispherical bowl of radius $R$ (simple back and forth motion). Now, I assume the oscillations are small and so the ...
- 1,294
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significance of maxima and minima of time varying kinetic energy of a system
Consider a system of particles where the kinetic energy of the system is varying with time. I'd like to know the significance (or meaning) of the time derivative of the kinetic energy being zero at a ...
- 2,172
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3
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In a 2D problem with a thrown object, why is the acceleration along the x-axis equal to 0?
I'm starting Physics, and I don't understand why the acceleration along the x-axis is zero for an object thrown near the surface of the Earth. This may be problem specific, but I wouldn't know since I ...
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2
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A tunnel inside the Earth (but not an ordinary tunnel)
I decided to dig a tunnel inside the Earth. In equatorial plane. It should be designed in such a way that it follows the Coriolis effect. That means if, say a stone is dropped from rest into the ...
- 2,717
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3
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How do you produce electricity from a wind mill?
How does a spinning windmill produce electricity?What is the principle behind the windmill?
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523
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What are examples of Solutions of Newton's Laws that have analogs to Solutions of the Wave Equation?
The idea is to give examples of processes that deal with properties of a particle that have clear wave analogues.
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2
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Mechanical work to required battery power
I have a very practical question where I've calculated the mechanical work needed by a simple mechanical system by solving the line integral $W = \int_C \ F \ dx$. However, since I have a black spot ...
- 333
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6
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Stronger than Newton's laws?
According to the Newton mechanics, the force is responsible for changing the body velocity, and the body mass is the body inertia - a property to resist to the applied force. These two things make a ...
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Two spheres (A physics olympiad problem)
Browsing an archive of problems of a local physics olympiad, i stumbled upon a problem which seems not a very trivial.
Given two identical metal spheres in vacuum, with mass $m$ and radius $R$. One ...
- 2,717
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Forget Hooke's law. Why does a spring exert a force?
Forgetting Hooke's law for a minute why, from a microscopic perspective (preferably quantum) on up to a macroscopic one, does a spring under tension exert a force?
I was thinking that there might be ...
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Simulation of physics of chains/ropes in force fields resources?
I'm thinking about a project to tackle, and I'd like to make a simulation that allows the user to define a rope or chain of length L, pin it at arbitrary points r1, r2.... etc. and draw the resulting ...
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How fast do I need to go in order to avoid being seen by the Police?
I was driving down the road at roughly the speed of traffic. I saw a police officer parked on the side of the road, and also noticed that a Semi was traveling in the lane right next to him. This got ...
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3
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Why trimming the mainsail gives a larger torque to head up
I am learning sailing on a 5m catamaran (Nacra 5).
I am familiar with basic aerodynamics and the physics of the sail and keel.
We learned that when sailing closed hauled, too tight a mainsail tends ...
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Upper equilibrium point of a pendulum with small perturbations
I remember the following fact about a rigid pendulum:
The point when the pendulum weight is lowest is a stable equilibrium while the point where the pendulum weight is highest is an unstable one.
But ...
- 2,082
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3
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Equations for bent paper
I have a paper which lies on a flat surface. The paper is fixed on one side and the opposite side can slide in the direction of the opposing side. As side end slides toward the other, a "bump" forms. ...
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Modeling and Simulating Pendulum Motion
I've been having difficulty creating a mathematical model of a pendulum in my code. While there is plenty of info on general equations describing a the motion of a pendulum, I seem to be having ...
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Distance traveled by a projectile into a block
This problem is part of one of the free response questions on the 2011 AP Physics C Mechanics exam [PDF of free-response questions] that was administered last week.
(Sorry the problem statement is so ...
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