# Tagged Questions

General questions about the way objects move and interact. This tag should be used when the tags for certain kinds of mechanics (newtonian-mechanics, classical-mechanics, quantum-mechanics, etc.) are too specific.

282 views

### What, if anything, makes forces the “cause” and acceleration the “effect”? [closed]

We typically say that forces cause acceleration inversely proportionate to mass. Would it be any less correct to say that acceleration causes forces proportionate to mass? Why? (Note that the ...
267 views

### 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 ...
109 views

### 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 ...
206 views

### 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 ...
126 views

### 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 ...
1k views

### How do you produce electricity from a wind mill?

How does a spinning windmill produce electricity?What is the principle behind the windmill?
131 views

### 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 ...
604 views

### 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 ...
197 views

### purely hypothetically, could a quantum being appear to be human? I ask for the purpose of research on a novel [closed]

I am a budding novelist, and im researching a few things for a fictional narrative. the characters are from other dimensions and i want to present a coherent scientific structure. any suggestions ...
319 views

### 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 ...
502 views

### Does decoherence explain all instances of wave function collapse?

Specifically, how can decoherence explain the appearance of flecks of metallic silver on a photographic plate when exposed to the very weak light of a distant star? EDIT: Perhaps the advocates of ...
326 views

### 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 ...
387 views

### 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 ...
1k views

How is the shape of the blades of an air fan determined? Trial and error, or is there a theory behind it? What are they trying to maximize, volume of air dislocated per rotation?
675 views

### Formula for implementing a simulated weighing scale

Background: I'm on the pledge drive committee for a non-profit group, and there is an issue that is always contentious within this group; thankfully the group is amicable, although competitive, about ...
4k views

### Which Mechanics book is the best for beginner in math major?

I'm a bachelor student majoring in math, and pretty interested in physics. I would like a book to study for classical mechanics, that will prepare me to work through Goldstein's Classical Mechanics. ...
5k views

### What is the difference between Newtonian and Lagrangian mechanics in a nutshell?

What is Lagrangian mechanics, and what's the difference compared to Newtonian mechanics? I'm a mathematician/computer scientist, not a physicist, so I'm kind of looking for something like the ...
187 views

### Does angry birds have randomness or just chaos [closed]

Does angry birds have randomness or just chaos?
154 views

### Why can you assume that the angular momentum vector of a top will always track its axis of rotation?

My favorite physics 101 textbook (Giancoli) explains precession in terms of a spinning top whose axis is tilted from the vertical. The way the book sets things up, $L$ (angular momentum) points along ...
995 views

### Distribution of forces

(little background: I'm trying to develop a small, quick 'n dirty static physics engine to determine whether a stacking of boxes is stable). If I have a 3D rigid box (with the bottom in the ...
2k views

### What are the Constraints on Building a Tower to Space?

When I was a boy I used to daydream about building a tower so tall that the top of it would stick out of the top of Earth's atmosphere project into near space. There would perhaps be a zero gravity ...
505 views

### How do anti-lock brakes know when to brake?

When you come to a stop normally, the brakes don't pulse when you stop. Since the car can only know its speed by the rotation of the wheels, how can it distinguish between the car is stopped normally ...
475 views

### what energies do the wheels of a moving car posses?

I saw this question in a test. I would have answered kinectic energy due to rotation and translation. It that correct. Else what is the answer? Oh no, i forgot to mention it was objective type ...
2k views

### Convert running speed uphill to equivilent speed on flat

Given a certain running pace uphill, I want to be able to determine an equivalent pace running with no elevation change. Assumptions: similar effort in both cases (say for example running at 90% max ...
510 views

### Most elegant/fundamental formulations of the laws of classical mechanics?

Newton tried to do it with three laws/statements. While the first can be derived from the second, the three form a pretty nice framework. Later on, I've encountered Lagrangian Mechanics, which ...
843 views

### Is the Woodward effect real?

Did anyone ever heard about this?I've never seen any serious physicist talk about "mass fluctuations". Here is the man in his own words http://www.intalek.com/Index/Projects/Research/woodward1.pdf ...
5k views

### What is the difference between a moment and a couple?

In mechanical engineering, the torque due to a couple is given by $\tau = P\times d$, where $\tau$ is the resulting couple, $P~$ is one of the force vectors in the couple and $d$ is the arm of the ...
310 views

### How does solar activity affect the ISS?

Currently the sun is launching some intense solar flares. http://www.guardian.co.uk/science/2011/feb/17/solar-flares-northern-lights-uk Th article I've linked also mentions how a "coronal mass ...
554 views

### Impulse - distance question

What impulse should be applied to an object of mass m, having a known coefficient of friction u to get to a distance d ? Thanks! Update 14.02.2011: I still wasn't able to find an answer to this. ...
446 views

### How to determine velocity and mass of asteroid 2011 CQ1

As you may have seen, a small asteroid had a near miss with the earth a few days ago. As a physics teacher teaching momentum, I think this could be an excellent problem for my students, however, I ...
361 views

### Shaking a jar of balls

A jar is filled with two types of balls, red and green. Red balls have radius $r_1$ and mass $m_1$, green balls have radius $r_2$ and mass $m_2$. If initially the balls are randomly placed throughout ...
2k views

### Homework Help - Physics / Calculus III / Force-Work Problem

I have tried to answer the problem correctly, and I have found similar (and even an identical) problem in my textbook, yet I still can't seem to yield the correct answer with this specific problem. ...
577 views

### Mechanical energy problem

Question goes: "An anvil hanging vertically from a long rope in a barn is pulled to the side and raised like a pendulum 1.6 m above its equilibrium position. It then swings to its lowermost point ...
327 views

### Will tensile strength keep a cable from snapping indefinitely?

Trying to secure a wall hanging using magnets; me and a coworker came up with an interesting question: When the hanging is hung using 1 magnet, the weight of it causes it to quickly drag the ...
166 views

### Non Classical mechanic answer to : length of time a thrown object spends in rest before falling down? [closed]

When an object is thrown upwards, when it eventually comes to rest and starts falling, for how long is it stationary? What about an particle in electric field having an initial velocity towards it's ...
316 views

### Terminal velocity for falling in a shaft

One falls slower in a mine shaft than in free air. This is due to collisions with the walls. How should one model the terminal velocity in the presence of such collisions?
2k views

### Why can't a piece of paper (of non-zero thickness) be folded more than “n” times?

Updated: "In order to fold anything in half, it must be π times longer than its thickness, and that depending on how something is folded, the amount its length ...
270 views

### Lagrangian density of linear elastic solid

I need the general expression for the lagrangian density of a linear elastic solid. I haven't been able to find this anywhere. Thanks.
6k views

### How did Newton discover his second law?

I've always assumed/been told that Newton's 2nd law is an empirical law — it must be discovered by experiment. If this is the case, what experiments did Newton do to discover this? Is it related to ...
2k views

### Why do rockets have multiple stages?

What is the advantage for rockets to have multiple stages? Wouldn't a single stage with the same amount of fuel weigh less? Note I would like a quantitative answer, if possible :-)
2k views

### Why is the bell, well, bell shaped?

What is the significance about the bell shape, when its hit at the rim it rings/produces sound better than other shaped objects? If so could anyone explain a little bit on it. EDIT: From the ...
431 views

### Moving from point A to point B in zero-gravity

Here's an proposal on how to get from point A to point B in zero-gravity without using any propellant and the question why it wouldn't work: A closed tube, filled with water and a round (solid) ...
1k views

### How long it will take for a upright rigid body to fall on a ground

Let's suppose there is a straight rigid bar with height $h$ and center of mass at the middle of height $h/2$. Now if the bar is vertically upright from ground, how long will it take to fall on the ...
788 views

### Why does Guillotine have of 45 degreed blade rather than the one parallel to the ground? [closed]

What is the point of having 45 degreed blade in Guillotine? After all, the pressure-Force over Area-blade imposes to the neck is the same either way.
205 views

### What happens when I move a very long bar? [duplicate]

Possible Duplicate: Faster than light information Hey there, I just want to make clear that I'm new here and I don't know whether this is the right place to ask such a hypothetical ...
893 views

### Equivalent spring-constant for infinite square grid of springs

Consider an infinite square grid, where each side of a square is a spring following Hooke's law, with spring constant $k$. What is the relation between the force and displacement between two points? ...
555 views

### Oscillate a swing without touching the ground

Is is possible to set a swing into oscillations without touching the ground? This occurred to me while watching the second pirates movie. There is a scene where the ship's crew is suspended in a cage ...
329 views

### Cheetah prosthesis efficiency

I want to compare the much-talked about Cheeta running prosthesis to a the normal running process in terms of force and energy, but I don't know where to start. How would you start the comparison? A ...