# Questions tagged [string]

This tag is for non-relativistic material strings, such as, e.g., a guitar string. PLEASE DO NOT USE THIS TAG for relativistic strings and string theory.

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### How does the tension exactly act on the pulley?

Everyone says that $T_1$ and $T_2$ will act on the patch of pulley wrapped by the string. So do they act on arcs $AB$ and $CD$ respectively? If not, then how do they act? And if $T_1 = T_2 = T$, they ...
27 views

### What's the relationship between the positions of each mass in this concentric pulley? [closed]

Reference image at the end of the post. This is supposed to be solved using lagrangian formalism, but I'm struggling to find a relationship between $x_1$ and $x_2$ (there has to be one: the discs are &...
1 vote
13 views

### String vibration under periodic boundry conditions

I wish to known which are the vibration modes (in case that they exist) of an string vibrating under periodical boundary conditions (PBC). The question is related to molecular dynamics simulations of ...
559 views

### Is it really the Tension in the rope that pulls the pulley?

We always solve questions by drawing two tension forces at the diametrically opposite end, to show that tension is pulling the pulley, but from the research I have done, it is clear that it is not ...
45 views

### Falling stones attached by a string

Suppose we have two stones, one "big" (A) and one "small" (B), the smaller one over the big one at the moment of free fall. Suppose also that a string of a certain length is ...
992 views

### Transverse waves in a rope: Why does tension not increase?

So I've gone down the "waves" rabbit hole... I found out that the propagation speed of transverse waves in ropes depends on the tension in the rope. At a first glance, this intuitively ...
106 views

### How does massless particles arise in string theory when it starts with the assumption that all strings are composed of massive little pieces? [duplicate]

I saw in a lecture (see after 00:55:00) by Leonard Susskind, he write Polyakov action, saying that the string is supposed to have divided into several pieces with mass (he says that it is the non-...
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### Hooke's law and modulus of elasticity [closed]

I'm having trouble in understanding the following question which state modulus of elasticity in Newton. Is it possible that we can define modulus of elasticity in Newton somehow?? I am getting a wrong ...
1 vote
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### How do I find the constraint relation in this question?

Let $\alpha_1$ and $\alpha_2$ be the angular acceleration of top pulley and bottom pulley respectively while $a$ is the acceleration of centre of mass of bottom pulley. Then, $$\alpha_2r+a=\alpha_1r$$ ...
1 vote
81 views

### Cosmological expansion around ropes between galaxies

The FLRW cosmology is a fair approximation of cosmological expansion for space between galaxies and clusters, however within galaxies themselves, is known to not hold, and in fact observational ...
33 views

### Elemental problem of dynamics

Ok, let's go fast. If we have the following system, and we want it to be static (acceleration = 0), we will always and obligatorily need friction, or any other force to oppose those existing in the ...
30 views

### How do i calculate the tension in string here? [closed]

How do I calculate the tension of string? I have already calculated the acceleration of the 2 masses which is 3m/s. But I can't seem to arrive at the answer which is 9.00N. Sorry if this was a ...
31 views

### Determine which ropes are under tension in pulley systems

These questions is gotten from beer and Johnston vector mechanics, the solution manual reveals: i) the two portions ropes are under tension in (a) - this I understand ii) only two out of the three ...
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We know a thread or string experiences the same tension force throughout the whole length,but when we tie a knot in the thread,why does the tension force become different in the two sides of the knot? ...
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### Can tension change in a massless string if there are three forces acting on it?

Consider this massless string, which has three forces acting on it, two on the ends and one at the middle: Would the tension change at the midpoint where the $5 \text{N}$ force is applied? I think it ...
64 views

### What does it really mean to have infinite tension?

Let an object of mass $m$ lie on a wire. The wire is much smaller in mass than the object and so its mass can be negligible. As the object lies on the wire, the wire is pulled down to a position where ...
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### Analogies for the Behavior of the Wave Function

In an overview of Quantum Field Theory, I recently heard the behavior of an elementary particle compared to that of a plucked (bowed) string. Although the stable states of excitation of the string are ...
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### What subfields, or specific works/articles, would cover the "the physics of ropes"?

Undergraduate student here. Yes, this is very similar to a post I made a year ago, however, as this topic is now pertinent to the research I plan to undertake, and now that I have distilled the idea a ...
1 vote
67 views

### Why does a pulley rotates?

I have beening dealing with problems involving pulleys. It seems like a pulley is something that changes the direction of a string, and it rotates when the string moves on it. Why do we need something ...
1 vote
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### Confusion about tension and disk

Why are the two tensions not equal? I thought tensions on two ends of a cord are always equal. And are the motion of the disk caused by friction force due to the motion of the cord and disk surfaces?
1 vote
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### How to find accelerations/tensions here? [duplicate]

$m_{2} = 6 m$, $m_{1} = m$, $m_{3} = 3 m$ I used conservation of string length and got $a_{3} + a_{pulley} + a_{1} = a_{2}$ and $T_{1} = 2 T_{2}$, but I have a feeling it's wrong because if the pulley ...
29 views

### Need help confirming answers; Laws of Motion Problem

I just recently started reviewing my old textbooks for fun (don't judge), and I'm having trouble wrapping my head around tension forces in this review question for the Laws of Motion chapter. Before ...
1 vote
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### Why can't I use vector addition in this way here? [duplicate]

I know that this exact question has been asked here a number of times, but none of the answers sit right with me. The question says that the ends of the strings are pulled with a velocity of "u&... 112 views

### Exceeding speed of light in simple mechanical setup

Suppose a system as shown. If i pull block $m1$ and $m2$ (the two side masses) with speed $v$ and theta is the angle mentioned the speed of central block becomes vsectheta in vertical direction. If ...
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### A paradox to law of conservation of angular momentum

I came across the following problem: A small body of mass $M$ tied to a non stretchable string moves over a smooth horizontal plane. The other end is being drawn into a hole with constant speed. Find ...
37 views

### How does the net vertical force equation for a non-relativistic string derived?

In the following image from "A first course in string theory", we get the net vertical force of a string, dFv. While I understand the first equation, I don't understand how the second ...
1 vote
112 views

### How does the work done by a cord-over-a-pulley work? [closed]

I can use the integral of power with respect to time to solve problem. In this situation, what is the work done on the cart by the tension? Please do not use calculus.
1 vote
40 views

### What is the constraint acceleration equation of the system? [closed]

In the question there is a hint given, $y_A$ is not constant. From this hint I began to look at the pulley that holds $m_1$ and $m_2$, and can see that it has the same tension as $m_3$. From this ...
378 views

### What is the centripetal force when a bar rotates about its center of mass?

Scenario 1: The centripetal force acting on the ball towards the center of rotation is $\frac{mv^2}{r}$, where $m =$ mass of the ball, $v =$ magnitude of the instantaneous linear velocity of the ball,...
1 vote
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### Sound of string in a guitar

I study about waves and I learn that the velocity of a transverse wave in a string depends only on the tension and the density of the string. The frequency depends only on the source (whatever ...
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### Why is the acceleration of $m_2$ downward?

The magnitude of the acceleration of $m_2$ is $a$. It does not say in the question whether it is downward or upward. But the answer assumes it is downward. So I think either it is badly framed, or it ...
743 views

### What physical state of a wound string corresponds to a 'tuned' string?

Many instruments can be strung with strings comprised of a solid core and wound with another metal wire, e.g. the viola. I was wondering about the physical state of wound strings when they are 'in ...
146 views

### What is Tension in a string? How is it produced at a molecular level?

I can't understand the direction of tension. Why is the direction of tension at the ends of a string away from the object or block of mass? Can someone tell me what happens internally in a string? ps: ...
1 vote
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### What is the difference between these two falling rope motion?

Here are two falling rope problems in Marion's Classical Dynamics Textbook and both equations of motion given by manual solution. (There is a typo in the third line of solution 9.15, one of the terms ...
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### Work of the tension acting as the centripetal force in a circular motion of varying radius

Consider this arrangement: A point mass (its mass being $m$) is executing uniform circular motion on a frictionless horizontal plane. A string passing through the center of the circle and attached to ...
88 views

### When to use positive and negative $\rm g$ when calculating force?

My teacher gave us a question the other day: A $20\ \rm kg$ sign is supposed to hang from a pair of wires attached to the wall and support beam as shown in the diagram. The worded that will be used ...
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### Motion of non-homogeneous string fixed at both ends when pulled at a point

How to solve displacement and velocity profile of string which has density $d_{1}$ till distance $x$ and density $d_{2}$ from $l - x$ to $x$ fixed at both ends and when given initial displacement and ...
66 views

### Determining the equations of motion for a 2 DOF in 2 directions

I'm having trouble understanding a 2DOF system that moves in 2 directions. The problem asks me to determine the EOM for a system shown in the attached image: My first idea is to transform this system ...
1 vote
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### Constraint rate change problem [closed]

Two small rings O and O' are put on two vertical stationary rods AB and A'B' , respectively. One end of the inextensible threads tied at point A'. The thread passes through ring O' and it's other end ...
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### What happens when I grab a rope after jumping down?

If a body has a mass of 54 kg and falls straight down for 11.33m before grabbing a rope, what are the forces acting here? What part does the coefficient of friction between the rope and hand play?
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### When the tensile force is equal to the weight of the body

I have met a lot of problems where the tensile force is different from $mg$, while in this one problem in the solutions it is written only that the tensile force is equal to mg. So if someone has time ...
41 views

### Why while solving some specific problems like this we need the second-order variation like this one?

Consider a string which is fixed at both ends and we are giving it a small amplitude by tauting it and moving to and from. If someone wants to derive the wave equation of the string, they would first ...
1 vote
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### What is positive rotation direction of a pulley in the Atwood machine?

In the Atwood machine the mass $m_1$ hangs on the left and $m_2$ hangs on the right, with $m_2 > m_1$. When released from rest the system accelerates clockwise which we define to be the positive ...
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### What exactly happens, step by step, in string harmonics?

I know that a string when plucked shows harmonics but I do not really understand how. Like, I understand resonating air column and how standing waves are formed there, but I can't seem to get the step ...
406 views

### Why does plucking a string produce harmonics instead of a pulse?

If I jerk a wire from the free end fixed at an end and then fix the free end, it will produce a pulse travelling back and forth between the two ends. So in a string fixed at both ends, if we pluck it ...
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### Why do we feel weightless while free fall but not while hanging with a string?

So why do we feel weighted by hanging on a string as their is no normal force acting on the object only tension force on the string not on object
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### Angular Momentum Conservation..04

A stone is attached to a string and is revolved in a circular path with constant angular velocity $\Omega$. In this state its angular momentum is $L$. If the length of the string is reduced to half ...
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### What the reason for not taking contribution of gravity when a wave is propagating?

Suppose a wave is being transmitted from one end of rope to other , for small amplitudes the wave speed is √T/u , suppose now that amplitudes of wave are not small and that gravity is also need to be ...
Question The ends of a light elastic string of natural length 0.8m and modulus of elasticity $\lambda$ N are attached to fixed points A and B which are 1.2m apart at the same horizontal level. A ...