Questions tagged [statics]
Analysis of a system of bodies where all forces are in balance and there is no motion relative to a chosen reference frame.
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Why is the shape of a hanging chain not a "V"?
From Wikipedia:
To answer this question: "What is the shape of a chain suspended at both ends?" we can use the variational principle that the shape must minimize the gravitational potential energy.
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Hanging a hammer from a table and a stick so that its midpoint is outside of the support of the table
I came across this pic on the internet today. At beginning I thought it is just not possible because the centre of mass is way off so gravity will generate torque making the stick and hammer fall. ...
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Why won't a tight cable ever be fully straight? [duplicate]
I posted this picture of someone on a zipline on Facebook.
One of my friends saw it and asked this question, so he could try to calculate the speed at which someone on the zipline would be going when ...
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Why are arched bridges stronger than flat bridges?
To keep the question brief: in bridge design, why is the arch structure favoured compared to a simple flat one?
In other words, how does the curved platform alter the force decomposition of the load ...
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Why does the curve of a hanging chain not minimize the area below it?
If we have a chain of fixed length hanging from two points we know that it will form a curve that minimizes the chain's potential energy.
If we imagine the chain as having many small segments, then ...
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Why does the weighing balance restore when tilted and released
I'm talking about a Weighing Balance shown in the figure:
Press & Hold on onside of the horizontal beam and then release it. It makes some oscillations and comes back to equilibrium like shown in ...
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Plank overhanging off a building — where does the normal force go?
Suppose there is a (uniform rectangular) plank of mass $m$ and total length $L$ that is on top of a building with some piece overhanging or jutting out past the building. Assume it isn't past the ...
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What holds up the lowest point of a rope following a catenary curve?
A catenary curve is the curve followed by a rope suspended at both ends in uniform gravity.
I thought I would try to solve it myself because it seemed like a good challenge, but almost immediately I ...
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How does this tensegrity table work?
I have assembled below a desk toy which seems to defy laws of physics at first glance (objects can be placed on top of it up to a certain limit, since it is already under strain).
The toy is in fact ...
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Why can't a rope be pulled completely straight?
I have found several discussions on how to calculate the sag of rope that is tied off at two points (like a tightrope), and I understand it to a certain extent. What I can't wrap my head around is how ...
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Is the weight of something being dropped the same as the force of something being static?
If I have a block of let's say: 10 kg and I put it over my foot in a static manner, what we know from static mechanics is that my foot is opposing the weight of the 10 kg mass with the same force in ...
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Two people are holding either end of a couch, is one person exerting more force than the other?
I was carrying a couch with my flatmate yesterday, and I started thinking about this.
Often when carrying objects like this, one person will be taller and has thus lifted his end of the couch higher ...
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What is the role of pillars in bridges?
As I can see in the picture, there are so many pillars which are holding the bridge. This picture gave a question to me that what are these pillars doing below the bridge?? An appripriate answer ...
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Hydrostatic friction: why do water droplets stay at rest on an inclined glass surface?
This is a non-expert question on a (seemingly simple) text-book topic.
The question is about "hydrostatic friction", defined as follows.
Consider a drop of water resting on a flat surface. ...
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Determining forces on a table with four legs and an object placed on a random point
We have an object with weight $D$ at a given location on table wit with four legs ($F_1$ to $F_4$). What is the force applied on each leg? (for simplicity, I'm just using the same labels $F$ and $D$ ...
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Moment of Inertia, why $r^2$and not $r$?
So my engineering mechanics book includes a brief discussion on area moments of inertia.
Unfortunately, the ensuing chapter is predominately computational in nature. I don't have a thorough grasp of ...
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What is the force on the arms in a pushup?
What force do the arms have to generate to do a pushup? Let us look a this simplified model:
The body can be represented by the green plank of mass B. Its angle to the ground is $\theta$.
This ...
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Does an elastic massive hanging chain take the shape of a parabola?
This is a problem in "Differential Equations: A Modern Approach with Wavelets" by Krantz:
If the length of any small portion of an elastic cable of uniform density is proportional to the ...
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Why does the beam in a weighing balance get tilted proportional to the weights added to each pan?
I'm talking about a beam balance(a simple weighing balance with a beam and two pans hung on either side)
As answered in a previous question, the beam comes back to the original position when one ...
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Newton's Third Law As Discussed in Den Hartog's Mechanics
A question, but some preliminaries in order to avoid answers involving concepts not yet introduced in the text I'm reading--Den Hartog's Mechanics.
Near the very beginning of his book, Den Hartog ...
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The Physics Behind the American Death Triangle [closed]
I've heard a lot about the American Death Triangle and how it is awful for belaying. The Death Triangle is set up as such: you have two anchor points with a single rope or line running through both ...
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Can the weight distribution of a partially supported rigid mass be determined?
Consider a rigid block of mass $M$ and length $L=L_1+L_2$ partially hanging off a table, as shown below:
If the center of mass is precisely on the ledge ($x=0$), then it's not hard to see that the ...
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If 1 screw can support 120 lbs, how much weight can 25 screws support?
While this is about a home improvement project, I figure the core part is interesting physics.
My situation is this: I am hanging 500 lbs of drywall (two layers weighing about 250 lbs each) on 5 ...
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Forces and torques about the CENTER OF MASS of a physical pendulum
I'm currently stumped by the following situation. Say we've got a rectangular physical pendulum (think ruler with a hole-punch at one end).
It's trivial to analyze the motion of the pendulum with the ...
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What determines the (minimum) angle at which a domino falls over?
Dominoes, when placed upright, remain that way. Sometimes, even if you tip them a little bit, they will go back to their upright position.
However, if you tip them too far, they will fall over.
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Force between two charged rods?
Suppose that we have two rods of length $l_1, l_2$ connected at one end but free to rotate. These rods have charge density $\lambda$ uniformly distributed, so the total charge of rod $i$ is $\lambda ...
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Physics of the Planche
The planche is a gymnastics exercise where the athlete holds their body parallel to ground supported only by their hands.
People sometimes train for this exercise using a progression of ``planche ...
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Hanging chain in a planet's gravitational field
The curve for a chain hanging between two poles in a uniform gravitational field is known as the catenary.
Is there known an expression for the curve of a hanging chain on a planet of mass $M$ which ...
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Shape of a string/chain/cable/rope/wire? [closed]
The height of a string in a gravitational field in 2-dimensions is bounded by $h(x_0)=h(x_l)=0$ (nails in the wall) and also $\int_0^l ds= l$. ($h(0)=h(l)=0$, if you take $h$ as a function of arc ...
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Is this static mechanical balance possible without trickery?
On damnlol.com, I came across this picture:
http://www.damnlol.com/hello-god-i-have-a-fault-to-report-7549.html
My question is: Is this possible without glue? If not are there similar situations in ...
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Solving a statically indeterminate truss
I'm trying to solve the statically indeterminate truss shown below and I'm having a little trouble.
$H$, $P$ and $\beta$ are given. The material is aluminum (density is 2700 ${kg/m^3}$) and has a ...
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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 ...
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Equilibrium of a tower of 2-d blocks
In the image below,
All the blocks are frictionless & identical with side of unit length, height $h$, weight $w$ & center of gravity at their geometric centers.
The 2 lowest blocks are on ...
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Number of decays in a chain reaction
It is widely known that the probability of $n$ decays from one system to another $A \rightarrow B$ (e.g., electrons decaying from one atomic energy level to another or muons decaying into neutrinos ...
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David Morin's question: Mountain climber [closed]
This question is from Introduction to Classical Mechanics, David Morin.
A mountain climber wishes to climb up a frictionless conical mountain. He wants to do this by throwing a lasso (a rope with a ...
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Cylinder rolling down an inclined plane held by a string
A cylinder of mass M and radius R is in static equilibrium as shown in the diagram. The cylinder
rests on an inclined plane making an angle with the horizontal and is held by a horizontal string
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Why is the normal contact force horizontal on an inclined ladder?
There is only one force acting on the ladder which is its weight and it acts vertically downwards. Then why does the normal contact force from the vertical wall act horizontally on the ladder? There ...
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How can we differentiate a statics and dynamics problem?
Here is the problem I confused a lot.
A uniform rod of mass m and length l is hanging vertically from the pivot O. A horizontal force F acts at the lower end of the rod. If F always remains ...
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Why don't couple forces violate Newton's First Law?
If you have some random object at rest and you apply a couple to it, the net force acting on it is zero. However because a moment acts on it, it starts to rotate.
So you had an object at rest, a net ...
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How are force couples free vectors?
According to wiki,
In rigid body mechanics, force couples are free vectors, meaning their
effects on a body are independent of the point of application.
I'm having some trouble understanding this ...
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Why does a force divide equally on multiple supports/legs?
Imagine a bridge spanning across two pillars/supports with a weight of $80N$. {Any external forces affecting the system are assumed to be negligible.}
We know that the force exerted on these 2 ...
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Morin 2.16 - Balancing a semi-infinite stick
Problem 2.16 from Morin's Classical Mechanics:
Given a semi-infinite stick (one that goes off to infinity in one direction), determine how its density should depend on position so that it has the ...
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Vectorial sum of two parallel and discordant forces
To obtain the resultant force, colored in purple, of two parallel and concordant forces $\vec{F_1}$ and $\vec{F_2}$, I apply a vector $\boldsymbol f$ (colored in brown) to the bar $AB$; similarly ...
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Proof of the Law of the Lever
Is there an accepted proof of the Law of the Lever?
I know of several attempted proofs, such as Archimedes's, but these either are incomplete or do not transfer to the setting of Newton's Laws. (It ...
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Hooke's Law question
I have the following question to answer :
A force of $160 \ \mathrm{N}$ stretches a spring $0.050 \ \mathrm{m}$ from its equilibrium position.
A. what is the spring constant of the spring?
The ...
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Why the dust on the wing cannot be cleaned by air flow? [duplicate]
I recently saw that the dust on the airplane's wings are frequently cleaned by some human workers (generally after every trip, workers clean the fine dust on wing's surface by hands). But I have a ...
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What are the conditions for a heap of identical hard spheres to be in a static equilibrium?
It is common to see a heap of conical shape formed by a large number of similar size hard spherical objects, for example, a heap of pebbles, sand etc. Suppose we want to model this system as a ...
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Understanding moments as forces?
I was watching this lecture on analysis of stress for mechanics of materials. At time 7:20, the lecturer says that in equilibrium, the sum of forces and "moments" in each direction (x,y,z) must be ...
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Bowling ball on a rubber sheet
After reading a layman's guide to general relativity, I began to wonder what shape a bowling ball on a large rubber sheet would produce. For simplicity, I would like to assume that Hooke's law applies ...
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A simple (?) problem of static equilibrium
A question on the site made me consider this simple problem, that unexpectedly seems to be indeterminate.
Consider a perfectly rigid, homogeneous bridge of mass $m$ with three pillars, one placed ...
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