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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55
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4answers
14k views

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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7answers
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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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2answers
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Why are arc bridges stronger than flat bridges?

To keep the question brief: in bridge design, why is the arc structure favoured compared to a simple flat one? In other words, how does the curved platform alter the force decomposition of the load ...
28
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7answers
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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 ...
19
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6answers
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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 ...
16
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3answers
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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 ...
11
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2answers
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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. If the ...
10
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6answers
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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 ...
9
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5answers
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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 ...
9
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4answers
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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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3answers
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Where does the extra equation come from to determine the forces from an object on a table?

I have a question about basic statics but somehow I cannot manage to find the answer on my own (btw, this is not a homework. It's been so many years since school for me...). The problem is very ...
9
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5answers
753 views

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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2answers
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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 ...
8
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4answers
567 views

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 ...
7
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1answer
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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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3answers
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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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3answers
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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 ...
6
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1answer
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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 ...
5
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2answers
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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. ...
5
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1answer
542 views

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 ...
5
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1answer
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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 ...
5
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1answer
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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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3answers
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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 ...
5
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2answers
246 views

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 ...
5
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1answer
212 views

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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5answers
514 views

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 ...
4
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1answer
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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 ...
4
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1answer
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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 ...
4
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1answer
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Hooke's Law question

I have the following question to answer: a force of160 N stretches a spring 0.050m from its equilibrium position. A. what is the spring constant of the spring? The equation for Hooke's Law in my ...
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2answers
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Static equilibrium question [closed]

I was doing some static equilibrium problems and I came across this problem which should be easy to solve, but is posing quite a challenge. I want to point out that this is not homework, just plain ...
4
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1answer
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Relationship between balloon size and pressure

I see numerous websites talking about how a small balloon is at higher pressure than a big balloon. This is a fun counter-intuitive factoid... unless it isn't a fact at all. Young-Laplace gives $$T =...
4
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1answer
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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 ...
4
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3answers
848 views

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 ...
4
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1answer
150 views

Why curvatures relieve stress?

Per example, we can stop a crack in a steel plate by drilling a hole at the end of it. Or we all know the famous example of popular science that airplane windows are round for the same reasons. Why ...
4
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4answers
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Blocks stacked on an incline connected by rope around pulley

Two objects A and B, of masses 5 kg and 20 kg respectively, are connected by a massless string passing over a frictionless pulley at the top of an inclined plane, as shown in the figure. The ...
4
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2answers
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Calculating torque adjustment for offset of pivot point

I'm trying to figure out how much to adjust torque, on a torque wrench when using an extension that offsets the pivot point. See this calculator for a diagram of what I'm talking about: The trouble ...
4
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2answers
174 views

Tension in a string, at an angle [closed]

This was a question on a mechanics exam. Part i wants me to assume that the tension in both parts of the string is the same. Even though I got the correct answer( 18.9N) by assuming so, I don't ...
4
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2answers
59 views

Does mass affect how likely an object is to topple over?

I was at work recently pulling around some pallets when I had a thought. I wanted to know if the mass of the pallet had any influence on how likely it is for the pallet to topple over when going ...
4
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1answer
643 views

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 ...
4
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1answer
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Shape of a string/chain/cable/rope/wire?

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 ...
4
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2answers
324 views

Can structural trusses in centrifuges be helpful at all?

Imagine a rotating space station for the purpose of creating artificial gravity. Generally, such a space station is portrayed as a ring or a cylinder. Images of these space stations often have lines ...
4
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1answer
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Application of Archimedes Principle

A candle is floating in a liquid placed in a container. The container is a cylinder of diameter $D$, and the candle is of width $d$. ($D>d$) The height of the liquid from the bottom of the ...
3
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3answers
333 views

What is a “Center Of Mass” issue of a Gorillapod?

I read somewhere that a Gorillapod may have "Center Of Mass" issues when used with the long lenses. So, I wish to understand what is a "Center Of Mass" issue? I have to clarify that I am NOT a ...
3
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5answers
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How much force would be necessary to hold a 6-foot steel staff weighing 20 lbs from one end of it, parallel to the floor?

This isn't a homework problem. I'm asking cause I'm wondering how strong a person would have to be to pull it off. Their arms would be extended, both hands gripping the staff at one end, and the staff ...
3
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1answer
481 views

Forces acting on two balls in a jar [closed]

Consider the following situation: Two balls are inside a jar with diameter $D$. The balls are uniform in mass and both have the same diameter $d$. Express the forces that acts at the colored dots in ...
3
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3answers
252 views

How does torque work?

Why can we assume the the net torque is equal to zero when we pick one of the points as the axis of rotation. For example if we selected the left leg of the table for the axis of rotation why can we ...
3
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2answers
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Mass required to prevent sign falling over with a set wind load - activity stations for disabled children

I'm currently working on my thesis and I'm stuck on a question. I'm designing activity stations for disabled children to be used for equine therapy. The stand is 9ft tall and I've calculated the the ...
3
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2answers
369 views

Why are truss members two force members and why are the forces directed along the axis of the member?

In Hibbeler's book, Engineering Mechanics: Statics, on pages 264-265 it is mentioned that two assumptions are made in the design of a truss: All loadings are applied at the joints The members ...
3
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2answers
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Why are some jenga pieces easier to remove than others?

Jenga is a game place with wooden blocks stacked on top of one another in an alternating pattern. Players take turns removing blocks from any layer and placing them on top. As the game progresses ...
3
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3answers
613 views

Does a cross really have a smaller area moment of inertia than a square of the same area?

I need to calculate the area moment of inertia of a cross for a homework assignment. The cross is a symmetric plus sign: _ | | |== ==| | | - ...