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 Won't a Tight Cable Ever Be Fully Straight?

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 ...
8
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3answers
332 views

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 ...
8
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2answers
5k views

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 ...
6
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3answers
748 views

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 ...
6
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2answers
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Hydrostatic friction: why do water droplets stay at rest on an inclined glass surface?

Tjis 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 ...
5
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2answers
992 views

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
296 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 ...
5
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1answer
135 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 ...
4
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1answer
402 views

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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3answers
262 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 ...
4
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3answers
752 views

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 ...
4
votes
1answer
662 views

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
1k views

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 ...
4
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2answers
206 views

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 ...
4
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4answers
675 views

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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1answer
2k views

If 1 screw can support 120 lbs, how much weight can 25 screws support?

While this is mainly about my personal home improvement project, I figure this would be a good opportunity for some of you to apply your physics skills to a real-world situation. My situation is ...
4
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3answers
1k 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 ...
4
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1answer
2k views

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 ...
4
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1answer
923 views

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
245 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
717 views

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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3answers
160 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: _ | | |== ==| | | - ...
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2answers
299 views

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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2answers
3k views

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 ...
3
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1answer
987 views

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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3answers
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Deadlifting the rear end of a car with a lever. What it equates to in actual weight?

Consider this youtube video of a guy lifting a car. (skip to 2:15). I checked, and these cars (PT Cruiser) weigh $1417\:\rm{kg}$. What would the equation be to estimate how much weight he is actually ...
3
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1answer
542 views

Statics of Rigid Bodies — Can there be two possible solutions?

I've been working on a question and there seem to be two possible solutions. My own solution does not match the one given in the book. However, after resolving forces and taking moments with both ...
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3answers
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Angles on swing sets

I'm building a swing set for my children. All of the designs I've seen involve building two A-frames and connecting them at the top with a crossbar/beam from which hang the swings. The A-frames are ...
3
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1answer
163 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 ...
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1answer
1k views

Statics question: When do I need to find support reactions?

We're finding forces (and whether they're in tension or compression) using the Method of Joints and Method of Sections. I don't understand why sometimes it's necessary to find the support reactions, ...
3
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2answers
40 views

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 ...
3
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0answers
422 views

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 ...
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0answers
130 views

Obtaining point of application of Ground Reaction Force with use of a hyperstatic load-cell array platform

I'm looking for the theory of an experiment that is giving me a hard time to perform. I have an instrument composed of a rigid horizontal square plate rigidly supported under each corner by a ...
2
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2answers
192 views

Why is moment dependent on the distance from the point of rotation to the force?

The formula for moment is: $$M = Fd$$ Where F is the force applied on the object and d is the perpendicular distance from the point of rotation to the line of action of the force. Why? Intuitively, ...
2
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1answer
364 views

Poisson effect formula for large deformations

English Wikipedia in the Poisson's ratio article gives an equation for large deformation: $$ \frac{\Delta d}{d}=-1+\frac{1}{\left(1+\dfrac{\Delta L}{L}\right)^\nu} $$ I couldn't find any reference ...
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2answers
616 views

Intuitive explanation of moments as they relate to center of mass

I would appreciate it if someone could give me an intuitive explanation of moments. I understand that that the center of mass could be thought of as the point which an object would balance on a ...
2
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1answer
398 views

Curvature of a cable supporting a suspension bridge in the regime where the mass of the cable is non-negligible

Assume I have a chain of uniform mass, $M_c$, suspended between two points such that it will follow the trajectory of a catenary curve when only supporting its own weight. Let's also say that this ...
2
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2answers
285 views

What is the shape of a clamped bent bar?

How would I figure out the Cartesian graph that describes a bar clamped flat for a length on one end with downward force being applied to the other? I have an idea that the bar will try to average ...
2
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1answer
71 views

Finding the Shape of a Hanging Massive Wire

This came from a physics wave book. I have a static, massive wire on the x-axis, with a y-displacement due to a force per unit length $F_y$. I start with the equation $F_y = ...
2
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1answer
227 views

Tension in a closed loop around an object, carrying stuff

Sorry when this sounds like a first year physics homework question, but it's actually not. It arose during a discussion regarding safe practices for rock climbing anchors... So here we go: Imagine ...
2
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1answer
2k views

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 ...
2
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1answer
44 views

Sign of gravitational force

I'm reading Lanczos's The variational principles of mechanics, and on pp. 80-81 there is an example involving a system made up of $n$ rigid bars, freely jointed at their end points, and the two free ...
2
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1answer
68 views

Relative strength of a dome

Is there a rough way to determine how the height of a dome affects the load that dome could support? For instance, assuming the bases of two domes are 24" in diameter, and one dome is 2" high while ...
2
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1answer
92 views

Can all systems be put in equilibrium?

I'm in a first year statics course. We have spent the whole semester solving for forces and moments so that the system is in equilibrium. When we are given a system, we immediately begin solving for ...
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2answers
243 views

A man pulling himself up on a platform

Consider the following image: Is it possible that the man pulls himself up (suppose the weight of the platform is negligible), or just lift the right part of the platform up? How to analyse this ...
2
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2answers
2k 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 ...
2
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1answer
84 views

Hanging a brick free over an edge by stacking them

National Geographics TV has a series called "None of the above". In one episode the presenter shows that by stacking 4 bricks (here shown as 'xxxxxxxx') you can have one of the bricks completely hang ...
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0answers
190 views

Mechanics of Materials (pressure and temperature)

A solid right cylinder of rock core is surrounded by four rods made of mild steel (all-thread rods). The rods are placed equidistantly around the core in a square formation. The tops and bottoms of ...
2
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1answer
84 views

Summing moments, where have I gone wrong?

Because I don't have enough rep to post images, the question is question 3 part iii) from here: http://www.mei.org.uk/files/papers/m209ju_jk32.pdf The surface at the contact point B is rough. The ...
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2answers
1k views

How do you weigh a box on a scale whose limit is too low?

As you will see I know nothing about physics and after being asked to solve a physics problem in a recent interview wanted to ask it of professionals and see what the response would be: I have a set ...