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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From Where Did The Table Get Energy? [duplicate]

When a block in resting on a table (the table is on Earth) the table stops the block from going more down, from where does it get the energy to apply a force opposite to gravity?
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Beam attached to a wall

Suppose I have a rigid beam attached to a wall, how can the reaction force on the point the beam is attached to generate a static equilibrium on the beam? ...
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1answer
210 views

Virtual work - Tension in a string

I am confused about the way Tension or Thrust is used in equation of virtual work. I mean whether to use $ T\cdot dr$ or $-T\cdot dr$ (work done by tension). For instance in the below case : A ...
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1answer
670 views

Ladder and friction (wall and floor) [closed]

Here there is a problem and the proposed solution: UPDATE: I can't understand why getting the maximum friction to solve this kind of problem when it is known that static friction, depending on other ...
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2answers
359 views

What is the maximum force you can exert on the ground with one leg? [duplicate]

You are standing on hard floor. Clothes weigh negligibly. You move only your leg. Everything else remains stationary. Assume you are made of the same material as the average human. Can the force ...
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Base length and Mass required to stabilize a large heavy sign [closed]

I have an 8ft x 7.5ft sign that is about 4inches thick and weighs about 150lbs. We can assume the whole thing is made of the same density I am building the stand for this and I cannot anchor it to ...
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1answer
49 views

Is there a name for this phenomenon?

Imagine I have a cylindrical pipe closed on both ends with lids. I fill it with sand and compress the sand tightly. Now I hold the cylinder vertically and remove the bottom lid. The sand will counter ...
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1answer
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Tipping angle of a box [duplicate]

how do you calculate the tipping angle of a box? That is the maximum angle a which a box can reach before tipping over. help, please!
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4answers
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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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1answer
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3 people carry a log of wood, which one is carrying the most weight [closed]

I saw this on a youtube video. The answer is that the one in the middle is carrying 25 % more than the other two. Can someone explain this?
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Varignon's theorem (Principle of moments) failure?

Varignon's theorem states that "If many coplanar forces are acting on a body, then the algebraic sum of torques of all the forces about a point in the plane of the forces is equal to the torque of ...
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1answer
66 views

A question about temperature in the concept of a macroscopic statistical-mechanical system

I recently have come across a question while working on Statistical Mechanics. The question itself was quite straight forward (no this is not a "do my homework" question in case you were wondering) ...
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Power in 1st Gear

The max power an engine puts out is what determines the max speed a car travel at right, obviously it's more nuanced than that because there are losses in the drive train, and the shape of the car ...
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320 views

Finding the equation of a chain hanging between two points

Say I have a connected at either end to two points, $A(x_A, y_A)$ and $B(x_B, y_B)$ of length $l$, where $l \leq \sqrt{(x_B - x_A)^2 + (y_B - y_A)^2}$, how would I go about finding an equation of its ...
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1answer
294 views

Conditions for Static Equilibrium of Rigid Body [duplicate]

When there are forces acting on a rigid body, the two conditions that has to be satisfied for the body to be in static equilibrium are: 1. The sum of all forces must be zero (translational equilibirum)...
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1answer
376 views

Work done in lifting a object from underwater if it displaces $W'$ of water? [closed]

A spherical shot of W gm weight and radius r cm, lies at the bottom of cylindrical bucket of radius R cm. The bucket is filled with water up to a depth of $h\,\mathrm{cm}$ $\left(h > 2r\right).$ ...
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1answer
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Why can't a pyramid stay balanced on a vertex? [duplicate]

A pyramid, such as a tetrahedron/3-simplex, or any other isohedron, falls from some height and lands on a vertex. It will eventually end up with a face to the ground. Why can't it stay balanced on a ...
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balancing a weight on a table without tipping over

I have a statics problem. Here is a model for a table: a circle of radius 1.5 ft a pole in the center a base in the shape of an X. I place a weight on the table on the edge. How much weight can ...
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1answer
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Equation to model change in leverage point

Is there a mathematical model for the situation below? First, I want to know how much force I would need to lift this block some distance using this wooden board. The block is situated against a wall ...
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1answer
556 views

Static equilibrium- when do you use forces and when do you use torques?

I'm working on an answered physics question in the book but I'm having trouble understanding why the problem could be solved by setting net torque equal to 0 and not net force. You are asked to find ...
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1answer
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Rotating a cube to the verge of tipping over [closed]

The problem: A $100\,\mathrm{kg}$ cubical box lies on the floor. A child pushes horizontally at the top edge. What is the magnitude of force to put the box on the verge of tipping over, given that ...
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1answer
91 views

How to solve problem with homogenous ball at equilibrium? [closed]

The answer according to solution manual: $F_A=566N F_B=283N$ I am not asking for you to solve this exercise, this is not a place for homework, I know. I need help how to think so I can solve this ...
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Does a car effectively become heavier the faster it moves?

Just for clarity, I'm not asking in relativistic terms. Assuming the engine's pistons aren't horizontally opposed, would the vertical force of the pistons move the car downward? Or would it not ...
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What stresses should be considered to determine the effective stress and FOS?

This question I tried to solve by considering the torsional shear and transverse shear and axial loading. But the answer is not clear. But the answers that I got by different trial and error method ...
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1answer
64 views

Why exactly do two parallel equal magnitude forces acting on a rigid body produce no rotational motion?

How exactly do we justify that two parallel forces $\vec{\mathbf{F}}_1$ and $\vec{\mathbf{F}}_{2}$ having the same magnitude, acting on a rigid body, do not produce a moment? Yes, if we consider any ...
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Can we take rotational equilibrium of only one part of a body?

Can we take rotational equilibrium of only one part of a body? Such as in a step ladder, where BD is a rope. Can we take rotational equilibrium of AC about C, and not the full ladder?
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How does a balancing toy work?

I know that the balancing toys have their center of mass under the axis on which they are balancing. That's why they stay still. But when we give a little tap on it, it re-balances itself. But how ...
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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 ...
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1answer
738 views

Why frictionless joints are assumed in pin-joint truss analysis?

In pin-jointed truss analysis, we consider some assumptions. One of them is that they have friction-less joints. Why do we have this assumption? What would happen if the joints were to have ...
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3answers
278 views

How is weight distributed in a stack of boxes? [duplicate]

Consider the following stacks of 2D boxes with uniform density: ...
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391 views

Why is the horizontal component of the reaction force at one of the hinges (which has a rounded bottom) neglected?

In the textbook "Engineering Mechanics: Statics (7th edition)" by James L. Meriam and‎ L. G. Kraige, Chapter 3: Equilibrium, in the section titled: "Examples of Free Body Diagrams", I found an example ...
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68 views

Is this system statically indeterminate?

In the static configuration of rectangular blocks shown below, we will assume that the gravitational force acting on each block is $1$. Each also has uniform density. We will also assume that: $(x_1,...
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Weight of objects

I recently observed that on a weighing machine(rectangular) a object weighed the same as the half portion of the same object with it's other half hanging out of the boundary of the machine (The ...
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1answer
433 views

Friction between ladder and wall (statics)

I was given the following task: My problem is the following: For $A$ and $B$ I’ve got friction forces $R_A$ and $R_B$. I can calculate those by using the formula: $R_A = N_A \cdot \mu $ $\mu $ is ...
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1answer
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Force-couple system equivalence: 3 moment equations

In force-couple system equivalence, We can show that two systems are equivalent if the resultant force in both are equal and that it has the same line of action. So Rx1 = Rx2, Ry1= Ry2, Ma1 = Ma2 ...
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443 views

Static Equilibrium: Normal Force

Consider the following situation where an object (for example a person) is at rest at point D such that the system is in static equilibrium: Why is there no normal force present at point D? To my ...
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3answers
347 views

Equilibrium of a rigid body

Why is it necessary for a body to have the net torque acting on it be balanced along with the forces for it to be in equilibrium? Isn't torque just some special case of force like rotation is a ...
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1answer
108 views

Statics: What it means if a rope snaps

There is a drawbridge that is held up by a hinge attached to a vertical wall on one side, and by a vertical rope on the other side. Say a horse starts walking from the wall towards the rope and at a ...
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1answer
206 views

Tension exerted by strings at corners of a rope grid [closed]

I am a software developer and helping someone with an art installation. It consists of a grid made of ropes. Grid is $12 \cdot 12$ meter and internal squares are $1 \cdot 1$ meter. Grid is suspended ...
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Express $\tan{\theta}$ in terms of $\alpha$ and $\mu_s$ [closed]

Problem: Determine the least angle $\theta$ where equilibrium is possible. Express $\tan{\theta}$ in terms of $\alpha$ and $\mu_s$ Attempt: I started of with drawing out all the acting forces: ...
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Conceptual Question about Suspension Bridge Forces

Two walkways are suspended from vertical rods. In the first scenario, one long rod supports both walkways and is attached to the ceiling. In the second scenario, two shorter rods are attached to each ...
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Conservation of percent difference?

I was doing a lab to experimentally determine coefficients of static friction between two surface. This was the basic set up: The mass of the block is a known quantity. I was checking the maximum ...
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1answer
281 views

Truss analysis how to calculate for a single beam?

Suppose I want to calculate the tension/compression force only in 4. Is there an easier way to calculate it or do I have to calculate everything with the joint method? I don't immediately see zero-...
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1answer
186 views

Why is shear stress equal to half of yield stress

Why is the value for acceptable shear stress equals to half of yield stress? $$ \tau = \frac{\sigma_{yield}}{2} $$ P.s Along the math behind it would be possible to explain this with visual consepts? ...
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Hard mechanics problem: Beam resting on three springs [closed]

I have trouble with following problem (ii): A uniform beam $AOB$, $O$ being the mid point of $AB$, mass M, rests on three identical vertical springs with stiffness constants $k_1$, $k_2$ and $...
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2answers
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What friction vector direction will be in this case?

Two bodies 1 (at the top) and 2 (the rightmost one) with same masses are connected to each other with lightweight non-expandable thread thrown over sheave. The block A has an acceleration a. I need to ...
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1answer
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Is there a quick way to convert between two coordinate systems?

I am solving a few Statics problems that ask to find the resultant moment that a given force produces about a given axis. The problem has two sets of coordinate systems (x, y, z) and (x1, y1, z) where ...
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Reliable, intuitive method for working out the direction of static friction?

I've seen Consistent method for finding direction of static friction which was the closest question I could find, but I'd like something better. I need a method which : 1) Intuitively makes sense 2)...
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1answer
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How to know what method to use when finding resultant forces?

I am quite confused as to what method should be used when finding resultant forces. Here are two examples of a question that asks to find the resultant force. One requires the parallelogram law to ...