# 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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### Hanging a Hammock

I have a hammock that I want to hang using a tree on one end and a wooden post set in a bucket of cement on the other end. How would I determine how heavy the cement needs to be so that it would ...
43 views

### 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 ...
126 views

### 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 ...
584 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 ...
976 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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### 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 ...
8k views

### Why is skidding considered kinetic friction when braking on a car?

So according to my textbook applying the brakes hardly to essentially lock up the wheels causes skidding which is kinetic friction so the breaking distance is longer. However, if the brakes were "...
122 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 ...
384 views

In the attached image the force output of the plank is calculated to be the work done (FDf) divided by the height the block rises (Dw). So the force the plank applies on the block is (FDf)/(Dw). But ...
296 views

### If the output force to input force increases when we use a lever, what decreases? (mechanical advantage) [closed]

I'm stuck on this one, does anybody have any idea?
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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 ...
118 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?
863 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 ...
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### 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 ...
5k views

### Distribution of weight of an inclined object

We have a rod, 1m long and negligible height compared to the length, mass 10Kg uniformly distributed (center of gravity in the center). It is tilted theta degrees to the horizontal, and rests on two ...
412 views

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

Consider the following stacks of 2D boxes with uniform density: ...
514 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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### 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-...
242 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? ...
104 views

### 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 ...
38 views

### 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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### Determine resultant couple moment by summing moments

The question states: Determine the resultant couple moment by (a) summing moments about point $O$ and (b) summing the moments about point $A$. I used scalar analysis for solving this ...
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### Why don't underground tunnels collapse on themselves?

I understand that underground tunnels don't collapse because the forces acting on it must be in equilibrium but I don't know which forces are present and I don't know how they are built.
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### How does an Anti-tilt chair work?

These chairs are designed as per European Educational Standard Part 1 and 2. These are claimed by the manufacturer as Anti tilt, so basically it makes it relatively difficult for a child to fall when ...
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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 ...
238 views

### Membrane shape under pressure

If I have a circular membrane that is fixed by the edges and then I apply a uniform pressure to the membrane (let's say the membrane is the top of a circular chamber that has a rigid wall and flat ...
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### Projection of force [closed]

So to find the projection along the cable in question 124 we used CA (not AC). Why? I thought that we always start from the end point of the Force, which is AC and not CA.
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### How to determine tipping point for a single pedestal dining table?

How to determine minimum weight to tip over a single pedestal dining table? We have a client who wants a rectangular table, 72" L x 42" W, and the MDF top will weigh approximately 100 lbs and the ...
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### Determining tension in a rope between two masses using only force arrows (i.e. no calculations)

The situation sketched above is often used for problems involving tension. Let's say we know the masses $m$ and want to determine the tension $T$ in the rope (no friction, massless rope). Since ...
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### Moment of a force about a given axis (Torque) - Scalar or vectorial?

I am studying Statics and saw that: The moment of a force about a given axis (or Torque) is defined by the equation: $M_X = (\vec r \times \vec F) \cdot \vec x \ \ \$ (or \$\ \tau_x = (\vec r \times ...
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### Is there any situation where one can measure force WITHOUT motion?

I am drafting a patent application where one part of the invention measures a force. I'd like to distinguish this from the prior art, which measures a displacement. But for all the force measurement ...