# Questions tagged [friction]

The resistance a surface or object encounters when moving over another.

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177 views

### What would be the optimal diameter of gold grains if you want to fill a pool with them and swim in it?

I was wondering, if I were to fill a pool with gold/gold dust, what would be the optimal diameter of the grains to minimalize the friction to a point at which you would be able to swim in it ((and if ...
448 views

### Why does the overhand knot jam but the figure-8 knot doesn't?

After tensioning a rope with an overhand knot in it, it is often very hard if not impossible to untie it; a figure-8 knot, on the other hand, still releases easily. Why is that so? Most "knot and ...
192 views

### When does water decrease or increase friction?

Water often acts as a lubricant e.g. a wet floor or road, but sometimes the reverse happens e.g. putting on wet clothes. I understand that surface tension is involved e.g. microscope slides sticking ...
91 views

### Is friction required for knots (not hitches or bends) to hold fast to a rope

Using rope with idealized coefficient of surface friction approaching zero, that is in all other ways typical (typically stretchable, compressible, flexible and twistable), is it possible to tighten a ...
106 views

### Why do dental drills need such high rpm?

Dental drills have always been cutting edge technology when it comes to high rotational speeds. According to Wikipedia modern drills reach more than 180 000 rotations per minute (rpm), which is 3000 ...
94 views

### What would the ideal amount of gravity be for an Olympic sprinter?

The goal here is to get the sprinter running as fast as possible. How much of an effect would more or less gravity have on the top speed of an Olympic sprinter, say... Usain Bolt? 6 ft 5 in (1.95m) ...
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### Transition from damping to runaway in non-Markovian friction

I am studying the dynamics of a particle with history-dependent friction. That is, the particle velocity $v(t)$ is governed by $$\frac{d v}{dt} = -\int_0^t M(s)\, v(t-s) \,ds ,$$ where $M$ is the ...
207 views

### How are two objects heated by friction?

Let’s say that we have two objects made of different materials, both at room temperature. Surfaces of the objects are rubbed together, producing frictional heat. Is the heat distributed evenly, i.e. ...
58 views

### Do photons have “friction” passing through fluctuating gravitational waves?

In the 1998 biography, A Beautiful Mind, by Sylvia Nasar, John Nash is said to have entered Einstein's office to discuss his ideas that photons have friction. Nash had an idea about "gravity, ...
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### Problem regarding friction

I require a clarification on some steps in the solution of the following problem. Problem: The coefficient of static friction between two blocks shown in figure is μ and the table is smooth.What ...
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### Explain why the magnets move towards eachother in this setup

I have two bar magnets (actually stacks of small magnets, but I believe they should behave the same) attached to a stack of (presumably conductive) coins, and not diametrically opposite. When I ...
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### Basic question on friction

Consider the whole system initially at rest. Suppose there is friction between the blocks, but not between the lower block and the floor. Suppose I apply a Force $F$ to the left over $M$, with $F$ ...
62 views

### Physical origin of gradient-like system with nonautonomous damping term

I am in mathematics major, and I am considering the equation $$\ddot u(t) + \gamma(t) \dot u(t) + \nabla \phi (u(t)) = g(t),$$ where $\phi: \mathbb{R}^n \to \mathbb{R}$ is of class $C^1$, convex and ...
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### Circular motion, kinetic friction and tension

Hi: I've read this post: Uniform Circular Motion w/ Tension and Friction and helped me a lot. I have a similar problem: At one end of the rope ($R = 1 m$) is tied a mass ($m = 3 kg$) and the ...
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### What IS the precise angle of repose of Cadbury Creme Eggs, anyways?

So, in What If, made by xkcd guy Randall Munroe, an off-handed joke about his famous love of Cadbury Creme Eggs was made. In the image's mouseover text, Randall jokes his life's dream is to own ...
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### Dissipation and first law of thermodynamics

Consider the following situation: a certain gas is contained in a well-insulated cylinder with a well-insulated piston head. Now, in this case the piston is not frictionless. In order for the piston ...
1k views

### Thermodynamics and Newton's second law

Is it correct to say, that the Newton's laws (or a Newtonian system) is reversible if the friction isn't considered (the fact that the time is of second order $\frac{d^2x}{dt^2}$) and an isolated ...
252 views

### Is it possible to eliminate Van der Waals interactions?

I came to know that the friction force actually depends on the surface contact area due to weak interactions (adhesion due to Van der Waals forces) between the atoms of both materials increasing in ...
40 views

### Dimensional analysis for the angle of friction (angle of response)?

I am supposed to find the angle of response θ for a cone shaped pile of sand given that the friction between the sand granulas is μ. This is easily calculated as $θ = \tan(μ)$ with the help of a free-...
17 views

### Why does the reduction of “sloshing” in tankers also reduce brake distance?

After watching this video, I was confused at why reducing the peak slosh force made braking easier, after all it's the same mass inside the truck, plus, shouldn't the overall force over the whole ...
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### Pulling force required to get stationary vehicle moving

Background There are many examples from humans pulling heavy vehicles, ranging from trucks to airliners. When researching the physics behind this, most results pointed to the reduced rolling ...
37 views

### Does coefficient of friction depend on temperature?

So I was thinking of brakes in a car the other day. When brakes are applied, the forward force is converted to heat in the brake disks. When brakes are cool (at the beginning) they have the ability to ...
28 views

### What wears out a rubber sole of a shoe faster - sliding against dry or wet clay?

This question arises from my daily experience as a tennis player. I'm a novice to tribology. I couldn't quite come up with anything to answer that question. Any help? I know for certain that the ...
35 views

### When tyres skid on a dry road, is it because friction is too low or because the rubber is not strong enough?

All other things being equal, a car with wider tyres is less likely to skid on a dry road than one with narrower tyres. As answers on related topics have made clear, the simple formula $F=\mu N$ does ...
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### Is the squeal a manifestation of the Painleve Paradox?

When a DJ does a rewind (suddenly reverses the direction of the vinyl’s rotation under the needle), a sharp squeal is produced. This squeal is neither simply the music played backwards (as a film ...
25 views

### Friction points on a Roberval balance scale

I need to make a simple yet sensitive balance scale with a single pivot point, as shown. I can also make a Roberval balance scale which requires 6 pivot points, as shown. But wouldn't having 6 pivot ...
107 views

### Statistical definitions of heat and work

My understanding of heat and work from a statistical mechanics perspective (mainly extracted from Schrödinger's Statistical Thermodynamics) is as follows. We have some system, let us say a box of gas....
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### How does friction cause a spinning top to fall?

I am wondering how friction can create a torque to change the angular momentum and cause a spinning top to tilt, like in the diagram of a close up of the rounded tip of a top while the angular ...
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### Work done by friction on an inclined plane

A rough inclined plane is placed on a cart moving with a constant velocity u on horizontal ground. A block of mass M rests on the incline. Is any work done by force of friction between the block and ...
92 views

### Is friction an emergent phenomenon?

The microscopic origin of the frictional forces is due to contact between irregularities of the surfaces as seen here [1]. Electromagnetic interactions are the fundamental forces responsible for ...
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### How does different shuffleboard powder affect the disks' speed

I do know there is friction and surface tension involved here but am not sure on how exactly does different kinds of shuffleboard powder affect the disks' speed. So taking into the account the ...
165 views

### Being Frictionless, surface of contact

I hear frequently references to a smooth surface, or frictionless pulley. Can being frictionless be obtained if only one of the 2 surfaces has 0 coefficient of friction? Or it is for the contact of ...
354 views

### what is the relation between friction, clearance/tolerance, roughness and mechanical properties in a prismatic joint?

Consider an axisymmetric model of a prismatic joint: Assumptions: The circular shaft moves linearly, only in longitudinal/axial direction The shaft has a diameter of $D$ The clearance between ...
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### Minimizing mechanical energy subject to fixed angular momentum

I may be in error, but this problem (ch. 4, no. 14 on p. 203), from Symon's Mechanics, 3rd edition, seems wrong to me: A mass $m$ of gas and debris surrounds a star of mass $M$. The radius of the ...
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### Experiment on friction coefficient

Here you can see the results of the experiment about a friction coefficient: The mean of the friction coefficient becomes 0.262 but when I do a linear regression in the form of y=mx the slope is 0.31....
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### Extension of the Capstan Equation

The Capstan Equation relates the holding force and the load force for a string wrapped around a cylinder. However, most people who work with heavy machinery know that if you wrap a rope around a post ...
225 views

### Effect of forces inside a sphere

What would be the effect of applying two parallel forces of the same value at opposite points of the equator inside a sphere? Common sense tells me that it wouldn't move, but it seems that it could ...
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### Lagrangian Mechanics + Account for friction of block

$\newcommand{\dd}{\mathrm{d}}$ I am trying to work out the Lagrangian mechanics for a pendulum problem in order to animate it. I'm working on one of the examples in the Wikipedia page on Lagrangian ...
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### Coulomb's static friction in multidimensional case - decide which mass begins to move

Consider a system of N coupled oscillators, under the effect of elastic forces, damping, dynamic and static friction and an external force; for simplicity, let's suppose $N=3$. The friction model is ...
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### Can iron filings on a magnet be removed using a stronger magnet?

After some experiments, I had some NdFeB permanent magnets "contaminated" with iron filings, sticking to it's poles surfaces/edges - you may have seen that before. There are some working ways to ...
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### Friction of an object falling between two walls

I'm simplifying this problem into 2D for easier understanding. Lets say we have 2 vertical walls separated by distance x. Inside these walls I place an object that is only slightly smaller than x in ...
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### Is there quantitative theory of cutting with edge or blade

I wonder if there is some simple theory of what determine efficiency ( speed, energy end force required ) of cutting by edge ( blade , knife, sword ) At least something phenomenological like in case ...
2k views

### Why friction force is force of constraint?

My understanding about constraint force is that it is a force which limits the geometry of particle's motion. For example, situations such as the particle trapped in a track or limited in domain can ...
219 views

### Is there any place you can safely skydive without a parachute?

While trying to explain to someone how a kitten walked away from a fall from a skyscraper I got to wondering if there's any place a human could do that and I find myself out of my depth. The only ...
581 views

### Why did increasing the Ackermann geometry in my race car make it faster in corners?

Ackermann geometry is used to account for the different radius arcs that the front tires follow when the steering wheel is turned from center. It's often expressed as a percentage: e.g. 25% Ackermann, ...
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### Sliding to Rolling

In a problem where a sphere is initially sliding and then begins to roll after having travelled some distance. The thing that confused me is that the work done by friction on the sphere is not simply ...
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### Block attached to spring oscillating on a surface with friction

Consider a block of mass $m$ moving with initial velocity $v_o$ attached to a spring with spring constant $k$, on a terrain which has a coefficient of kinetic friction $\eta$ and coefficient of static ...
29 views

### Friction in tyre with low air pressure

I was driving a normal hatchback with right side tire with nearly half of advised pressure. Now everything was fine until i experienced that when surface is wet and i try to take turn on incline, ...
The objective is to prove that the Lagrangian: L'=\frac{2\dot x+\lambda x}{2\Omega x}\tan^{-1}(\frac{2\dot x+\lambda x}{2\Omega x})-\frac{1}{2}\ln(\dot x^2+\lambda \dot{x } x + \omega^2x^2), \qquad \...