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57

A completely clean silica surface has a very high surface energy and a very high coefficient of friction. However silica surfaces adsorb pretty much anything at the least excuse, so glassware from your cupboard will have a layer of various molecules adsorbed from its environment, and this greatly reduces the coefficient of friction. Dishwash powder is ...


18

It's so simple because it's only a first order approximation model to how friction actually works. There are several other models, but to use them you usually need more parameters or other pieces of information about the system (for example, if there are fluid lubricants involved, the pattern of the surface, the materials involved, etc). The model $F_f = ...


8

The reason that you get slip at even the smallest forces results not from the fact that the tire is slipping against the ground, but that the tire is elastic. There is no way to completely eliminate slip with an elastic tire. Let's see why this is. To measure the slip, lets put twenty little green splotches of die evenly spaced on the circumference of the ...


6

There are several components to "rolling friction". First off, there is the wheel bearing that will inevitably have some friction (though a well-designed bearing has very little). Next, if ordinary pneumatic (or simply hard rubber) tires are used, the weight of the "load" will cause the rubber to deform. Rubber is a "viscoelastic" material, meaning that ...


6

"Rolling friction" is a misnomer. Never be confused by it. Its not friction. Its ROLLING RESISTANCE-the correct name. It comes into play because objects are not perfect rigid in real life. It comes into play because of the deformation of shape of objects when in contact. When an object rolls on another WITHOUT slipping, there is a surface contact between ...


4

(whatever I know about rolling friction ) In mechanics, friction plays a major role both in the laboratory and industrial worlds. Friction is the resistance to the sliding, rolling, or flowing motion of an object due to its contact with another object. Rolling friction is caused primarily by the interference of small indentations formed as one surface ...


4

It's so simple because it just happens to be a very simple, idealistic model for explaining how friction acts. In reality, friction is significantly more complicated - for instance you have static/dynamic friction and possibly lubrication (leading to the whole field of tribology). A mathematical model can be as simple as you want it to be; however, there ...


3

I did some research and it seems that superhydrophobic coating can be used to decrease drag forces to a certain extent because the coating repels the water. It creates an air layer between the surface and the water that causes slip through two-phase flow. This allows the ship to slip past the water.


3

In the comment you talked about the one that everyone knows where you just loop a string around itself and pass it through the loop.. I believe you refer to the overhand knot If you just pull on both ends, it will become tight - but once the "hole" in the loop has gone down to nothing, there is nothing to make the knot continue tightening (unless ...


3

You are wrong at assuming constant friction. Rolling Friction increases when you increase speed of the car (See the formulae at the bottom). Also, aerodynamic drag increases with the square of speed (See the formula at the bottom). So, at higher speed, the car engine needs to counter higher rolling friction and air drag to maintain that speed. While the ...


3

In a perfect vacuum, on a frictionless road, you could just turn off the engine and the car would keep moving, never slowing down. However, in the real world, there are several effects that exert a force on a moving car, slowing it down, such as: rolling drag between the tires and the road surface, fluid drag from the air that the car moves through, and ...


2

Dynamic friction is constant, it doesnt change with speed. That is why the trick works. If you pull the cloth fast enough, the friction force will act for such a short time that it will not be enough to pull the stuff above it. In the case of the weels, the frictions force is also constant, but you make it last longer per unit of lenght because the wheels ...


2

Imagine a weel rotating clockwise (the car moving to the right). At the point of contact with the floor, that point on the weel will be moving to the left, so the friction will be towards the right (it is what actually makes the car move forward). Regarding why rolling friction is usually larger that the static one. It is because the static friction ...


2

Simple example of impulsive force of friction: a marble hitting a hard surface at an oblique angle. During the (short) impact (which most would agree is an impulse) the normal force will be very large - and a component of friction will appear along the surface. When the marble is no longer in contact with the surface it will be rotating - showing that a ...


2

The frictional force would be in the forward direction if the object is a rotating object. Let us say the object we are talking about is a wheel. At a point of time, the frictional force between the wheel and the surface will be only at the point of contact of the wheel with the surface i.e the bottom most point of the wheel, say A. With respect to the ...


2

The effect is real. The heat in the bowl causes the production of steam in the cavity between it and the table. Depending on the temperature, this can be a far more powerful effect than mere thermal expansion of the air. The liquid between the bowl edge and the table acts like a liquid seal for a reasonably smooth and planar table/bowl interface - liquid ...


2

When considering motion in a fluid, or of a fluid, there are two types of forces to consider. Everyone immediately thinks of viscous forces, which arise from the viscosity of the fluid, and as you say these disappear in a superfluid. However there are also inertial forces that arise because the fluid has a mass. Accelerating the fluid requires a force just ...


2

Considering that the $10kg$ block does not accelerate from its initial position (ie the mass does not move), you need not have kinetic friction coefficient $\mu_k$ to solve the problem. Just use the static friction coefficient alone. Free body diagram: where $R=10 g \cos (30^\circ)$ and $\mu=\mu_s$. For equilibrium (zero acceleration), $$Mg-T=0$$ $$ ...


1

First find the force of gravity on the block, in coordinates relative to the inclined plane. "Down" relative to the plane, $F=ma$ yields $$F=10kg*10\frac{m}{s^2}*cos(\pi/6)=70.7N$$ "Left" relative to the plane, we have $$F=10kg*10\frac{m}{s^2}*sin(\pi/6)=50N$$ Since the tension on the string means the force of gravity on the hanging block is the same as the ...


1

I have considered the wet friction idea too. Ice when compressed will form a liquid layer where the pressure is applied. Yet one thing many seem to overlook is that the surface of the ice that the curling stones move over is not smooth. to prepare the ice for curling we first use a scraper to smooth it and level it. the scraper is a very sharp knife with a 5 ...


1

You can get answer to your first question here. When tires are inflated less area of the tire comes in contact with the ground, and thus there is less kinetic friction.


1

Let's assume that the box you have is perfectly closed and has a fixed amount of air in it. When the bird inside starts flapping it's wings it creates disturbances in the air present inside the box. The air molecules may start dancing in complex ways and it is difficult to completely describe this motion qualitatively. Consider the whole system (box + air ...


1

The bird is hovering in the box. The only way for it to hover is to increase the pressure underneath its wings and decrease the pressure above its wings. This pressure differential times the area of the bird will balance the exact weight of the bird. The pressure differential may be thought of as a net downward impulse given to the air molecules by the ...


1

It depends on the coupler design, and wether the coupler is in tension or compression. If you have a buffer-and-chain coupler there is simply no way you will uncouple the train when it is running without using explosives. Maybe a link and pin coupler can be released if you jiggle the throttle while extracting the pin. If you have a Janey / SA3 / AAR / ...


1

perhaps depends on the knot, but a reef knot would untie itself as it is held together by friction.... Many knots would just untie - maybe all. Reef knot. For example, people who go fishing would not be able to use the reef knot to tie thin nylon line together, but this knot works well for larger rope and is taught to sailors. Edit after comment. A ...


1

You should find out if the force is enough to overcome static friction between the slab and the block. Suppose it is not enough. Then the slab and the block will have the same acceleration, and you can calculate this acceleration. Then you can calculate the maximum static friction between the slab and the block. If this static friction is enough to move the ...


1

You went wrong when you considered the intersection to be a pivot point. The sticks are hinged at the ground. Those are the points around which the sticks will pivot. 1) The stick on the right will fall to the ground unless the force of friction is big enough to prevent it. How big does that force need to be? 2) The stick on the left creates the force of ...


1

Since the direction of friction is specified by the direction of motion, it must be the opposite direction, right? Wrong. You can go back to whoever told you that and yell at them. :-P Seriously though: the direction of friction actually has nothing to do with the direction of motion. One really obvious way to see this is that you can make the ...


1

Friction always oppose the natural tendency of the object to move in a given direction. In this case, if the car enters a frictionless sloped curve with a hight enough speed, the tendency will be to go up the slope becaue it ir trying to go straight but as the slope is curved, it will end up going up (in addition to straight). Now, if you add friction, this ...



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