# Tag Info

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

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 ...

0

This is not true, when the box is sealed you are weighing the total mass of the box and everything in it, not the weight of the bird plus the weight of the box

0

Assuming is a black box seen from the outside, consider the following situation: After a while, the bird tries to fly higher and pushes the ceiling of the box upwards. Will the weight of the box decrease? Then, after a while, the bird dies and falls to the floor. Will the weight of the box increase? If the box is sealed, it cannot change the time averaged ...

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 ...

6

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 ...

0

Both graphs have 0% force at 0% slip. That implies that no force is being applied to the tire. As applied force is increased until a certain point the increase in slip remains nearly linear because the coefficient of friction remains nearly constant. Thus effective force also increases nearly linearly. After that point the coefficient of friction gets ...

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 ...

0

Driving through standing water takes a lot of energy and wants to slow the car down, so to maintain same speed as on a dry surface, you require more fuel, i.e. your mpg will drop. I have instant reading mpg on my BMW 335D and have noticed what would other wise be 30mpg on dry surface can drop to 25 or loess on wet surface. I'm talk surfaces with pooling ...

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 ...

0

My analysis is that the object would not rotate at any velocity. My reasoning is that if the object is completely flat on the floor then there would be no net torque. Say r is the distance from the bottom left corner (as in your diagram) to the point at which the net frictional force acts (directly beneath center of mass, at a point where the object and ...

0

yes, erosion by wind, air, dust, sand, is a form of friction. But wahtering can also happen for other causes, such as chemical reactions like corrosion, oxidation, etc.

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 ...

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 ...

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 ...

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 ...

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 ...

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 ...

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 ...

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.

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 ...

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 ...

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 ...

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 ...

0

You are actually completely right, and then at the last minute you look the wrong way!!! Lets examine the last bit. If you were to continue applying a net force on an object for an infinite time it would indeed accelerate infinitely. There is nothing wrong here. The key point is the NET force, that is, the net amount of force, and its direction, that is ...

1

It is due to tensile internal forces (and the center of mass doesnt change position). And you are correct, the friction doesnt change. Look at the graph for the forces acting on the small element of paper in contact with the floor. We have two equations to describe vertical and horizontal forces at equilibrium: $N-mg-T\cos{\alpha}=0$ and ...

1

I believe a major factor in it is failure of the central pivot's friction to resist the larger torque generated by the longer lever arms. So, yes, this is a matter of friction, but not only on the feet. What resists "first" is the friction of the central pivot. If that fails, then feet friction comes into play.

1

Frictional force is horizontal. Use angle to get horizontal force component due to gravity on paper. That's plenty!

0

yes it would, (well if it was moving down a ramp) like you said, friction would also come to play, but it should go faster because the more weight you add the more downwards momentum and if it's moving dow a ramp then it should turn that to forward momentum.

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 ...

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 ...

1

You have ignored the car friction with air! If we assume the car as an aerodynamic body (air flows on the car surface smoothly and does not separate) then the air friction on the car is proportional to the square of car speed.

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 ...

0

You are right By Newtons Second law $$\ F = m (v-u)/t$$ Once you have attained the speed of $$\ 100 m/s$$ You only need to supply the force to counteract the friction. But attaining to this speed requires more force and thus more energy. We can conclude that riding on the same road at a constant speed of any magnitude does't require more energy. Let ...

2

The rate of temperature change will be the power per unit mass times the specific heat. So if you have a certain mass of water $M$ flowing per second, at a velocity $v$, losing $\Delta P$ pressure per second, then work done is $v\Delta P A$ and $A = \frac{M}{\rho v}$ . Then with a heat capacity $c$ (about 4.2 kJ/kg/K for water), and the relationship between ...

1

You can always imagine torque as being represented by two opposing forces at either side of the object. In this case imagine the torque being generated by two forces of magnitude F at the top and bottom of the disk, the top one pointing forwards and the bottom one, backwards. In order to generate torque $\tau$, these two forces must be of magnitude ...

0

See https://www.andrew.cmu.edu/course/24-352/Handouts/logdecrement.pdf and http://en.wikipedia.org/wiki/Logarithmic_decrement For a damped oscillation you measure the peaks, and record the decreasing amplitudes. From that you calculate the log decrement $$\delta = \frac{1}{n} \ln \left( \frac{x_0}{x_n} \right)$$ where $x_0$ is the initial amplitude and ...

0

The way I see it movement of a rolling ball in a bowl shaped like half a sphere may be quite tricky to model exactly and the equations may not be straightforward The one thing that is certain is that damping causes energy to be lost from the motion and you can measure the ammount of energy whenever the ball gets to the highest point of the oscillation. So ...

1

The friction between a solid and liquid is a function of viscosity. The best way to answer this is with a model setup called Couette flow where a fluid sandwiched between two plates is sped up by the movement of the top plate: Image source: University of Virginia, Physics 152 taught by Michael Fowler The friction force $F$ that the fluid exerts on ...

2

If there is not enough friction to keep the vehicle in its circular path, it will skid. The force needed for the circular path is the centripetal force: friction (the force keeping the car on the road) must be greater that that. Now the no-slip condition (centripetal force < friction) implies = $\frac{mv^2}{r} < mg\mu$ . Your equation follows by simple ...

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