Why is rolling friction always less than sliding friction?
927320
Think of common sense and think of a wheel with rolling friction and think of using a giant slab of rock being dragged on the ground. Which is easier to move from point A to point B. Yes, the wheel has a certain surface area which is quite minimal hence you need traction with wheels if you are going fast.
379528
Friction and erosion.
101281
well with most engines, the larger they are, the greater the frictional losses are.  a LTD (low-temperature-differential) stirling can operate on such a small temperature difference because they are so small and light, and because there is very little losses (air drag around the wheel, and friction in the bearings/moving parts).\n\nin general, the heavier something is, the greater the sliding or rolling friction is.  the frictional force of a mass sliding on a surface is proportional to the mass of the object (actually, proportional to the force with which it presses down on the surface).\n\non the other hand as you increase the size of the stirling engine, you find a case of diminishing returns in terms of the work you can extract.  have a look at the very first diagram on this page:\n\nhttp://www.cse.iitk.ac.in/~amit/courses/371/abhishe/main2.html\n\nthat is a closed curve that represents the stirling cycle.  the work that can be extracted from one cycle is the area inside the box.  notice that the top and bottom parts of the box are curved.  these are isotherms or lines of constant temperature, and those temperatures are the two temperatures in the differential that drives the stirling engine.  notice that as you increase volume, they get closer and closer together.\n\nso think about what happens if you scale up the size of your engine.  the box that defines the stirling cycle grows towards the right, but because those isotherms are getting closer and closer together, the larger you make it the less additional work you get.  at some point you  will stop gaining any measurable amount of work at all!\n\nso there is a trade-off at work here - on the one hand, a larger engine means more work can be extracted - but more frictional losses, too.  \n\nsince the amount of friction grows linearly with the size of the engine, and the amount of extra work grows less-than-linearly with the size of the engine, then the best ones are clearly the small ones!  if the amount of energy extracted grew at the same rate as the amount of energy lost through friction, then all sizes would be equivalent.  \n\nps - this relationship also holds true in 4-stroke otto cycle internal combustion engines, and pretty much any thermodynamic engine - and it's why big engines typically have several cylinders rather than one humongous one.  bigger cylinders gain you less and less.
739032
You've left out some information. All one can deduce from what you've given is that the roll has unwound 12 turns after 1 hour. Since we don't know what the thickness of the tape is we have no idea what the reduced diameter is or how much tape unrolled. Sorry.
462841
When the calculations were set up for determining coefficient of friction 1 was set as the upper limit. There can be no coefficient larger than 1. Highly smooth surfaces of the same shape (for example, two rectangular blocks) are in almost perfect contact with each other so that almost every molecule of one is in direct contact with every molecule of the other (that's a bit of an exaggeration unless the surfaces are micropolished, but it will serve). The extremely large surface area in contact produces enormous frictional resistance to sliding movement. It helps to understand this if you can make the distinction between smooth and slick. Think for example about trying to slide one heavy glass plate over another without any lubrication between them. They "stick" to each other like glue.
205045
obviously you have to use the least steep slope, which will cause your marble to move slower. i think you are trying to find ways to slow down the marble itslef??? you would want to increase friction, and this can be done by applying something onto the marble or onto the slope (my favourite would be honey or syrup) im not sure if that is what you were looking for?? good luck
21718
This question sounds like your teacher/ta/whoever wants you to make 3 assumptions (or they already did and you just didnt mention it):\nThey have the same mass (otherwise he would have to specify the masses of each)\nAir resistance is negligible (pretty common)\nThey all roll without slipping (otherwise they all get to the bottom together)\n\nIf you make these assumptions, the order is based on their respective moments of inertia about the center.  They each move downward at a speed based on their rotation, so the order they reach the bottom will be smallest rotational inertia first. Sphere, disk, ring.
153998
If you examined almost any surface under very high magnification, what looked smooth to the naked eye would look like endless mountain ranges in all directions.  If you then place two such surfaces together using weight or another force to join them and tried to move one surface relative to the other, the opposing "mountains" would have to slide up and over each other or even break (shear) the mountain tops.  The resulting resistance to motion is called friction.  When mountain tops are sheared, the energy forcing the motion is converted to extremely high local heat at the molecular level as bonds between millions of atoms and molecules are broken.  Friction causes heat and can quickly destroy an engine bearing that loses its lubrication.\n\nThe high heat of friction can actually cause a lubricant (oil, etc.) to break down and bond (as olifins) to the metal parts making smoother surfaces, especially when the parts are held apart by a film of circulation oil that carries away excess heat to a heat exchanger.  Grease (which is often a mixture of soap and oil) is thicker than oil and can fill the microscopic mountain valleys so that the parts slide on opposing layers of grease reducing friction and heat.\n\nWhen you push on a heavy crate, you must apply enough force to overcome the static friction (sometimes called stiction) and start the crate moving.  Continued motion creates dynamic friction generating a constant supply of heat.\n\nIf you place any object on a board and lift one end, all materials (combinations of board (incline) and object (load) will slide before the board reaches an angle of 45 degrees.  The higher the board angle, the higher the coeficient of friction.  This method can be used to calculate the coefficients of friction for various combinations of materials (board and load).  Ice on teflon has perhaps the least coefficient of friction and is very slippery.  The objects and board should not serve as cogs or act as adhesives to get valid results.\n\nFamily friction (strife) is the only kind of frictioon that can be reduced by flowers or a hug and a kiss.  That may cover most of the other types of friction, right?
883514
There are 2 types of friction: kinetic and static\nKinetic friction slows a sliding object to a stop.\nStatic friction prevents a stationary object from moving. It is usually much stronger than kenitic friction. (That's why it is tough to get a heavy, stationary cardboard box to slide on the carpet, but once it's moving, it becomes easier)\n\nWith your automobile example, the tires on the car roll on the ground. It's important to note that they roll... not slide, because sliding object experience kinetic friction forces, while rolling objects experience static friction forces. (This is why you get a carpet burn if you slide on the carpet, but not if you roll).\nStatic friction prevents the tire from "Sliding" on the road, even though the axle of the car is forcing it to rotate. The tire applies a "horizonal" force on the road, attempting to slide (and squeel), but the road applies an equal and opposite static frictional force. The result is a "roll".\n\nIf for some reason, the you slam on the breaks and the breaking force applied to the tire exceeds the maximum static friction force between the road and the tire, the tire will slide (skid). The resulting kentic frictional force stopping the car is much smaller than the previous static frictional force stopping the car. As a result, skidding cars slow down less rapidly than reglar breaking cars. This is why anti-lock breaks were invented - to keep cars from skidding in a stop.\n\nI really hope this helps. Tried to keep it short and sweet.
282006
Answer is 'acts in the direction opposite of motion."\n\nFriction always opposes motion. Therefore first one is wrong.\nFriction is equal to mu times the weight. and mu is not equal to one.Therefore second  one is wrong.\n Sliding friction is less than static friction\nTherefore Third   one is wrong.
570511
both.  the friction, as well as creating its own heat, slows the airflow, which aggrivates the compression along the leading edges.\n\nI'm pretty sure.  don't put this on a physics test and then blame me.
858739
Use Bernouillis equation and you will have your answer. You can use the equivalent lengths method to find the friction.
933078
You cannot get away from friction.  Material properties are also not going to be exact.
528746
Average Kinetic coefficient of friction (m) cannot be calculated as such, \n      since  m can lie anywhere between 0 and 1.\n   \n      Thus, its value is probably more near 0.5 than many values. That's all that can be infered.
609398
FRICTION is the energy generated when two bodies have some physical interaction with each other. \n\nGRAVITY is the force of a mass which pulls an object towards its center.
937470
Getting started, have to overcome static friction.  Therefore, force applied = static friction force just before movement\n\nF = umg\nF = 75N\nm = 20kg\ng = gravity\nfind u, plug and play\n\nsame problem for sliding friction
1026758
friction is equal to the normal force (such as weight) times the coefficient of friction. f = c of friction X normal. if you try to push a cardboard box on the ground, its kinda easy. now imagine putting a hundred pounds in that box. its harder to push becuase there's more friction holding the box back.