Ever wonder why your hands get warm when you rub them together quickly or why rubbing two sticks together can eventually start a fire? The answer is friction! When two surfaces rub against each other, they naturally resist each others' movement at a microscopic level. This resistance can cause the release of energy in the form of heat, warming your hands, sparking a fire, and so on.[1] The greater the friction, the more energy released, so knowing how to increase the friction between moving parts in a mechanical system can potentially allow you to generate lots of heat!
Steps
Creating a More Frictive Surface
- Create a “rougher” or more adhesive point of contact. When two materials slide or rub against each other, three things can happen: small nooks, crannies, and irregularities on the surfaces can catch on each other; one or both surfaces can deform in response to the motion; and, finally, the atoms within each surface can interact with each other.[2] For practical purposes, all three of these effects do the same thing: generate friction. Picking surfaces that are abrasive (like sandpaper), deform when pressed (like rubber), or have adhesive interactions with other surfaces (like tacky glue, etc.) is a straightforward way to increase friction.
- Engineering textbooks and similar resources can be great tools when picking which materials to use to generate high friction. Most standard building materials have known "friction coefficients" — that is, measures of how much friction they generate with other surfaces. Sliding friction coefficients for just a few common materials are listed below (higher coefficient equals greater friction):[3]
- Aluminum on aluminum: 0.34
- Wood on wood: 0.129
- Dry concrete on rubber: 0.6-0.85
- Wet concrete on rubber: 0.45-0.75
- Ice on ice: 0.01
- Press the two surfaces together harder. One fundamental principle of basic physics is that the friction an object experiences is proportional to its normal force (for our purposes, this is basically the force with which it presses into the object it's sliding against).[4] This means that the friction between two surfaces can be increased if the surfaces are pressed into each other with greater force.
- If you've ever used a set of disc brakes (for instance, on a car or bike) you've observed this principle in action. In this case, pressing the brakes pushes a set of friction-generating pads into metal discs attached to the wheels. The harder the brakes are pushed, the harder the pads get pressed into the discs and the more friction is generated. This can stop the vehicle quickly, but can also release lots of heat, which is why a set of brakes is usually quite hot after heavy braking.[5]
- If one surface is in motion, stop it. Up until now, we've focused on kinetic (or "sliding") friction — the friction that occurs between two objects or surfaces as they rub against each other. In fact, this friction is different from static friction — the friction that occurs when one object starts to move against another. Essentially, the friction between two objects is greatest right when they start moving against each other. Once they're already in motion, friction decreases. This is one of the reasons why it's harder to start pushing a heavy object than it is to keep moving it.[6]
- Try this simple experiment to observe the difference between static and kinetic friction: place a chair or another piece of furniture on a smooth floor in your house (not rug or carpet). Make sure the furniture doesn't have protective "foot pads" or any other sort of material on the bottom that might make it easy to slide across the floor. Try to push the furniture just hard enough so that it starts moving. You should notice that as soon as the furniture starts moving, it immediately becomes slightly easier to push. This is because the kinetic friction between the furniture and the floor is less than the static friction.
- Remove lubrication between the two surfaces. Lubricants like oil, grease, petroleum jelly, and so on can greatly reduce the friction between two objects or surfaces. This is because the friction between two solids is generally much higher than the friction between those solids and the liquid between them. To increase friction, try removing any lubricants from the equation, using only "dry", un-lubricated parts to generate friction.
- To see the friction-reducing potential of lubricants, try this simple experiment: Rub your hands together as if they're cold and you want to warm them up. You should immediately notice them heat up from the friction. Next, put a fair amount of lotion in your palms and try the same thing. Not only should it be easier to rub your hands against each other quickly, but you should also notice much less heat.
- Remove wheels or bearings to create sliding friction. Wheels, bearings, and other "rolling" objects experience a special kind of friction called rolling friction. This friction is almost always much less than the friction generated by simply sliding an equivalent object along the ground — this is why these objects tend to roll, rather than slide along the ground. To increase the friction in a mechanical system, try removing wheels, bearings, and so on so that parts rub against each other rather than roll against each other.[7]
- For instance, consider the difference between pulling a heavy weight along the ground in a wagon versus pulling a similar weight in a sled. A wagon has wheels, so it's easier to pull than a sled, which drags against the ground, generating lots of sliding friction as it goes.
- Increase the fluid viscosity. Solid objects aren't the only things that can generate friction. Fluids (liquids and gases like water and air, respectively) can also generate friction. The amount of friction a fluid generates as it passes against a solid depends on several factors. One of the easiest of these to control is the fluid viscosity — that is, what's commonly called its "thickness". Generally, highly viscous fluids (ones that are "thick", "gooey", etc.) generate more friction than fluids that are less viscous (ones that are "smooth" and "liquid").
- For instance, consider the difference in effort you might experience when blowing water through a straw versus blowing honey through a straw. Water, which isn't very viscous, is very easy to suck into and blow out of a straw. Honey, on the other hand, is quite a bit more difficult to move through a straw. This is because honey's high viscosity generates lots of resistive friction as it's forced through a narrow tube like a straw.[8]
Increasing Fluid Drag
- Increase the area exposed to air. As noted above, fluids like water and air can generate friction as they move against solid objects. The friction force that an object experiences as it moves through a fluid is called drag (this is sometimes referred to as "air resistance", "water resistance", etc.) One of the properties of drag is that object with bigger cross-section — that is, objects that present a bigger profile to the fluid as they move through it — have greater drag. The fluid has more total space to push against, increasing the friction on the object as it moves through it.
- For instance, let's say that a pebble and a sheet of paper both weigh one gram. If we drop both at the same time, the pebble will fall straight to the floor, while the paper will slowly drift to the ground. This is the principal of drag in action — the air pushes against the big, wide face of the paper, producing drag and causing it to pass through the air much more slowly than the pebble, which has a relatively small cross-sectional area.
- Use a shape with a greater drag coefficient. While the cross-sectional area of an object is a good general indication of how great its drag will be, in fact, drag calculations are slightly more complicated. Different shapes interact with fluids in different ways as they pass through them — this means that some shapes (for instance, flat plates), can have a greater drag than different shapes (for instance, spheres) made out of the same amount of material.[9] Since the quantity that measures the relative amount of drag a shape makes is called a "drag coefficient", shapes with high drags are said to have large drag coefficients.
- For example consider an airplane wing. The shape of a typical airplane wing is called an airfoil. This shape, which is smooth, narrow, rounded, and sleek, passes through the air easily. It has a very low drag coefficient — 0.45. On the other hand, imagine if an airplane had sharp-edged, boxy, prism-shaped wings. These wings would generate much more friction because they wouldn't pass through without great resistance. In fact, prisms have a higher drag coefficient than airfoils — about 1.14.[10]
- Use a less streamlined body flow. In a phenomenon related to the different drag coefficients of different shapes, objects with bigger, boxier "body flows" generally generate more drag than other objects. These objects are made with harsh, straight edges and don't usually get skinnier toward the back of the object. On the other hand, objects with streamlined body flows are narrow, have rounded edges, and usually taper off towards the back of the object — like the body of a fish.
- For example, consider the way the average family sedan is constructed today versus the way it was constructed decades ago. In the past, most cars had a boxy appearance and were constructed to have lots of straight, square edges. Today, most sedans are much more streamlined and incorporate lots of gentle curves. This is on purpose — streamlined body flows make a car have less drag, decreasing the amount of work that the engine must do to move the car (and thus increasing the fuel economy).[11]
- Use a less permeable material. Some types of materials are permeable to fluids. In other words, they have holes in them that the fluid may pass through. This effectively reduces the area of the object that the fluid is able to push against, lowering the force of drag. This property holds true even if the holes are microscopic — as long as the holes are large enough to let some of the fluid pass through the object, the drag will be reduced. This is why parachutes, which are designed to create lots of drag to slow the speed of the user's fall, are made out of strong, light silk or nylon and not cheesecloth or coffee filters.
- For an example of this property in action, consider the fact that a ping pong paddle can be swung faster if a few holes are drilled in it. The holes let air pass through as the paddle is swung, greatly reducing the drag and allowing the paddle to move faster.
- Increase the speed of the object. Finally, no matter what shape an object is or how permeable the material it's made from is, the drag it creates will always increase as it goes faster. The faster an object goes, the more fluid it has to move through, and, thus, the greater drag it experiences. Objects moving at very high speeds can experience very high friction due to drag, so these objects usually must be very streamlined or they will fall apart under the force of the drag.
- For instance, consider the Lockheed SR-71 "Blackbird", an experimental spy plane built during the cold war. The Blackbird, which could fly at speeds greater than mach 3.2, experienced extreme drag forces at these high speeds in spite of its streamlined design — extreme enough, in fact, that the metal fuselage of the plane would actually expand from the heat generated by the friction of the air in mid-flight.[12]
Tips
- Don't forget that extremely high friction can release lots of energy in the form of heat! For example, you won't want to touch a car's brake pads right after you've slammed on the brakes!
- Keep in mind that high drag forces can cause structural damage to an object passing through a fluid. For instance, if you stick the flat side of a thin piece of plywood into the water while you're cruising in a speed boat, there's a good chance it will be shattered.
Related wikiHows
Sources and Citations
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