Have you ever wondered why your hands get warm when you rub them together quickly or why by rubbing two sticks you can start a fire? The answer is friction! When two surfaces rub against each other, they naturally resist each other on a microscopic level. This resistance can cause energy to be released in the form of heat, warming hands, starting a fire, and so on. The greater the friction, the greater the energy released, so knowing how to increase the friction between moving parts in a mechanical system can potentially allow you to generate a lot of heat!
Steps
Method 1 of 2: Create a Surface with More Friction
Step 1. Create a rougher or more adhesive contact point
When two materials slide or rub against each other, three things can happen: the small niches, irregularities and protuberances of the surfaces can collide; one or both surfaces may deform in response to motion; finally, the atoms of the surfaces can interact with each other. For practical purposes, all three of these effects produce the same result: they generate friction. Choosing surfaces that are abrasive (like sandpaper), deform when crushed (like rubber), or that have adhesive interactions with other surfaces (like glue, etc.) is a direct method of increasing friction.
- Engineering manuals and similar sources can be great tools for choosing the best materials for creating friction. Most building materials have known coefficients of friction - which measure the amount of friction generated in contact with other surfaces. Below you will find the dynamic friction coefficients for some of the more common materials (a higher coefficient indicates more friction:
- Aluminum on aluminum: 0, 34
- Wood on wood: 0, 129
- Dry asphalt on rubber: 0.6-0.85
- Wet asphalt on rubber: 0.45-0.75
- Ice on ice: 0.01
Step 2. Press the two surfaces together with more force
A fundamental principle of basic physics is that the friction on an object is proportional to the normal force (for the purposes of our article, this is the force pressing towards the object against which the former is sliding). This means that the friction between two surfaces can be increased if the surfaces are pressed against each other with more force.
If you have ever used disc brakes (for example in a car or bicycle), you have observed this principle in action. In this case, pressing the brake pushes a series of drums that generate friction against the metal discs attached to the wheels. The deeper you squeeze the brake, the greater the force with which the drums are pressed against the discs and the greater the friction generated. This allows the vehicle to stop quickly, but also causes significant heat production, which is why many brakes are usually very hot after heavy braking
Step 3. If a surface is moving, stop it
Up until now, we have focused on dynamic friction - the friction that occurs between two objects or surfaces that rub against each other. In fact, this friction is different from static - the friction that occurs when one object starts moving against another. Basically, the friction between two objects is greater when they start to move. When they are already in motion, the friction decreases. This is one of the reasons why it is harder to start pushing a heavy object than it is to keep moving it.
Try this simple experiment to see the difference between dynamic and static friction: Place a chair or other piece of furniture on a smooth floor in your home (not on a carpet). Make sure that the piece of furniture does not have protective felt pads or any other material on the bottom that would make it easier to slide on the ground. Try to push the furniture hard enough to make it move. You should notice that as soon as it starts to move, it will quickly become easier to push it. This is because the dynamic friction between the furniture and the floor is less than the static friction
Step 4. Eliminate the lubricants between the two surfaces
Lubricants like oil, grease, glycerin and so on can greatly reduce the friction between two objects or surfaces. This is because the friction between two solids is usually much higher than the friction between the solids and the liquid between them. To increase friction, try to remove lubricants from the equation, and use only "dry", non-lubricated parts to generate friction.
To test the friction effect of lubricants, try this simple experiment: Rub your hands together as if you feel cold and want to warm them. You should immediately notice the frictional heat. Then, sprinkle a generous amount of the cream on your hands and try to do the same thing. Not only will it be much easier to rub your hands together quickly, but you should also notice less heat production
Step 5. Eliminate wheels or bearings to create sliding friction
Wheels, bearings and other "rotating" objects follow the laws of rotating friction. This friction is almost always much less than the friction generated simply by sliding an equivalent object along a surface - this is because these objects tend to roll and not slide. To increase friction in a mechanical system, try removing wheels, bearings, and all rotating parts.
For example, consider the difference between pulling a heavy weight on the ground on a wagon versus a similar weight on a sled. A wagon has wheels, so it is much easier to tow than a sled, which slides against the ground, generating a lot of friction
Step 6. Increase the viscosity of the fluid
Solid objects aren't the only ones that create friction. Fluids (liquids and gases such as water and air, respectively) can also generate friction. The amount of friction generated by a fluid flowing against a solid depends on many factors. One of the simplest to check is the viscosity of the fluid - that is, that is often referred to as "density". Generally, very viscous fluids ("thick", "gelatinous", etc.) generate more friction than less viscous ones (which are "smooth" and "liquid").
Consider, for example, the effort it takes to drink water through a straw and the effort it takes to drink honey. It is very easy to suck up the water, which is not very viscous. With honey, however, it is more difficult. This is because the high viscosity of honey creates a lot of friction along the narrow path of the straw
Method 2 of 2: Increase Fluid Resistance
Step 1. Increase the area exposed to the air
As mentioned earlier, fluids such as water and air can generate friction as they move against solid objects. The frictional force that an object undergoes during its movement in a fluid is called fluid dynamic resistance (in some cases this force is referred to as "air resistance", "water resistance", etc.). One of the properties of this resistance is that objects with a larger section - that is, objects that have a wider profile to the fluid through which they move - suffer more friction. The fluid can push against more total space, increasing the friction on the moving object.
For example, suppose that a stone and a sheet of paper both weigh one gram. If we drop both of them at the same time, the stone will go straight to the ground, while the paper will slowly flutter downwards. This is the principle of fluid dynamic resistance in action - the air pushes against the large and large surface of the sheet, slowing its movement much more than it does with the stone, which has a relatively small section
Step 2. Use a shape with a higher fluid drag coefficient
Although the section of an object is a good "general" indicator of the value of the fluid dynamic resistance, in fact, the calculations to obtain this force are slightly more complex. Different shapes interact with fluids in different ways during movement - this means that some shapes (for example, a circular plane), can undergo much greater resistance than others (for example, spheres) made from the same amount of material. The value that relates form and effect on drag is called the "fluid dynamic drag coefficient" and is higher for forms that produce more friction.
Consider, for example, the wing of an airplane. The typical wing shape of airplanes is called an airfoil. This shape, which is smooth, narrow, rounded and streamlined, cuts through the air with ease. It has a very low drag coefficient - 0.45. Imagine instead if an airplane had sharp, square, prismatic wings. These wings would generate a lot more friction, because they couldn't move without offering a lot of air resistance. Prisms, in fact, have a much higher drag coefficient than the airfoil - about 1.14
Step 3. Use a less aerodynamic body line
Thanks to a phenomenon related to the drag coefficient, objects with larger, squared off flow lines usually generate more drag than other objects. These items are made with rough, straight edges and usually don't get slimmer in the back. On the other hand, objects that have aerodynamic profiles are narrow, have rounded corners and usually shrink in the back - like the body of a fish.
Consider for example the profile with which today's family sedans are built versus what was used decades ago. In the past, many cars had a boxy profile and were built with many sharp and right angles. Today, most sedans are much more aerodynamic and have a lot of gentle curves. This is a deliberate strategy - the airfoils greatly decrease the drag encountered by cars, reducing the amount of work the engine has to do to propel the car (thereby increasing fuel economy)
Step 4. Use a less permeable material
Some types of materials are permeable to fluids. In other words, they have holes that fluids can pass through. This effectively reduces the area of the object against which the fluid can push, reducing drag. This property is also true for microscopic holes - if the holes are large enough for some fluid to pass through the object, the resistance will be reduced. This is why parachutes, designed to create a lot of resistance and slow down the fall rate of those who use them, are made with strong nylon or light silk fabrics and breathable nonwovens.
For an example of this property in action, consider that you can move a ping pong paddle faster if you drill a few holes in it. The holes let air pass through the racket when it is moved, greatly reducing drag
Step 5. Increase the speed of the object
Finally, regardless of the shape of the object or its permeability, the resistance always increases in proportion to the speed. The faster the object goes, the more fluid it has to pass through, and consequently, the higher the resistance. Objects that move at very high speeds can experience very high resistance, so they usually have to be very aerodynamic or will not withstand the resistance.
Consider, for example, the Lockheed SR-71 "Blackbird", an experimental spy plane built during the Cold War. The Blackbird, which could fly at speeds greater than 3.2, suffered extreme aerodynamic drag at those speeds, despite its optimal design - the forces were so extreme that the airplane's metal fuselage expanded due to the heat generated by friction. of the air in flight
Advice
- Don't forget that extremely high friction can cause a lot of energy in the form of heat! For example, avoid touching the car's brakes after using them a lot.
- Remember that very strong resistances can cause structural damage to an object moving through a fluid. For example, if you put a plank of wood in the water while driving on a speedboat, there is a good chance it will crack.
