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 coefficients indicate greater friction): 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
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 on a car 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. [4] X Research source On a bike, the brake pads press onto the metal frame of the tire to stop them from rotating.
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.
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.
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.
For instance, consider the difference in the 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. [7] X Research source
Viscosity of most fluids can be increased by lowering the temperature of the fluid. For example, a marble falls more slowly through cold molasses than molasses at room temperature.
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.
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. Objects with bigger, boxier “body flows” generally generate more drag than other objects. 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 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.
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. [8] X Research source
