Friction is a force that resists motion

One important force engineers have to consider when making a machine that either moves or has moving parts is friction. Friction is the force that resists motion. Friction is caused by solids, liquids, or gases rubbing against each other and creating forces in the opposite direction.

You might know from personal experience that it is a lot easier to slip and fall when you are skating on ice than when you are walking on grass. It is also much easier to move on a slip-n-slide than on a sidewalk. Smoother and slicker surfaces like ice generate much less friction than rough and dry surfaces like sidewalks.

There are many different kinds of friction, including static friction, kinetic friction, and fluid resistance. In science, “fluids” are things that can flow, which include both gases (like air) and liquids (like water). Fluid resistance is also called drag. The forms of friction are grouped by the type of solids, liquids, or gases that are interacting with each other and how they are moving.

Dry friction occurs between dry solid surfaces

Static friction and kinetic friction are both kinds of dry friction. Static friction is made between two dry objects that are not yet moving. When you push a heavy box, but it does not move, you have created static friction. The static friction between the ground and the box resisted the motion you were trying to cause and made the box stay still. If you were able to overcome this force and make the box move, it would still feel difficult to keep it moving. This is because of kinetic friction, the friction between two moving surfaces. Kinetic friction is always less than static friction. Therefore, it is easier to keep the box moving than it is to initially push the box.

Higher amounts of dry friction are created by more weight being pressed into another object. Therefore, the heavier the object, the more friction it creates. You can create additional friction by pushing down hard on an object. The more force pushing the surfaces of the objects together, the greater the friction force becomes.

Values of dry friction are represented by coefficients of friction. If one object is harder to move than another object of the same weight on the same surface, it is said to have a higher coefficient of friction. The object that can be moved more easily has a lower coefficient of friction.

Fluid friction depends on viscosity, which is the thickness of a fluid

Honey is thicker and more viscous than and flows much more slowly.
Viscosity is the density and thickness, of a fluid. The thicker a liquid is, the higher the viscosity and the more friction it creates. Thicker and more viscous fluids like honey and maple syrup are known for moving slower. This is a result of fluid friction slowing them down. Water has a lower viscosity and lower fluid friction. That is why it is faster to pour water than it is to pour syrup.

Fluid resistance is friction created when objects move through gases and liquids

Top: Umbrella and runner with a parachute, both items create drag by catching moving air. Bottom: Aerodynamic cycling helmet created to reduce wind resistance (drag); dolphins are hydrodynamic - shaped well to swim through water with minimal drag
When an object moves quickly through a gas, like air, or a liquid, like water, the gas or liquid pushes back and creates a resistance, commonly known as drag. At slow speeds this might not be very noticeable, but as you speed up, it becomes a strong force. Drag is a common problem among fast forms of transportation like cars, trains, boats, planes, rockets, and even bicycles. The higher the amount of drag, the more energy has to be used to make the vehicle move.

Engineers try to lower the amount of air resistance created by objects by making them more aerodynamic. This means they shape the object in such a way that it moves through the air more easily. Ways to make something more aerodynamic include minimizing flat surfaces and shaping objects with curves and channels such that they funnel the air around them. Small changes in shape can have large changes in the amount of air resistance they cause. Similarly, engineers design boats to reduce the amount of resistance the boat encounters when moving through the water. Something that is shaped to move through the water efficiently is called hydrodynamic.

Friction makes machines less efficient

Ball bearings and lubricants are used to reduce friction in moving parts
With machines, efficiency is a measure of how well the machine is running. A machine with higher efficiency wastes less energy. Machines that have parts that rub against each other or against the ground need parts like wheels and ball bearings that reduce the friction created when two objects rub against each other. If the amount of friction is high, energy that could be used to power the machine will be wasted trying to overcome this friction. This machine will have a lower efficiency. Engineers can also add things like lubrication to help make parts slide more easily. Lubricants include oil, grease, and water. An example of a lubricant is the water on a slip-n-slide. The water coating on the slide makes it slicker and lubricated, reducing friction when you go sliding down it.

One research institute estimates that at least one-third of the energy put into a car is used to overcome friction. Only about 21 percent of all the energy put into the car is actual used to make it move. The rest of the energy is wasted in friction, heat, and inefficiencies in the car.

Friction can be a helpful force

Friction does have positive uses! Friction helps to create traction which allows us to walk without immediately falling down. Our shoes have bottoms on them with different patterns of ridges and materials that help create more friction with the ground. Without friction we would be unable to create the force needed for us to walk.

Additionally, friction can be used to start a fire. If you have ever watched any survival shows, you may have seen people rubbing two sticks together to start a fire for themselves. This is because the rubbing of the sticks together creates friction which in turn creates heat. If they are able to generate enough friction, a fire can be started.

Friction also creates forward motion in cars. As the wheels on a car turn, the rubber material of the tires and the hard asphalt ground create a friction force that moves the car forward. If this friction was not generated, the wheels of the car would spin without moving the car at all. If you have ever seen a car stuck in the snow or mud, the tires may spin around and around and not help the car go anywhere, because the tires cannot get a firm purchase on any solid ground. The goal of tires is to create as much friction with the ground as possible to be able to move the car forward.

Friction is an important concept in engineering. Friction resists motion and often causes a waste of useful energy. Although it is often something to be overcome in the design of a machine, it can also be helpful in everyday life.