Friction

Friction is the force which resists movement between objects. Friction is not considered to be a fundamental force - a force that is necessary in order for the universe to function. Friction does not only occur in visible objects but also within air and water.

This story was created for the Google Expeditions project by Vida Systems, now available on Google Arts & Culture.

Friction by Vida Systems

Friction does not only occur in visible objects but also within air and water.

Many human made objects need to take into account friction in their design and either mitigate its influence or use it to its advantage. 

History

Humans have been aware of the presence of friction for thousands of years. The invention of the wheel and ball bearings indicate knowledge that lessening friction helps move objects along.

Ancient Greek philosophers were interested in friction and were aware that there are at least two types of friction - static and kinetic. It wasn’t until the Renaissance that scholars really began to study and understand this force. 

DaVinci

As part of his broad range of studies Leonardo DaVinci was interested in friction and developed the basic laws of friction. However these notes were never published and remained undiscovered for hundreds of years.

Amontons

Guillaume Amontons rediscovered the basic laws of friction 200 years later in 1699. His laws only apply to dry friction, like a brick sitting on an inclined plane.

Coulomb

Charles-Augustin de Coulomb continued this work in 1785, verifying Amonton’s laws and expanding on them. 

Modern work

During the 1950’s friction was studied at a microscopic level. The development of the atomic force microscope allowed scientists to study friction at an atomic level, verifying Amontons first law.

Static friction

Static friction is the force between a resting object and the surface on which it is resting, for example a brick sitting on a ramp. In order to move the brick static friction needs to be overcome. Once the brick is moving, kinetic friction needs to be overcome. 

To move the brick and overcome static friction one of two things needs to occur; the incline of the plane needs to increase or something needs to physically push the brick.

Bikes

Riding a bike requires static friction to be overcome. This is achieved by pushing the bike, or using the pedals.

Balancing rocks

The art of rock balancing uses static friction to its advantage. Rocks balanced usually have to have three points of contact and use static friction to keep the rocks in place.

Inclines

One of the most common examples of static friction is an stationary object on an incline. In order to overcome static friction the angle of incline needs to increase or a physical force needs to be applied to the stationary object.

Closer Look by Vida Systems

Closer look

Although the surfaces of the objects may look smooth at a microscopic level you can see that the surface is very uneven, allowing static friction to occur.

Rolling friction

Rolling friction is the force that slows down a rolling object. A number of factors influence the amount of rolling friction that occurs including the material the wheel is made out of and the type of surface it is trying to roll across.

Rolling friction can be demonstrated when rolling a ball across the floor. The ball will eventually come to a stop, without rolling friction the ball would keep travelling until it hit another object.

Size and shape

The size and shape of the rolling object will affect how much rolling friction needs to be overcome. Wider tires on a bike increases rolling friction, giving bikers more control over rough terrain. 

Tread

Tread on car tires are designed to increase rolling friction. Without high rolling friction a car could slip and lose traction, causing accidents.

Bowling

The relatively heavy bowling bowl slides easily along a bowling lane. This is due to the smoothness of both the bowling ball and the highly polished lane, drastically reducing rolling friction.

Weight

The weight of the rolling object will affect the amount of rolling friction. For example a large truck will have greater rolling friction than a car because the truck has a heavier load pressing down on the wheels, creating greater rolling friction.

Kinetic friction

Kinetic friction is the force acts between two moving surfaces. Kinetic friction can only occur when at least one object is moving. Kinetic friction is similar to rolling friction as they both act to stop the moving object. 

Kinetic friction creates heat, in some instances this heat generation needs to be overcome. In others the heat generation is embraced.

Rubbing hands

Rubbing your hands together is an example of kinetic friction. The friction between your hands creates heat, warming you up.

Windscreen wipers

Kinetic friction occurs when windscreen wipers are used. The water helps lubricate the glass, allowing the wipers swipe smoothly. Using windscreen wipers on dry glass causes the wipers to shudder due to high kinetic friction.

Sliding

When an object slides across a surface it encounters kinetic friction. This is different to rolling friction where the object is round in shape.

Skiing

To go faster wax can be added to the bottom of skis which reduces kinetic friction. The snow itself also slides around under the skis, creating kinetic friction. Gravity is the main force that helps you overcome kinetic friction when downhill skiing. 

Fluid friction

Fluid friction is the force that resists and object moving through liquid or gas. This type of friction can be separated further into viscous resistance (resistance to an object moving through a liquid) and air resistance (resistance to an object moving through gas). 

Thickness

The thickness of the liquid present will change the amount of viscous resistance encountered. Stirring a straw through a thickshake is much harder than stirring a straw in a glass of water.  

Hail

Hail falling out of the sky encounters air resistance. This resistance means that the hail can only travel up to a certain speed while falling - called terminal velocity. 

Honey

Honey is a non-Newtonian fluid - a fluid that behaves differently to water. When stirred continually fluid friction decreases (it becomes more ‘fluid like’). Another non-Newtonian fluid, cream, creates more friction when stirred continually.

Reducing friction

Friction can be both a useful force and a hinderance. Friction is useful because, without it, nothing would stay still! On the other hand, too much friction means that objects couldn’t move at all.

In many situations the amount of friction needs to be reduced to either help an object move, reduce wear and tear on an object or reduce the amount of heat produced. There are a number of methods used to reduce friction between objects.

Streamlined

A bulky, square shaped object needs much more force to get it to move through water and air compared to a streamlined object. This is why many marine animals have similar shaped bodies and why racing cars have streamlined designs.

Smooth

Smoother surfaces will reduce friction, allowing objects to travel with less resistance. Elite swimmers have special swimsuits which are engineered to reduce fluid resistance and allow them to move faster through the water. 

Lubrication

Lubrication is a way of making the surface of an object smoother. Lubricants are a slippery substance which provides a slippery barrier between two moving parts. Oil on a squeaky hinge stops the two parts rubbing together.

Water

Water is a type of lubricant. This is why spilled water can be a hazard as it reduces the amount of friction between the floor and your foot.

Reduce contact

Wheels are designed to reduce the amount of friction between the ground and the object needing to be moved. Rolling friction is also easier to overcome compared to static friction.

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