Friction

Introduction

Newton's First Law states that an object in motion tends to stay in motion. Objects we encounter in daily life, however, do not keep moving forever. This is due to a force called friction that stops moving objects.

What is friction?

Friction is a force that resists motion. It occurs when one object is in contact with another object. When two surfaces are pressed together, the molecules of these surfaces come in contact and experience attractive forces between one another. In addition, rough materials can 'catch' on other materials, further impeding (stopping or slowing down) their motion. Friction is found almost everywhere. The only place where there is no friction is in a vacuum, as even air has friction.

Certain materials exert more friction than others. For example, a rough surface (such as a carpet) exerts more friction than a smooth one (such as an ice skating rink). The state of the material an object is in contact with also affects friction. There is more friction between two solid surfaces, for example, than between a solid object and a gas.

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Friction does not only stop objects from moving. It also causes objects to heat up or make a sound. When you rub your hands together, for example, they begin to feel warmer because friction is heating them up. In addition, rubbing your hands together makes a sound.

There are three main types of friction: sliding friction, rolling friction and fluid friction.

Sliding friction

Sliding friction happens when one objects slides over, or rubs against, another. It can be observed by trying to push a wooden box across a floor or table. This force keeps and object still and prevents it from moving when a force is applied.

Sliding friction is stronger when an object is stationary (this is called static friction) than when the object is in motion (this is called kinetic friction). When trying to move a box, it is often harder to get the box moving than it is to keep the box in motion.

Rolling friction

Rolling friction happens when a round object rolls from one place to another. This kind of friction can be seen in objects like balls and wheels. There are two types of rolling friction: starting friction and rolling friction.

Starting friction happens when a force is applied to a ball or wheel. It is affected by the static friction that is a part of sliding friction. If the force applied to a wheel is less than the force of static friction, the wheel will grip the surface and begin to roll. If the force is more than that of static friction, the wheel will slip. Rolling friction is a force that slows the progress of a ball or wheel once it is already in motion. It can be observed by watching a ball rolling through dirt. After some time, the ball will eventually slow down and stop.

Rolling friction is not as strong as sliding friction, which can be helpful in some situations. If you were to place round wooden dowels between the box from our last example and the floor, it would be much easier to push the box across the floor or table than it would be to simply slide the box.

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Fluid friction

Fluid friction is friction between a solid and a fluid such as a gas or liquid. The amount of friction depends on the fluid and its viscosity, or how thick it is. A high viscosity fluid will have more friction than a low viscosity fluid.

An example of fluid friction is fluids moving through a hose. Some fluids, such as water, will move through the hose easily because they have low viscosity and low friction. Other fluids, like grease, will have a harder time moving through the hose because they have a high viscosity and therefore high friction.

Friction is the cause of phenomena such as air resistance and water resistance, also known as drag. Air resistance slows down vehicles like cars and aeroplanes as they move. It also slows down thrown objects, such as balls. Water resistance slows down swimmers and other objects travelling through the water.

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Friction: helper or hindrance?

Friction is an important force in many day-to-day actions. Walking, for example, depends on friction. As your foot pushes backwards on the ground, friction keeps it from moving backwards, but the reaction force pushes you forwards. (Refer Chapter 4)

Friction can also be a hindrance. Some types of machinery have strong friction forces within them. Friction can cause wear, or damage to surfaces, so machinery can be damaged if friction forces are too strong. For cases like this, friction can be reduced using a lubricant, which is a material that reduces friction between two objects. Oil is an example of a lubricant.

In other cases, friction can slow down a vehicle or a person. Vehicles are designed to be shaped aerodynamically, or in a way that reduces friction between the air and the vehicle. Swimmers wear special suits that are very smooth to reduce friction between themselves and the water so they can move more quickly.