Friction definition in physics

Frictional forces are forces that act between the surfaces of two objects in contact with each other to oppose relative motion when the objects are moving relative to each other or to oppose the tendency to move when the objects are moving relative to each other.

Frictional forces exist in the motion of liquids and gases as well. Let us discuss the frictional forces that act between solid bodies in this lesson.

Friction force explained

We know from experience that if we put something like a pencil on a table and tap on it to get it moving, its speed will decrease until it comes to rest. If the same thing were put on a surface that was even slightly smoother than the table, it would roll farther before coming to a stop.

Since the surface exerts a force on the object to oppose its motion, the object gradually slows down and comes to rest as described above. Frictional force is the term used to describe this type of force acting in the opposite direction. When something is moving, friction acts as an obstacle.

Simple explanation for friction

If we use a very light force to move the table, it may not move. This is due to the floor exerting a force on the table that opposes the force we apply. The magnitudes of the two forces are the same, but their directions are opposite. They therefore cancel each other.

Let us now slightly increase the force we apply to the table. If the table still does not move, it means that the frictional force exerted by the floor has automatically increased to balance the force applied. The frictional force is a force that adjusts automatically to balance the force we apply.

However, if we continue to apply force to the table, it will begin to move at some point. This occurs because the frictional force cannot automatically adjust above a certain threshold. When we apply a force that exceeds the limit, an unbalanced force equal to the difference between the two opposing forces remains. This unbalanced force causes the table to move.

Static, limiting and dynamic states of frictional forces

Frictional forces are classified into three types based on the situations in which they act on a body. They are as follows:

  • The frictional forces that act when there is no relative motion despite the application of a force to the body.
  • The frictional forces exerted on a body when it initially starts to move. This force includes the slight additional force required to give it velocity.
  • The frictional forces that act on bodies during relative motion.

Simple activity to demonstrate the friction force stages

Let us perform the following activity to examine the differences between the three types of frictional forces mentioned above.

Attach a small ring to the wood block and the balance to the ring.

Place the block of wood on a horizontal table and pull with a light touch. The magnitude of the applied force would be read from the Newton balance. The force would initially be insufficient to move the block.

Increase the force on the object gradually. When the force is gradually increased, it will eventually begin to move. Find the force just as it is about to move.

Static friction

The maximum frictional force exerted by the table’s surface to oppose the motion is equal to the force required to initiate the motion. When the applied force is less than the maximum frictional force, the table’s surface exerts a frictional force on the body that is equal to and opposite to the applied force. The frictional force exerted on the body prior to motion is referred to as static friction.

Limiting frictional force

The static friction acting on the body increases as the applied force is gradually increased. However, the static frictional force can only increase to a certain limit. When the applied force exceeds this maximum value, the frictional force cannot increase any further to keep the body in equilibrium. As a result, the body begins to move and gains a small velocity. The limiting frictional force between two bodies is the maximum frictional force between their surfaces when they are in contact.

Dynamic friction

Dynamic friction between the two surfaces refers to the frictional force acting on the body after it begins to move. In other words, dynamic friction is the frictional force acting on moving bodies. The limiting frictional force is slightly less than dynamic friction.

Factors affecting the limiting frictional force

Frictional forces act between objects that come into contact. The limiting frictional force is determined by the nature of the surfaces in contact and the normal reaction between them, and not by the area of the contact surfaces.

When the normal reaction between the two forces increases, also increases the limiting frictional force.

How to find normal reaction?

Weight of the object = Perpendicular reaction force

Practical applications of friction

Most of the machines and instruments we use on a daily basis have multiple parts in contact with one another. When we use these machines and instruments, the parts slide against one another, creating frictional forces. As a result, when machines are operated, a large amount of additional work must be done against frictional forces, resulting in energy loss. This energy loss manifests as heat, raising the temperature of the object. We would be able to reduce energy loss and temperature rise if we could reduce frictional forces.

Methods of reducing friction

Lubricants such as graphite, lubrication oil, or grease are applied between the contact surfaces.

Contact surfaces’ roughness is reduced or polished.

Inserting balls capable of rolling between the contact surfaces. This is how ball bearings are made, which are used to connect most of the rotating parts of vehicles and machines to non – rotating parts.

Advantages of frictional forces

  • We can only walk on a surface because the frictional force exerted by that surface on our feet keeps us from slipping. We tend to slip and fall when walking on a wet or oily surface due to a lack of friction.

  • Grooves are etched into the tire’s surface to increase friction between the tire and the road surface. On wet roads, a water layer between the road and the tire reduces friction between the two surfaces, causing vehicles to skid. The purpose of grooves on the surface of tires is to allow water to pass through them.

  • Coir ropes are made by twisting together a large number of coir fibers. Even when a large force is applied to a rope, the fibers do not separate due to friction.

  • Because of friction, moving vehicles can be brought to a halt by applying brakes. The friction between the surfaces of the rubber break-pads and the wheels causes the bicycle to come to a halt.

  • Disc-breaks are used in modern automobiles to bring the vehicle to a halt. In such systems, the wheels are stopped from rotating by the frictional forces created by pressing the break-pads against a disc attached to the wheel.

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Anuradhika Lakmali

Anuradhika Lakmali is a co-founder of Science A Plus learning network. She is working as a government teacher and has interest in chemistry, biology, phisics and self development.