Meaning and types of friction

Introduction to Friction

Imagine you are pushing a heavy bag of garri across a cement floor. You will notice that you need to apply some force to make it move. If you stop pushing, the bag stops immediately. Why? There is an invisible 'enemy' force trying to stop the movement. We call this force friction.

Friction is a force that acts between two surfaces in contact that are sliding or trying to slide across each other. It always works in the direction opposite to the direction of motion.

In simple terms, friction is a 'stubborn' force that tries to prevent one object from rubbing against another.

Static and Kinetic Friction

There are two main types of friction you need to know for your exams: Static friction and Kinetic (or sliding) friction.

Static friction is the force you must overcome to start an object moving. Think of trying to push a broken-down Okada. The hardest part is getting it to start rolling. Once it starts moving, it feels a bit easier. The friction when the object is still is static friction.

Basically, it is the friction that keeps an object 'glued' to a spot until you push hard enough.

Limiting friction is the maximum value of static friction. It is the point where the object is just about to start moving.

Kinetic friction happens when the object is already moving. When you are finally pushing that Okada down the road at a steady speed, the friction acting against you now is kinetic friction.

In simple terms, it is the friction that tries to slow down an object that is already sliding.

Factors affecting friction

Friction depends on two main things:

The nature of the surfaces: A rough road has more friction than a tiled floor wiped with soap.

The normal reaction: This is related to how heavy the object is. A full bag of cement is harder to push than an empty one because it presses harder against the floor.

Worked Example

A wooden box is placed on a rough table. A horizontal force of 20 N is applied, but the box does not move. When the force is increased to 25 N, the box is just about to slide. What is the limiting friction?

1. Identify the state of the box: The box is 'just about to slide' at 25 N.

2. Recall the definition: Limiting friction is the maximum static friction before motion starts.

3. Conclusion: The limiting friction is 25 N.